CHAPTER 2

Introduction to , Bases and Salts

Classification of matter On the basis of a) composition – elements, compounds and mixtures b) state – solids, liquids and gases c) solubility – suspensions, colloids and solutions Types of mixtures – homogeneous and heterogeneous Types of compounds – covalent and ionic

What Is an and a ?

Ionisable and non-ionisable compounds An ionisable compound when dissolved in water or in its molten state, dissociates into ions almost entirely. Example: NaCl, HCl, KOH, etc.

A non-ionisable compound does not dissociate into ions when dissolved in water or in its molten state. Example: glucose, acetone, etc.

Arrhenius theory of acids and bases Arrhenius theory, theory, introduced in 1887 by the Swedish scientist Svante Arrhenius, that acids are substances that dissociate in water to yield electrically charged atoms or molecules, called ions, one of which is a hydrogen ion (H+), and that bases ionize in water to yield hydroxide ions (OH−). It is now known that the hydrogen ion cannot exist alone in water solution; rather, it exists in a combined state with a water molecule, as the hydronium ion (H3O+). In practice the hydronium ion is still customarily referred to as the hydrogen ion. Arrhenius acid – when dissolved in water, dissociates to give H+ (aq) or H3O+ ion. Arrhenius base – when dissolved in water, dissociates to give OH− ion.

Examples Acids

• (HCl) • Sulphuric acid (H2SO4) • (HNO3) Bases

• Sodium hydroxide (NaOH) • Potassium hydroxide (KOH) • Calcium hydroxide (Ca(OH)2)

Bronsted Lowry theory

any compound that can transfer a proton to any other compound is an acid, and the compound that accepts the proton is a base. A proton is a nuclear particle with a unit positive electrical charge; it is represented by the symbol H+ because it constitutes the nucleus of a hydrogen atom

According to the Brønsted–Lowry scheme a substance can function as an acid only in the presence of a base; similarly, a substance can function as a base only in the presence of an acid. Furthermore, when an acidic substance loses a proton, it forms a base, called the conjugate base of an acid, and when a basic substance gains a proton, it forms an acid called the of a base. Thus, the reaction between an acidic substance, such as hydrochloric acid, and a basic substance, such as ammonia, may be represented by the equation:

In the equation the ammonium ion (NH+4 ) is the acid conjugate to the base ammonia, and the chloride ion (Cl-) is the base conjugate to hydrochloric acid.

Example In the reaction: HCl (aq) +NH3 (aq) →NH+4(aq) +Cl− (aq) HCl – Bronsted acid and Cl− – its conjugate acid NH3 – Bronsted base and NH+4 – its conjugate acid

Physical test Given are two possible physical tests to identify an acid or a base. a. Taste An acid tastes sour whereas a base tastes bitter. The method of taste is not advised as an acid or a base could be contaminated or corrosive. b. Effect on indicators by acids and bases An indicator is a chemical substance which shows a change in its physical properties, mainly colour or odour when brought in contact with an acid or a base. Below mentioned are commonly used indicators and the different colours they exhibit: a) Litmus In a neutral solution – purple In acidic solution – red In basic solution – blue

Litmus is also available as strips of paper in two variants – red litmus and blue litmus. An acid turns a moist blue litmus paper to red. A base turns a moist red litmus paper to blue. b) Methyl orange In a neutral solution – orange In acidic solution – red In basic solution – yellow c) Phenolphthalein In a neutral solution – colourless In acidic solution – remains colourless In basic solution – pink

Indicators: Indicators are those substances which tell us whether a substance is acidic or basic by change in colour. For e.g., litmus solution. • Olfactory indicators: Those substances whose odour changes in acidic or basic media are called lfactory indicators. For e.g., clove, vanilla, onion. • Natural indicators: Turmeric, litmus (obtained from lichen) • Synthetic indicators: Methyl orange, phenolphthalein.

Olfactory Indicators Those substances whose smell or odour changes in acidic or basic solution are called olfactory indicators.

For Ex:Onion and Vanilla extract

SYNTHETIC INDICATOR

An indicator prepared from natural substances is known as natural indicator. Examples are Litmus, Turmeric, China rose petals, snowball plant, Red Cabbage and Grape Juice. SYNTHETIC INDICATOR. An indicator prepared from artificial substances is known as synthetic indicator Examples are Phenolphthalein and Methyl Orange

Acids Acids are substances that give up Hydrogen ions in solution. Acids are corrosive to metals while releasing Hydrogen gas, have a pH between 0 and 6.9 and are sour to the taste. There are many common substances that are acids: lemon juice (citric acid), vinegar (acetic acid), stomach acid, and soda pop (carbonic acid)

• They give sour taste.

• Acids turn blue litmus to red. This is used as confirmation test for the presence of acid.

• When acids react with metals, gases are evolved. Three Major Types of Acids. Usually acids can be divided into three major types. First one is binary acid, second one is oxyacid, and the last one is carboxylic acid. Binary acids are all written in “H-A” form, which means hydrogen bond to a nonmetal atom. Reactions with Acids • Acid + metal → salt + hydrogen. • For example: • Hydrochloric acid + magnesium → magnesium chloride + hydrogen. • Zinc and iron also react with hydrochloric acid. • Magnesium, zinc and iron also react with . ... • sulfuric acid + iron → iron(II) sulfate + hydrogen. Neutralisation reaction A neutralization reaction is when an acid and a base react to form water and a salt and involves the combination of H+ ions and OH- ions to generate water. The neutralization of a strong acid and strong base has a pH equal to 7. The neutralization of a strong acid and will have a pH of less than 7, and conversely, the resulting pH when a strong base neutralizes a weak acid will be greater than 7 When a solution is neutralized, it means that salts are formed from equal weights of acid and base. The amount of acid needed is the amount that would give one mole of protons (H+) and the amount of base needed is the amount that would give one mole of (OH-). Because salts are formed from neutralization reactions with equivalent concentrations of weights of acids and bases: N parts of acid will always neutralize N parts of base. .Similarity between Acids and Bases • Both acids and base react with water. They produce ions in water

• Both acids and bases acts as electrolytes, so are good conductors of electricity.

• Both of them changes the colour of the litmus paper. Classification of Acids Acids can be classified as organic or inorganic. Some of the more common organic acids are: citric acid, carbonic acid, hydrogen cyanide, salicylic acid, lactic acid, and tartaric acid

The main difference between mineral acid and is that mineral acidsare inorganic compounds composed of different chemical element combinations whereas organic acids are organic compound sessentially composed of carbon and hydrogen atoms. ... Ex:- Hydrochloric acid, sulfuris acid, ,nitric . Strong Acids

Strong acids dissociate completely into their ions in water, yielding one or more protons (hydrogen cations) per molecule. There are only 7 common strong acids.

• HCl - hydrochloric acid • HNO3 - nitric acid • H2SO4 - sulfuric acid (HSO4- is a weak acid) • HBr - hydrobromic acid • HI - hydroiodic acid • HClO4 - perchloric acid • HClO3 - chloric acid

Weak Acids

Weak acids do not completely dissociate into their ions in water. For example, HF dissociates into the H+ and F- ions in water, but some HF remains in solution, so it is not a strong acid. There are many more weak acids than strong acids. Most organic acids are weak acids. Here is a partial list, ordered from strongest to weakest.

• HO2C2O2H - oxalic acid • H2SO3 - sulfurous acid • HSO4 - - hydrogen sulfate ion • H3PO4 - phosphoric acid • HNO2 - nitrous acid • HF - hydrofluoric acid • HCO2H - methanoic acid • C6H5COOH - benzoic acid • CH3COOH - acetic acid • HCOOH - formic acid

A dilute acid is one that is not very concentrated. You can make an acid solution more dilute by adding water. ... Strong and weak characterize the acid's ability to dissociate in an aqueous solution, independent of concentration. Concentrated acid is an acid solution with a high molar concentration of hydrogen ions.

Dilution Dilution is the process of reducing the concentration of a solution by adding more solvent (usually water) to it. It is a highly exothermic process. To dilute acid, the acid must be added to water and not the other way round. Strength of acids and bases The terms strong and weak describe the ability of acid and base solutions to conduct electricity. If the acid or base conducts electricity strongly, it is a strong acid or base. If the acid or base conducts electricity weakly, it is a weak acid or base. An acid or base which strongly conducts electricity contains a large number of ions and is called a strong acid or base and an acid or base which conducts electricity only weakly contains only a few ions and is called a weak acid or base.

Bond Strength

The bond strengths of acids and bases are implied by the relative amounts of molecules and ions present in solution. The bonds are represented as:

ACID BASE

H-A M-OH where A is a negative ion, and M is a positive ion

Strong acids have mostly ions in solution, therefore the bonds holding H and A together must be weak. Strong acids easily break apart into ions. Weak acids exist mostly as molecules with only a few ions in solution, therefore the bonds holding H and A together must be strong. Weak acids do not readily break apart as ions but remain bonded together as molecules.

Water

Acids and bases in water When added to water, acids and bases dissociate into their respective ions and help in conducting electricity.

Difference between a base and an alkali Alkalis :

Any substance that produces OH- (hydroxide ions) in aqueous solution is called alkali.

Here are some examples:

1) Sodium hydroxide solution -

NaOH(aq)

2) Calcium hydroxide solution -

Ca(OH)2

(aq)

3) Ammonia solution -

NH3

(aq)

Bases:

Any substance that will neutralize an acid, but does not dissolve in water, is called a base.

Here are some examples:

Copper oxide, ferrous oxide and zinc carbonate are the good examples of base.

Reactions of Bases

Acid-Base Reactions. When an acid and a base are placed together, they react to neutralize the acid and base properties, producing a salt. The H(+) cation of the acid combines with the OH(-) anion of the base to form water. The compound formed by the cation of the base and the anion of the acid is called a salt.

Reaction of Base with Metals: When alkali (base) reacts with metal, it produces salt and hydrogen gas. Example: Sodium hydroxide gives hydrogen gas and sodium zincate when reacts with zinc metal. Sodium aluminate and hydrogen gas are formed when sodium hydroxide reacts with aluminium metal.3. Reaction with Non-metallic Oxides

Alkali is considered as strong base. Reaction of Base with Metals: When alkali (base) reacts with metal, it produces salt and hydrogen gas. Example: Sodium hydroxide gives hydrogen gas and sodium zincate when reacts with zinc metal. Base reacts with non-metallic oxides to form salt and water. 2NaOH + CO2 → CO2 → Na2CO3 + H2O Classification of Bases BASES an be classifies according to strength ( degree of ionization ) and molecular structure , Bases are classified according to their degree of ionization ( dissociation ) into strong bases and weak bases . Strong bases Bases which are completely ionized in the water, Their solutions are good conductors of electricity, They are considered as strong electrolytes . Examples : Potassium hydroxide KOH , Sodium hydroxide NaOH , Barium hydroxide Ba(OH)2 .

Weak bases Bases which are incompletely ionized in the water, Their solutions are bad conductors of electricity ,They are considered as weak electrolytes .

Examples : Ammonium hydroxide NH4OH

Importance of pH pH is an important quantity that reflects the chemical conditions of a solution. The pH can control the availability of nutrients, biological functions, microbial activity, and the behavior of chemicals. Because of this, monitoring or controlling the pH of soil, water, and food or beverage products is important for a wide variety of applications.

Agriculture and gardening

Soil is a complex system that involves many different factors that are affected by soil pH, such as microbial activity, fungal growth, availability of nutrients, and root growth[1].

Under acidic conditions, many minerals in soil become soluble, releasing toxic metals such as aluminum. Some nutrients, such as phosphorus and molybdenum, become less available at lower pH values. Under alkaline (basic) conditions, the soil can become deficient in nutrients such as zinc, copper, iron, manganese, boron and phosphorus.

Aquaculture and aquatic ecosystems

Water that has a pH that is too low or too high can be harmful to fish and other aquatic life. At low pH, toxic metals such as aluminum can enter the water in greater concentrations, some nitrogen-bearing chemicals become more toxic, and the metabolic processes of fish can become less efficient. Water with pH below 5 can inhibit reproduction or lead to death, and young fish and other aquatic organisms are especially susceptible. Water with a pH below 6.5 can inhibit growth.

Food Industry n the food industry, pH is measured to test for quality, to control microbial activity, to control the taste and other properties, and to prolong the shelf life of food. In milk, pH is tested to check for impurities or infection. The pH is also affected by the souring of milk and maturation of cream, and the pH determines whether cheese will be soft or hard. The pH of cream also determines whether butter will be sour or sweet. For production of yogurt, the pH of cultured milk is kept low to maintain a desirable environment for appropriate microbial activity.

Salts When acid and base neutralize, salts are formed. Strong acid and strong base combines to form neutral salt.

NaOH + HCl → NaCl + H2O Eq.1. Formation of Neutral Salt Strong acid and weak base combine to form acidic salt. For Example, Hydrochloric Acid and ammonium hydroxide combine to form ammonium chloride. Other examples, sodium hydrogen carbonate, sodium hydrogen sulphate etc. HCl + NH4OH → NH4Cl + H2O Eq.2. Formation of Acidic Salt Similarly, weak acid and strong base combine to form basic salt. For Example, Acetic Acid and sodium hydroxide combine to form sodium acetate. Other examples are calcium carbonate, potassium cyanide etc. CH3COOH + NaOH → CH3COONa + H2O Eq.3. Formation of Basic Salt The most common salt is table salt or sodium chloride (NaCl).

Salts

Salts A salt is a combination of an anion of an acid and a cation of a base. Examples – KCl,NaNO3,CaSO4,etc. Salts are usually prepared by the neutralisation reaction of an acid and a base.

Common salt Sodium Chloride (NaCl) is referred to as common salt because it’s used all over the world for cooking.

Family of salts Salts having the same cation or anion belong to the same family. For example, NaCl, KCl, LiCl. pH of salts A salt of a strong acid and a strong base will be neutral in nature. pH = 7 (approx.). A salt of a weak acid and a strong base will be basic in nature. pH > 7. A salt of a strong acid and a weak base will be acidic in nature. pH < 7. The pH of a salt of a weak acid and a weak base is determined by conducting a pH test.

Preparation of Sodium hydroxide Chemical formula – NaOH Also known as – caustic soda Preparation (Chlor-alkali process): Electrolysis of brine (solution of common salt, NaCl) is carried out. At anode: Cl2 is released

At cathode: H2 is released Sodium hydroxide remains in the solution.

Bleaching powder

Chemical formula – Ca(OCl)Cl or CaOCl2 Preparation – Ca(OH)2(aq)+Cl2(g)→CaOCl2(aq)+H2O(l) On interaction with water – bleaching powder releases chlorine which is responsible for bleaching action.

Baking soda Chemical name – Sodium hydrogen carbonate Chemical formula – NaHCO3 Preparation (Solvay process) – a. Limestone is heated: CaCO3→CaO+CO2 b. CO_2 is passed through a concentrated solution of sodium chloride and ammonia: NaCl(aq)+NH3(g)+CO2(g)+H2O(l)→NaHCO3(aq)+NH4Cl(aq) Uses: 1. Textile industry 2. Paper industry 3. Disinfectant

Washing soda Chemical name – Sodium hydrogen carbonate Chemical formula – NaHCO3 Preparation (Solvay process) – a. Limestone is heated: CaCO3→CaO+CO2 b. CO_2 is passed through a concentrated solution of sodium chloride and ammonia: NaCl(aq)+NH3(g)+CO2(g)+H2O(l)→NaHCO3(aq)+NH4Cl(aq) Uses 1. In glass, soap and paper industries 2. Softening of water 3. Domestic cleaner

Crystals of salts Certain salts form crystals by combining with a definite proportion of water. The water that combines with the salt is called water of crystallisation.

Plaster of paris Gypsum, CaSO4.2H2O (s) on heating at 100°C (373K) gives CaSO4. ½ H2O and 3/2 H2O CaSO4. ½ H2O is plaster of paris. CaSO4. ½ H2O means two formula units of CaSO4 share one molecule of water. Uses – cast for healing fractures.