Nature’s Chemistry May 2017
DUNCANRIG SECONDARY SCHOOL
CHEMISTRY DEPARTMENT
National 5 Chemistry
Unit 2 Nature’s Chemistry Key Facts and Theory
Hydrocarbons, Homologous Series, Alcohols, Carboxylic Acids, and Energy from fuels May 2017 version
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Nature’s Chemistry May 2017
Hydrocarbons
A hydrocarbon is a compound made up of only hydrogen and carbon atoms, e.g. hexane, C6H14, is a hydrocarbon but acetone, C3H6O, is not a hydrocarbon because acetone contains atoms of oxygen as well.
Since both carbon and hydrogen are non-metal atoms, hydrocarbons are made up of molecules with the atoms joined by covalent bonds, i.e. by the sharing of electron pairs.
Carbon is in Group 4 of the Periodic Table and so atoms of carbon have four electrons in the outer shell (energy level) and form four bonds.
Hydrogen atoms have one electron in the first shell (energy level) and forms one bond.
What is meant by a hydrocarbon?
Complete the following table.
Name Formula Hydrocarbon (yes or no)
octane C8H16
ethylamine C2H5NH2
glucose C6H12O6
natural gas CH4
water H2O
carbon monoxide CO
alcohol C2H5OH
acetylene C2H2
How many covalent bonds are formed by … carbon atoms? … hydrogen atoms?
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The alkanes Activity 2.1
Methane, ethane and propane are the first three members of a series of hydrocarbons called the alkanes. The alkanes are a subset of the set of hydrocarbons.
Each member of the alkane series has a name that ends in –ane and a prefix that indicates the number of carbon atoms in the molecule, e.g. methane is the alkane with one carbon atom per molecule.
Prefix Number of carbon atoms in the molecule
meth - 1
eth - 2
prop - 3
but - 4
pent - 5
hex - 6
hept - 7
oct - 8
All the hydrocarbons in the alkane series are known as saturated hydrocarbons. A saturated hydrocarbon is one in which all the carbon to carbon bonds are single covalent bonds.
What is meant by a saturated hydrocarbon?
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Alkanes continued
The hydrocarbon with just one carbon atom is called methane.
In a molecule of methane, one carbon atom is joined to four hydrogen atoms to give a stable electron arrangement for all the atoms in the molecule.
The formula for methane is CH4. This gives the number of atoms of each element in the molecule but CH4 does not give any information about the arrangement of atoms in the molecule. Information about the arrangement of atoms is given by the full structural formula.
The full structural formula is ‘flat’. The actual structure of methane is based on the three-dimensional tetrahedral arrangement of electrons in a carbon atom:
Molecules of ethane have two carbon atoms joined by a single covalent bond.
Each carbon atom can form three bonds with hydrogen atoms.
Molecules of propane have three carbon atoms joined by single covalent bonds.
Three hydrogen atoms are attached to the end carbon atom; two hydrogen atoms are attached to the middle carbon atom.
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Methane, ethane and propane
A shortened structural formula can be used to show the grouping of hydrogen atoms round each carbon atom. The different ways of representing alkanes are shown below.
Name of Full structural Shortened Formula alkane formula structural formula
Methane CH4 CH4
Ethane
CH3CH3 C2H6
What information is given by the full structural formula?
Complete the table to show the different ways of representing propane.
Name of Full structural Shortened structural Formula alkane formula formula
Propane
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Alkanes continued
For the alkanes with four, five, six, seven and eight carbon atoms per molecule: write the name, write the formula, draw the full structural formula, and draw the shortened structural formula.
4 carbons Formula: Name:
Full structural formula: Shortened structural formula:
5 carbons Name: Formula:
Full structural formula: Shortened structural formula:
6 carbons Name: Formula:
Full structural formula: Shortened structural formula:
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Alkanes continued
7 carbons Name: Formula:
Full structural formula: Shortened structural formula:
8 carbons Name: Formula:
Full structural formula: Shortened structural formula:
Alkanes are an example of a homologous series. A homologous series is a family of compounds that can be represented by a general formula and have similar chemical properties.
The general formula for the alkanes is CnH2n+2. Each of the alkanes has two hydrogen atoms for every carbon atom plus two hydrogen atoms at the end.
Alkanes are a homologous series of saturated hydrocarbons
can be represented by the general fomula CnH2n+2 are very flammable and are commonly used as fuels are insoluble in water slowly decolourise bromine water (substitution reaction).
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Nature’s Chemistry May 2017
The Alkenes Activity 2.2
Ethene and propene are the first two members of a series of hydrocarbons called the alkenes. The alkenes are another subset of the set of hydrocarbons.
Each member of the alkene series has one carbon to carbon double bond. The name ends in –ene and a prefix (same as for alkanes) indicates the number of carbon atoms in the molecule, e.g. ethene is the alkene with two carbon atoms per molecule.
All the hydrocarbons in the alkene series are known as unsaturated hydrocarbons. An unsaturated hydrocarbon has (at least) one carbon to carbon double bond in each molecule.
What is meant by an unsaturated hydrocarbon?
Ethene and propene Carbon atoms can also join up by the sharing of two electron pairs, i.e. by forming two covalent bonds.
The two covalent bonds between two carbon atoms is called a double covalent bond.
Each carbon atom can only form two bonds with other atoms
The hydrocarbon with two carbon atoms joined by a double covalent bond is called ethene.
Molecules of propene have three carbon atoms joined by covalent bonds. One is a double covalent bond.
Two hydrogen atoms are attached to carbon atom 1; one hydrogen atom is attached to carbon atom 2; three carbon atoms are attached to carbon atom 3. The different ways of representing ethene and propene are shown below.
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Full structural formula Shortened structural Formula formula
C2H4
Full structural formula Shortened structural Formula formula
CH2=CHCH3 C3H6
For the alkenes with four, five, six, seven and eight carbon atoms per molecule: write the name, write the formula, draw the full structural formula, and draw the shortened structural formula
4 carbons Name: Formula:
Full structural formula: Shortened structural formula:
5 carbons Name: Formula:
Full structural formula: Shortened structural formula:
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6 carbons Name: Formula:
Full structural formula: Shortened structural formula:
7 carbons Name: Formula:
Full structural formula: Shortened structural formula:
8 carbons Name: Formula:
Full structural formula: Shortened structural formula:
Alkenes are another example of a homologous series. The general formula for the alkenes is CnH2n. Each of the alkenes has two hydrogen atoms less than the alkanes due to the double (rather than the single) covalent bond.
Alkenes are a homologous series of unsaturated hydrocarbons
can be represented by the general formula CnH2n are very flammable used to make alcohols and polymers are insoluble in water immediately decolourise bromine water (addition reaction)
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The cycloalkanes
Carbon atoms can join together with covalent bonds to form closed chains or ‘rings’. Alkanes with a ring of carbon atoms are called cycloalkanes.
Cycloalkanes are another subset of the set of hydrocarbons. Each member of the cycloalkane series has a name beginning with ‘cyclo’ to indicate that there is a ring of carbon atoms. The name ends in –ane to indicate that all the carbon to carbon bonds are single covalent bonds and a prefix (same as for alkanes) indicates the number of carbon atoms in the molecule.
The first member of the series is cyclopropane with three carbon atoms joined in a ring by single covalent bonds.
Each carbon atom is able to form two covalent bonds with hydrogen atoms.
The different ways of representing cyclopropane are shown below.
Full structural formula Shortened structural formula Formula
C3H6
What is meant by a cycloalkane?
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The cycloalkanes continued
For the cycloalkanes with four, five, six, seven and eight carbon atoms per molecule: write the name, write the formula, draw the full structural formula, and draw the shortened structural formula
4 carbons Name: Formula:
Full structural formula: Shortened structural formula:
5 carbons Name: Formula:
Full structural formula: Shortened structural formula:
6 carbons Name: Formula:
Full structural formula: Shortened structural formula:
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7 carbons Name: Formula:
Full structural formula: Shortened structural formula:
8 carbons Name: Formula:
Full structural formula: Shortened structural formula:
Cycloalkanes are another example of a homologous series. The general formula for the cycloalkanes is also CnH2n. Each of the cycloalkanes has two hydrogen atoms less than the corresponding alkane due to the bond that closes the ring.
Cyclolkanes are a homologous series of saturated, cyclic hydrocarbons
can be represented by the general formula CnH2n are very flammable and are commonly used as fuels are used as solvents are insoluble in water slowly decolourise bromine water (substitution reaction).
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Naming alkanes
Carbon compounds are given a systematic name according to an internationally accepted convention.
There are three different structures for the compound with molecular formula C5H12. Their shortened structural formulae are shown below.
A
B C
Structure A is called a straight chain hydrocarbon. Structures B and C are called branched chain hydrocarbons.
For naming straight-chain alkanes name them the same way as on pages 5&6.
Branches are named after the corresponding alkane with the –ane ending changed to -yl.
methyl group e.g. ethyl group
To name an alkane: 1. Select the longest continuous chain of carbon atoms and name it after the appropriate alkane. 2. Number the carbon atoms from the end of the chain nearer the branch. 3. Name the branch(es) and indicate the position(s) of the branch(es) on the chain. 4. Use ‘di’ and ‘tri’, etc. when the same branch is present more than once. e.g.
2-methylbutane
3-methylpentane
2,3-dimethylbutane 14
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Naming alkanes Name each the following alkanes.
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Naming alkenes
Alkenes are named in a similar way to alkanes.
1 Select the longest continuous chain of carbon atoms containing the double bond and name it after the appropriate alkene. 2. Number the carbon atoms from the end of the chain nearer the double bond and indicate the position of the double bond. 3. Name any branch(es) and indicate the position(s) of the branch(es) on the chain.
but-1-ene but-2-ene 3-methylbut-1-ene
Name each of the following alkenes.
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Homologous Series
A homologous series is a family of compounds which
fit a general formula have similar chemical properties show a gradual change in physical properties
Complete the following table.
Family General formula
alkanes
alkenes
cycloalkanes
Use the general formulas to complete the following table.
Number of carbon atoms in the molecule Formula
alkane 6
alkane 9
alkene 8
alkene 12
cycloalkane 7
cycloalkane 10
In each of the following lists of hydrocarbons identify the one that is in a different homologous series from the others.
ethane butene methane octane
C3H8 CH4 C7H14 C12H26
C2H4 C6H12 CH4 C4H8
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Cycloalkenes are hydrocarbons with one carbon to carbon double bond. They are another example of a homologous series.
Cyclohexene
Try and work out the general formula of the cycloalkenes
Homologous series- Physical changes
Complete the following tables using your data booklet and work out the state (solid, liquid or gas) of the hydrocarbon at 25oC.
Name Formula b.p. / oC m.p. / oC State at 25 oC
methane
ethane
propane
hexane
Name Formula b.p. / oC m.p. / oC State at 25 oC
ethene
propene
but-1-ene
pent-1-ene
hex-1-ene
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Name Formula b.p. / oC m.p. / oC State at 25 oC
cyclobutane
cyclopentane
cyclohexane
You can see from the data in the three tables that ‘as size of the molecule get bigger, the melting and boiling points increase’.
This pattern is due to the fact that as the size of the molecule increases, the strength of the intermolecular forces increases and so more energy is required to separate the molecules.
Size of molecule Strength of intermolecular forces
Also note that there is a regular change in melting and boiling points this is because there is a regular increase in mass of the molecule.
If we look at successive members of ANY homologous series, the formula differs by a –CH2 group, eg
Methane CH4 Ethane C2H6 Propane C3H8 Butane C4H10 and as a result the relative formula masses differ by 14.
Methane CH4 (16g) Ethane C2H6 (30g) Propane C3H8 (44g) Butane C4H10 (58g)
So there is a regular pattern in the melting and boiling points and you are often asked to predict the melting or boiling point of the next member of the homologous series.
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Isomers
Isomers are compounds with the same molecular formula but different structural formula.
They usually have different physical properties.
The following flow diagram can be used to decide whether or not two compounds are isomers.
Do molecules have the same formula?
YES NO
NOT ISOMERS Do molecules
have different structures?
YES NO
NOT ISOMERS ISOMERS same molecule
Examples:
different formulae 1. NOT isomers propane ethane
2. same formula
same structure
NOT isomers butane butane
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Isomers continued
same formula 3. same structure NOT isomers
2-methylbutane 2-methylbutane
4. same formula different structures
isomers
2,2-dimethylpropane 2-methylbutane
5. same formula different structures isomers but-1-ene but-2-ene
6. same formula same structure
but-1-ene but-1-ene NOT isomers
Isomers may belong to different homologous series, for example both propene and cyclopropane have the molecular formula C3H6 but they have a different structure. So they are isomers.
propene cyclopropane
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Isomers continued
Decide whether or NOT each of the following pairs of hydrocarbons are isomers.
1.
2.
3.
4.
5.
6.
7.
8.
9.
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Isomers continued
Decide whether or NOT each of the following pairs of hydrocarbons are isomers.
1. 2-methylpentane butane
2. 2,3-dimethylbutane hexane
3. 2-methylhexane 3,3-dimethylpentane
4. 2-methylbut-1-ene pent-2-ene
5. pent-1-ene methylcyclobutane
Many carbon compounds, other than hydrocarbons, have isomers.
Examples:
1. same formula different structures isomers
2. same formula same structure NOT isomers
same formula 3. different structures isomers
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Isomers continued
Decide whether or NOT each of the following pairs of carbon compounds are isomers
1.
2.
3.
4.
5.
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Reactions of alkenes Alkanes, alkenes and cycloalkanes are hydrocarbons.
Alkanes and cycloalkanes are saturated; all the carbon to carbon bonds are single covalent bonds. Alkenes are unsaturated; the molecules contain at least one carbon to carbon double covalent bond.
What colour change is observed when bromine (in solution) is added to … a saturated hydrocarbon? …an unsaturated hydrocarbon?
When bromine (in solution) is added to an unsaturated hydrocarbon, the brown colour of the bromine ‘disappears’, i.e. the bromine is immediately decolourised. (It is incorrect to say that the bromine solution goes clear; it is clear to begin with!)
In the reaction, the carbon to carbon double bond breaks and the bromine atoms add on the carbon atoms at either side of this bond. e.g. ethene with bromine
+ Br2
propene with bromine
+ Br2
This kind of reaction is called an addition reaction because of the way that bromine adds on to the alkene. When a halogen is added on across the double bond in an alkene, the product is called a dihaloalkane
The reaction with bromine is the way to distinguish an unsaturated hydrocarbon from a saturated hydrocarbon; the bromine is immediately decolourised by the unsaturated hydrocarbon.
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Reactions of alkenes
Alkenes can also react with hydrogen in an addition reaction. The corresponding alkane is formed. e.g. ethene with hydrogen
+ H2
This addition reaction is called hydrogenation.
What is the difference between a saturated and an unsaturated hydrocarbon?
What is the test for an unsaturated hydrocarbon?
Name the type of addition reaction taking place when hydrogen is added on across adouble bond.
Draw a structural formula formed by the reaction of bromine with each of the following hydrocarbons.
but-1-ene
but-2-ene
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Reactions of alkenes continued
cyclohexene
terpinoline
Draw a structural formula for the product of the reaction of but-1-ene and hydrogen.
Complete the following table.
Alkane Cycloalkane Alkene
General formula CnH2n+2 CnH2n CnH2n
Saturated or unsaturated
Reaction with bromine (yes or no)
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Reactions of alkenes continued
Three different hydrocarbons were treated with bromine solution. Each of the hydrocarbons contained six carbon atoms. The results are shown.
Formula Hydrocarbon Effect on bromine
C6H12 A decolourises quickly
C6H14 B no immediate change
C6H12 C no immediate change
Give the names and draw possible structures for A, B and C.
A Name :
B Name :
C Name :
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The alcohols
Alcohol is found in alcoholic drinks. However, more than one carbon compound can be classified as being an alcohol. Like the alkanes, alkenes and cycloalkanes, the alcohols make up a homologous series. Ethanol (‘eth-’ meaning two) is the alcohol found in alcoholic drinks.
The functional group of the alcohols is the hydroxyl group (-OH). This is the group that gives the characteristic properties to the alcohols. The atoms in a hydroxyl group are joined by a covalent bond and the group is part of a covalent molecule (compare with the hydroxide ion found in alkalis).
Each member of the alcohol series has a name which ends in -anol and a prefix which indicates the number of carbon atoms in the molecule, e.g. ethanol has the hydroxyl group attached to two carbon atoms.
The different ways of representing ethanol are shown below.
Full structural formula Shortened structural formula Formula
CH3CH2OH C2H5OH
For the straight-chain alcohols with one, three and four carbon atoms per molecule (and the hydroxyl group at the end of the carbon chain): write the name, write the formula, draw the full structural formula, and draw the shortened structural formula
1 carbon Name: Formula:
Full structural formula: Shortened structural formula:
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Alcohols continued 2 carbons Name: Formula:
Full structural formula: Shortened structural formula:
3 carbons Name: Formula:
Full structural formula: Shortened structural formula:
4 carbons Name: Formula:
Full structural formula: Shortened structural formula:
5 carbons Name: Formula:
Full structural formula: Shortened structural formula:
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Alcohols continued
6 carbons Name: Formula:
Full structural formula: Shortened structural formula:
7 carbons Name: Formula:
Full structural formula: Shortened structural formula:
8 carbons Name: Formula:
Full structural formula: Shortened structural formula:
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Alcohols continued
What is the group that gives the characteristic properties to the alcohol family of carbon compounds?
What is the general formula for the homologous series of alcohols?
Ethanol can be prepared in industry by the reaction of ethene with water. Since the double bond breaks as atoms are added on to the carbon atoms at either side, this is another example of an addition reaction.
+ H2O
ethene water ethanol
This kind of reaction is also known as hydration.
Why is the reaction of ethene with water called an addition reaction?
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Naming alcohols
From propanol onwards, isomerism can occur due to different positions of the hydroxyl group. e.g.
To name an isomeric alcohol: 1 Select the longest continous chain of carbon atoms containing the hydroxyl group and name it after the appropriate alcohol. 2 Number the carbon atoms from the end of the chain nearer the hydroxyl group and indicate the position of the group. 3 Name any branch(es) and indicate the position(s) of the branch(es) on the chain. e.g.
propan-1-ol propan-2-ol
3-methylbutan-1-ol
Some alcohols have more than one hydroxyl group. e.g.
ethane-1,2-diol propane-1,2,3-triol (ethylene glycol, (glycerol) found in antifreeze)
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Naming alcohols
Name each of the following alcohols.
Draw a structural formula for each of the following alcohols.
2,2 dimethypentan-1-ol
2-methylhexan-3-ol
Draw the full structural formula for each of the FOUR alcohols that have the formula
C4H9OH. Write the name of each alcohol.
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Carboxylic acids
Vinegar is an acidic solution. Ethanoic acid is the name of the solution that gives vinegar the sour taste. Carboxylic acids are used in the manufacture of soaps, preservatives, medicines (aspirin).
Ethanoic acid (‘eth’ indicating two carbons) is the second member of another homologous series called the carboxylic acids.
The group that gives the characteristic properties to the carboxylic acids is the carboxyl group:
Each member of the carboxylic acid homologous series has a name which ends in -anoic acid and a prefix which indicates the number of carbon atoms in the molecule, e.g. ethanoic acid has two carbon atoms including the carbon in the carboxylic acid group.
The characteristic acid group must always be at the end of a carbon chain.
The different ways of representing ethanoic acid are shown below.
Full structural formula Shortened structural formula Formula
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Carboxylic acids
For the straight-chain carboxylic acids with one, three and four carbon atoms per molecule: write the name, write the formula, draw the full structural formula, and draw the shortened structural formula
1 carbon Name: Formula:
Full structural formula: Shortened structural formula:
2 carbons Name: Formula:
Full structural formula: Shortened structural formula:
3 carbons Name: Formula:
Full structural formula: Shortened structural formula:
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Carboxylic acids continued
4 carbons Name: Formula:
Full structural formula: Shortened structural formula:
5 carbons Name: Formula:
Full structural formula: Shortened structural formula:
6 carbons Name: Formula:
Full structural formula: Shortened structural formula:
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Carboxylic acids continued
7 carbons Name: Formula:
Full structural formula: Shortened structural formula:
8 carbons Name: Formula:
Full structural formula: Shortened structural formula:
What is the group that gives the characteristic properties to the carboxylic acid family of carbon compounds?
What is the general formula for the homologous series of carboxylic acids?
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Carboxylic acids continued
With branched chain carboxylic acids, any branch(es) and the position(s) of the branch(es) on the chain are named in the same way as with alcohols. e.g.
3-methylbutanoic acid 2-methylbutanoic acid
Name each of the following carboxylic acids.
Draw a structural formula for each of the following carboxylic acids.
3-ethylpentanoic acid
3,3-dimethylhexanoic acid
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Reactions of Carboxylic Acids
Solutions of carboxylic acids have a pH less than 7. This means they can be involved in neutralisation reactions, the same as other acids such as hydrochloric, nitric and sulfuric.
Remember from Unit 1 that…Bases are substances which neutralise an acid, moving the pH towards 7. Examples of bases are metal oxides, metal hydroxides and metal carbonates. If the metal hydroxide is soluble in water it is called an alkali.
When an alkali, a metal oxide or an insoluble metal hydroxide react with an acid, a salt and water are produced.
acid + alkali salt + water
If a metal carbonate reacts with an acid, carbon dioxide gas is produced as well as the salt and water.
acid + metal carbonate salt + water + carbon dioxide
The name of the salt produced depends on the reactants used.
The first part of the salts name comes from the metal (or positively charged ion) from the base and the end part comes from the acid. The following table shows the ending of the salt name produced when the first eight carboxylic acids are reacted.
Carboxylic acid neutralised Ending of salts name methanoic methanoate ethanoic ethanoate propanoic propanoate butanoic butanoate pentanoic pentanoate hexanoic hexanoate heptanoic heptanoate octanoic octanoate
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Reactions of Carboxylic Acids continued
The word equation for the reaction between methanoic acid and the alkali, sodium hydroxide is shown below:
methanoic acid + sodium hydroxide sodium methanoate + water
Acids also react with some metals to produce a salt and hydrogen gas.
acid + metal salt + hydrogen
This is not classed as a neutralisation reaction as no water is formed. However the salts name is made up the same way.
Complete the following word equations
propanoic acid + lithium hydroxide + water
pentanoic acid + calcium carbonate + +
ethanoic acid + zinc oxide +
+ copper oxide copper octanoate +
+ magnesium magnesium butanoate + 41
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Uses of carbon compounds
Alcohols Alcohols are effective solvents for substances that are insoluble in water. Methylated spirits (or meths) is ethanol mixed with other chemicals. Alcohols with smaller molecules evaporate easily, making them ideal for cleaning solvents eg, cleansing wipes, hand gels and handwash.
The high flammability and the very clean flame with which they burn has resulted in alcohols being used as fuels. Ethanol is mixed with petrol for use as an engine fuel in countries where it can be economically produced in sufficient quantities, e.g. in Brazil. The ethanol can be obtained by the fermentation of sugar cane which can be considered as a renewable source of energy. Methanol is also used as a fuel in car racing.
Carboxylic acids Carboxylic acids are used in the preparation of preservatives, soaps and medicines. Vinegar is a solution of ethanoic acid. Vinegar is used as a preservative in the food industry. Foods in vinegar can be stored for a long time because the low pH prevents the growth of harmful bacteria and fungi.
Vinegar is also used in household cleaning products designed to remove the build-up of insoluble carbonates found on plumbing fixtures, e.g. taps. The acid reacts with the carbonates in a neutralisation reaction. It is useful as a household cleaner as it is non-toxic and can be used safely.
Give TWO uses for alcohols.
Why can ethanol be described as a renewable source of energy?
Give a use for vinegar … as a preservative in the food industry … as a household cleaning product.
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Uses of compounds continued
What are the THREE products of the reaction between vinegar and a carbonate compound?
Why is vinegar useful as a household cleaner.
Summary of consumer products
Alkanes
Used as fuels
Alkenes
Used to make alcohols ( in hydration reaction eg/ ethene + water ethanol) and to make polymers ( eg ethene polythene and propene polypropene)
Cycloalkanes
Used as fuels and as solvents
Alcohols
Used as fuels and solvents
Carboxylic acids
Used in the preparation of preservatives, soaps and medicines.
Vinegar (Ethanoic acid) used in household cleaning products. 43
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Energy changes
An exothermic reaction is one in which energy is released, e.g. when a fuel burns (combustion), heat (mainly) is given out to the surroundings. The flame in a Bunsen burner is a result of burning methane, i.e. methane and oxygen reacting to form carbon dioxide and water.
CH4 + 2O2 CO2 + 2H2O
In the chemical reaction, energy is required to break the covalent bonds in the reactant molecules … to overcome the forces of attraction holding the atoms in the molecules together. New covalent bonds can then be formed in the product molecules. When these are formed, energy is released.
In a reaction in which energy is released, i.e. an exothermic reaction, more energy is given out in the bond making than is required for bond breaking.
The opposite of an exothermic reaction is an endothermic reaction … one in which energy is taken in during the reaction.
What is meant by … an exothermic reaction? … an endothermic reaction?
Is energy released when covalent bonds … are broken? … are made?
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Energy changes continued
Which step, bond breaking / bond making, must involve more energy … in an exothermic reaction? ... in an endothermic reaction?
Give an example of an exothermic reaction.
Give an example of an endothermic reaction.
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Combustion of hydrocarbons and alcohols
When any hydrocarbon ( alkane,alkene, cycloalkane) or alcohol is completely combusted, carbon dioxide and water are formed.
HYDROCARBON OR ALCOHOL + OXYGEN CARBON DIOXIDE + WATER
You can prove water is produced by cooling the gases, using ice, and checking the boiling point of the liquid produced is 100 0C. Also the freezing point of water and the melting point of ice is 00C. The carbon dioxide gas will turn limewater from a clear to milky colour.
Word equations and balanced chemical equations can be written for the combustion of hydrocarbons and alcohols.
Word equation
Ethane + oxygen Carbon dioxide + water
This can be changed into a chemical equation by substituting the words for the formula of the substances ( remember that oxygen is a diatomic element and is written as O2 ).
Chemical equation
C2H6 + O2 CO2 + H2O
You need to balance the chemical equation by putting numbers in front of the formula.
C2H6 + 3 ½ O2 2 CO2 + 3 H2O
Or
2C2H6 + 7O2 4CO2 + 6H2O
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Examples for you to try Write the word and a balanced chemical equation for the complete combustion of 1. Propane
2. Methanol
3. Butene
4. Pentane
5. Ethanol
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Energy from fuels Activity 2.6 Alkanes and alcohols can be used as fuels, e.g. natural gas is mainly methane, ethanol is mixed with petrol for use in cars. A fuel releases energy on reaction with oxygen. This type of reaction, known as burning (or combustion), is exothermic. In an exothermic reaction, more energy is released in bond making than is required for bond breaking. The following apparatus can be set up to measure how much energy is given out when a fuel burns.
The energy released in the burning of a fuel can be calculated by using the heat energy to raise the temperature of a known mass of water.
The heat released = c m ΔT
where c = specific heat capacity of water = 4.18 kJ kg-1 oC-1 Remember to change m = mass of water (in kg) absorbing heat from g kg 3 (1 cm of water has a mass of 1g you divide by 1000 So 50cm3 50g 0.05kg)
ΔT = temperature change
Example: Calculate the heat released on the burning of a fuel that raises the temperature of 100 cm3 of water by 10.5 oC.
Heat released = c m ΔT
= 4.18 x 0.1 x 10.5 = 4.39 kJ
In the lab, the calculated energy released is less than the actual energy released because some energy is lost to the surroundings, e.g. the container for the fuel and the air.
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Energy from fuels continued For each of the following results, calculate the heat energy released by the burning of the fuel.
The temperature of 50 cm3 of water is increased by 12 oC.
The temperature of 250 cm3 of water is increased by 21.8 oC.
The temperature of 100 cm3 of water is increased by 8.4 oC.
The temperature of 500 cm3 of water is increased by 15.7 oC.
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Nature’s Chemistry May 2017
Energy from fuels continued
The energy released from the burning of different fuels can be compared by calculating the energy released for the burning of one mole of each.
Alcohol Structural formula Heat released / kJ mol-1
methanol CH3OH 727
ethanol CH3CH2OH 1367
propan-1-ol CH3CH2CH2OH 2020
Butan-1-ol CH3CH2 CH2CH2OH
There is a fairly constant difference between the heat released per mole for any two successive members of a homologous series. Since each pair differ by a - CH2 - group the bond breaking energy and energy of bond making with oxygen is approximately constant for this group in different molecules. Fill in the table by predicting a value for butan-1-ol.
Why is energy released in the burning of a fuel?
When calculating the heat energy released in the burning of a fuel in the lab, why is the experimental value less than the actual value?
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Calculating the specific heat capacity for liquids other than water
You could also be asked to calculate the specific heat capacity of a liquid other than water, given the temperatures and volume of other liquid.
Try this example,
The energy released when an alcohol burns can be used to heat liquids other than water.
The data below was collected when the energy released, by burning an alcohol, was used to heat a sodium chloride solution.
Energy released when the alcohol was burned (kJ) 13.3
Initial temperature oC 15
Final temperature oC 49
Mass of sodium chloride solution heated (g) 100
Calculate the specific heat capacity, in kJkg-1 oC-1, of the sodium chloride solution.
You may wish to use the data booklet to help you.
Show your working clearly.
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