Unit 7 Writing and Balancing Chemical Equations

UNIT 7 – WRITING AND BALANCING CHEMICAL EQUATIONS I. Counting Atoms: Before you can write or balance a chemical equation, you must first be able to count the atoms, which are present. A subscript multiplies only what is immediately in front of it. A coefficient placed in front of a molecule or a formula unit multiplies every atom in the formula by that number.

Example 7-1: How many oxygen atoms are there in a formula unit of iron(III) dichromate:

(b) in 5 formula units of iron(III) dichromate?

Example 7-2 : How many total atoms are there in 6 formula units of barium chloride dihydrate?

Example 7-3: How many elements are present in 3 formula units of aluminum bicarbonate?

II. Writing and Balancing Chemical Equations A chemical equation is exactly what it says it is—an equality between the reactants (which are substances written on the left side of the equation) and the products (which are substances written on the right side). An arrow pointing to the right serves as the = sign it is read “yields”.

A. HOW TO WRITE A BALANCED EQUATION (1) Write a correct formula for each of the reactants. Put a plus sign between the reactants to separate them. Note: A plus sign means “added to” or “reacts with”. (2) Draw a yield arrow (3) Write a correct formula for each of the products, putting a plus sign between them also. Note: On the product side, the plus sign means “as well as” or “in addition to”. (4) Balance the metals first, putting coefficients where necessary (5) Balance the polyatomics next (IF THEY STAY TOGETHER—that is, if there are the same polyatomics on each side of the equation) by adding a coefficient in front of the entire compound. (not in the middle of the compound) (6) If the polyatomic comes apart OR if there is no polyatomic present, balance the non-metals except hydrogen and oxygen next by adding coefficients where necessary (7) Balance the hydrogens and the oxygens (which were not part of the polyatomics) last NOTE: Coefficients CANNOT be placed in the middle of a compound.

B. STATE SYMBOLS There are four physical state symbols which are used as subscripts immediately following substances whose physical states are known or given. (s) – solid, which is used for solids or precipitates (ℓ) – liquid, which is used only for “true” liquids such as elements which are liquids at room temperature such as mercury and bromine; also for water, and for molten (melted) substances (g) – gas (aq) – aqueous, which means dissolved in water, denoted within the word equation as “solution”.

C. SIMPLE GUIDELINES for writing state symbols when the state is not given or known:

(1) all pure metallic elements (except mercury) will be designated (s) unless the reaction indicates otherwise

(2) water will be designated as (ℓ) unless it is a combustion reaction or the vapor state is indicated; pure bromine and pure mercury will be designated (ℓ); organic substances which are listed as “liquids” in the problem will be designated (ℓ); anything which is “molten” will be designated (ℓ)

(3) metallic oxides are always (s); non-metallic oxides are (g)

(4) acids are designated (aq); soluble ionic compounds are designated (aq) J.Schornick 2002-2003 pg. 1 of 7 UNIT 7 – WRITING AND BALANCING CHEMICAL EQUATIONS AQUEOUS Example 7-4:Write a balanced equation for the following reactions (a) A solid piece of zinc reacts with hydrochloric acid to produce a solution of zinc chloride and hydrogen gas as the products

(b) phosphoric acid is reacted with a solution of sodium hydroxide to produce a solution of sodium phosphate and water as the only products

(c) chlorine gas is bubbled through a solution of lithium iodide, and the products are found to be a solution of lithium chloride and solid iodine.

(d) solutions of silver nitrate and barium chloride are mixed, and the products are a precipitate of silver chloride and a solution of barium nitrate

III. Predicting Products

A. HOW TO PREDICT THE PRODUCTS OF A CHEMICAL REACTION:

To be able to predict the products of a chemical reaction, you must first be able to recognize what type of reaction it is. There are five specific types of reactions, which you need to be able to recognize.

(1) SYNTHESIS – means “putting together”; characterized by having two pure elements as its reactants and there will be only ONE product formed—a compound between these two elements; LIKE A MARRIAGE; A + B  AB

Example 7-5 : (a) Write a balanced equation for the reaction, which will take place when potassium reacts with chlorine gas.

(b) Write a balanced equation for the reaction, which will take place when aluminum is allowed to react with oxygen gas.

(c) If iron(III) nitride is the only product formed in this reaction, write a balanced equation for the reaction.

(2) DECOMPOSITION – means “breaking apart”; characterized by having only ONE reactant which simply comes apart into its elements or into smaller compounds (the products); REQUIRES ENERGY INPUT, and its usually written OVER the yield arrow; LIKE A DIVORCE; CD  D + C

Example 7-6 : (a)Write a balanced equation for the decomposition of lead(II) oxide by heat.

(b) Write a balanced equation for the decomposition (by electricity) of water

(c) Note: Sometimes the products will be given, if YOU cannot predict them. The following is an example: Write a balanced equation for the decomposition (by heat) of potassium chlorate into potassium chloride and oxygen gas

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(3) SINGLE REPLACEMENT – characterized by having an element and a compound as reactants and the products will be another element and another compound; there are really two types of single replacement reactions—one in which the positive ion of the compound is replaced by the lone element and one in which the negative ion of the compound is replaced by the lone element; LIKE A LOVE TRIANGLE; A + BC  AC + B or in some cases XY + Z  XZ +Y

Note: guys don’t dance with guys in chemistry and girls don’t dance with girls in chemistry!! i.e. the negative won’t react with another negative to form a compound, same holds true for 2 metals. KCl + I2 → KI + Cl2 – NOT– KCl + I2 → ICl + Li

Predicting if Single Replacement reactions occur or not: We have been assuming that if you can write an equation, IT WILL OCCUR, and that is not necessarily true. It is difficult to predict whether equations will actually take place or not, so we will assume that ALL REACTIONS EXCEPT DOUBLE AND SINGLE REPLACEMENT REACTIONS WILL TAKE PLACE as written.

Whether or not a single replacement reaction will take place will be based on the ACTIVITY SERIES of metals and on metals shown below. It is not necessary to memorize it because it will be provided for you on all quizzes and exams, but you must know how to use it AND YOU MUST REMEMBER TO USE IT!

ACTIVITY Rules: SERIES (1) Li Lithium is the most active metal in this series. Elements closest to lithium will replace K those furthest from lithium. (2) + Ba The top five most active elements (Li, K. Ba, Ca, Na) can replace the “H ” in water. Ca For these reactions it is helpful to think of water as an H+ and an OH- Na

Mg 2K(s) + 2 HOH(l)  2KOH(aq) + H2(g) Al Mn (3) All metals ABOVE “H” on the list can replace the H+ in acids. Zn Cr Fe Zn(s) + 2HNO3(aq)  Zn(NO3)2(aq) + H2(g) Cd (4) Co For non-metals, we will only deal with the halogens. Fluorine is the most active and Ni iodine is the least. The halogen closest to fluorine will replace the one furthest from Sn fluorine. Pb H How do I know if those pesky multiple oxidation Cu state elements go to their higher oxidation number Ag or their lower oxidation number? Hg Pt Au

Remember the Phrase: As Stan Fell, Huge Cups Cracked These metals go to their higher oxid. #, the rest go to their lower! Chemistry is so cool!!

***IF A REACTION DOES NOT TAKE PLACE, WRITE THE REACTANTS CORRECTLY, DRAW THE YIELD ARROW AND WRITE “NO REACTION.”

Example 7-10: Write a correct balanced equation for each of the following:

(a) a piece of zinc is added to a solution of silver nitrate

(b) tin(II) chloride solution is poured over magnesium

J.Schornick 2002-2003 pg. 3 of 7 UNIT 7 – WRITING AND BALANCING CHEMICAL EQUATIONS (c) iron(III) nitrate solution plus copper

(d) copper(II) chloride solution plus aluminum (e) sodium chloride solution plus calcium

(f) silver dropped into hydrochloric acid

(g) magnesium is added to phosphoric acid

(h) sodium plus water

(i) calcium fluoride plus iodine

(j) water is poured over gold

(k) sulfuric acid plus lithium

Example 7-7 : Write a balanced equation for magnesium is added to a solution of tin(IV) nitrate

(b) lithium is added to a solution of barium hydroxide

(c) magnesium chloride and hydrogen gas are the PRODUCTS of this reaction

(d) liquid bromine is poured into a solution of sodium iodide

(e) flourine gas bubbles through a solution of strontium bromide

(4) DOUBLE REPLACEMENT – characterized by having two compounds as reactants and two different compounds will be the products; LIKE A DO-SI-DO AND CHANGE PARTNERS.

A. For a double replacement to take place there must be a gas, precipitate, or molecular compound formed (most common = H2O) B. If, after predicting the products for a DR reaction, both products are soluble (AKA: aqueous, (aq) ) then the reaction doesn’t take place.

UMMM, How do I know if something is soluble or insoluble?

I can read your mind!! Here are the solubility rules that will help you predict if a compound is soluble (i.e. will dissolve in water) or insoluble. You might want to J.Schornick 2002-2003 do your homework 7.6! pg. 4 of 7 UNIT 7 – WRITING AND BALANCING CHEMICAL EQUATIONS

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C. Here are the solubility rules for ionic compounds: MEMORIZE them NOW so you won’t have to guess the physical state when you write an equation.

(1) any ionic compound whose cation is Group I A or ammonium is soluble, no matter what the anion is

(2) all chlorates, perchlorates, nitrates and acetates are always soluble, no matter what the cation is

(3) 1+ 2+ 2+ all chlorides, bromides and iodides are soluble EXCEPT Ag , Pb , and Hg2

(4) 1+ 2+ 2+ 2+ 2+ 2+ all sulfates are soluble except Ag , Pb , and Hg2 AND Ca , Sr , and Ba

(5) EVERYTHING ELSE (fluorides, sulfides, phosphides, phosphates, phosphates, nitrites, hydroxides, oxides, permanganates, bicarbonates, carbonates, sulfites, oxalates, bisulfates, bisulfites, cyanides, chromates, dichromates, phosphides, nitrides, etc) are INSOLUBLE UNLESS they begin with Group I A or ammonium (rule 1)

??Did I mention that it will make your life soooooo much easier if you memorize them NOW??? Example 7-8 : (a) Write a balanced equation for solution of barium chlorate and silver nitrate being mixed

(b) Write a balanced equation for the reaction of potassium cyanide plus tin(II) flouride

(c) calcium phosphate and aluminum bromide are the PRODUCTS of this reaction

(d) hydrochloric acid is mixed with a solution of calcium hydroxide

(e) copper nitrate is added to a solution of sodium sulfate.

NOTE: Show “dancers overhead”

(5) COMBUSTION – literally means “burning”, but we will take it to mean that hydrocarbon (compound with H and C) or an oxyhydrocarbon (compound with H, C, & O) reacts with oxygen. The products are always the same —carbon dioxide and water vapor, no matter what the beginning reactants are.

Reminder: ALWAYS WRITE OXYGEN AS A REACTANT IN A COMBUSTION REACTION.

Example 7-9 : (a) Write a balanced equation for the combustion of propane.

(b) Write a balanced equation for the burning of octane

(c) What is the balanced equation when methyl alcohol (CH3OH) is burned?

(d) cyclobutane reacts with oxygen

Disclaimer: Many things burn, but for right now we are only going to use carbon containing compounds in combustion

reactions. The products are always CO2 and H2O IF the compound burning or reacting with oxygen contains, C & H or C, H, & O.

J.Schornick 2002-2003 pg. 6 of 7 UNIT 7 – WRITING AND BALANCING CHEMICAL EQUATIONS  You will understand other types of combustion reactions when you take AP Chemistry, so until then, just enjoy the simplicity of Pre-AP.

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