Chemical Equations Chapter 10 Eugene Passer Chemistry Department Bronx Community College © John Wiley and Sons, Inc Version 1.1

Chapter Outline 8.1 The Chemical Equation 8.4 Types of Chemical Equations 8.2 Writing and Balancing Equations 8.5 Heat in Chemical Reactions 8.3 What Information Does an Equation Tell Us

Chemical equations provide us with the means to Chemists use chemical equations to describe reactions they observe in the laboratory or in nature. Chemical equations provide us with the means to summarize the reaction display the substances that are reacting show the products indicate the amounts of all component substances in a reaction.

The Chemical Equation

Chemical reactions always involve change. Atoms, molecules or ions rearrange to form new substances. The substances entering the reaction are called reactants. The substances formed in the reaction are called products. During reactions, chemical bonds are broken and new bonds are formed.

A chemical equation is a shorthand expression for a chemical change or reaction. A chemical equation uses the chemical symbols and formulas of the reactants and products and other symbolic terms to represent a chemical reaction.

Coefficients (whole numbers) are placed in front of substances to balance the equation and to indicate the number of units (atoms, molecules, moles, or ions) of each substance that are reacting.

coefficient 2 Al + Fe2O3  Fe + Al2O3

Conditions required to carry out the reaction may be placed above or below the arrow.

Al + Fe2O3  Fe + Al2O3 coefficient 2   heat

The physical state of a substance is indicated by symbols such as (l) for liquid.

2Al(s) + Fe2O3(s)  2Fe(l) + Al2O3 (s) (s) (l) In a chemical reaction atoms are neither created nor destroyed. 2Al(s) + Fe2O3(s)  2Fe(l) + Al2O3 (s) (s) (l) All atoms present in the reactant must also be present in the products.

Symbols Used in Chemical Reactions

Al + Fe2O3  Fe + Al2O3 Al + Fe2O3  Fe + Al2O3 Chemical Equation reactants products Al + Fe2O3  Fe + Al2O3 Al + Fe2O3  Fe + Al2O3 iron oxygen bonds break aluminum oxygen bonds form

symbol (aq) aqueous meaning after formula location

placed between substances symbol + meaning plus placed between substances location

symbol  heat meaning written above  location

symbol  gas formation meaning after formula location

Writing and Balancing Equations

Never change a correct formula to balance an equation. To balance an equation adjust the number of atoms of each element so that they are the same on each side of the equation. Never change a correct formula to balance an equation.

Steps for Balancing Equations

mercury(II) oxide → mercury + oxygen Step 1 Identify the reaction. Write a description or word equation for the reaction. Mercury (II) oxide decomposes to form mercury and oxygen. mercury(II) oxide → mercury + oxygen

The formulas of the reactants and products can never be changed. Step 2 Write the unbalanced (skeleton) equation. The formulas of the reactants and products must be correct. The reactants are written to the left of the arrow and the products to the right of the arrow. HgO  Hg + O2 The formulas of the reactants and products can never be changed.

Element Reactant Side Product Side Step 3a Balance the equation. HgO → Hg + O2 Element Reactant Side Product Side Hg 1 1 There is one mercury atom on the reactant side and one mercury atom on the product side. Mercury is balanced.

Step 3a Balance the equation. Count and compare the number of atoms of each element on both sides of the equation. Determine the elements that require balancing.

Element Reactant Side Product Side Step 3a Balance the equation. HgO  Hg + O2 Element Reactant Side Product Side O 1 2 There are two oxygen atoms on the product side and there is one oxygen atom on the reactant side. Oxygen needs to be balanced.

Step 3b Balance the equation. Balance each element one at a time, by placing whole numbers (coefficients) in front of the formulas containing the unbalanced element. A coefficient placed before a formula multiplies every atom in the formula by that coefficient.

Element Reactant Side Product Side Step 3b Balance the equation. HgO  Hg + O2 2 Element Reactant Side Product Side O 1 2 Place a 2 in front of HgO to balance O. There are two oxygen atoms on the reactant side and there are two oxygen atoms on the product side. Oxygen (O) is balanced.

Step 3c Balance the equation. Check all other elements after each individual element is balanced to see whether, in balancing one element, another element became unbalanced.

Element Reactant Side Product Side Step 3c Balance the equation. 2HgO  Hg + O2 Element Reactant Side Product Side Hg 2 1 Count and compare the number of mercury (Hg) atoms on both sides of the equation. There are two mercury atoms on the reactant side and there is one mercury atom on the product side. Mercury (Hg) is not balanced.

Element Reactant Side Product Side Step 3c Balance the equation. 2HgO  Hg + O2 2 Element Reactant Side Product Side Hg 2 1 Place a 2 in front of Hg to balance mercury. There are two mercury atoms on the reactant side and there are two mercury atoms on the product side. Mercury (Hg) is balanced.

Element Reactant Side Product Side  THE EQUATION IS BALANCED 2HgO  2Hg + O2 Element Reactant Side Product Side Hg 2 2 O 2 2

sulfuric acid + sodium hydroxide → sodium sulfate + water Balance the Equation sulfuric acid + sodium hydroxide → sodium sulfate + water

H2SO4(aq) + NaOH(aq) → Na2SO4(aq) + H2O(l) Balance the Equation H2SO4(aq) + NaOH(aq) → Na2SO4(aq) + H2O(l) 2 Reactant Side Product Side SO4 1 1 Na 1 2 O 1 1 H 3 2 2 2 4 Place a 2 in front of NaOH to balance Na. There is one Na on the reactant side and there are two Na on the product side.

 THE EQUATION IS BALANCED  H2SO4(aq) + NaOH(aq) → Na2SO4(aq) + H2O(l) 2 2 Reactant Side Product Side SO4 1 1 Na 2 2 O 2 1 H 4 2 2 4 Place a 2 in front of H2O to balance H. There are 4 H on the reactant side and two H on the product side.

butane + oxygen → carbon dioxide + water Balance the Equation butane + oxygen → carbon dioxide + water

 THE EQUATION IS BALANCED  C4H10 (g) + O2 (g) → CO2(g) + H2O(l) 2 8 10 13 Reactant Side Product Side C 8 8 H 20 20 O 2 26 26 Place a 13 in front of O2 to balance O. There are now 26 O on the product side.

What Information Does an Equation Tell Us

The meaning of a formula is context dependent. The formula H2O can mean: 2H and 1 O atom 1 molecule of water 1 mol of water 6.022 x 1023 molecules of water 18.02 g of water

In an equation formulas can represent units of individual chemical entities or moles. + Cl2 2HCl → 1 molecule H2 1 molecule Cl2 2 molecules HCl 1 mol H2 1 mol Cl2 2 mol HCl

Formulas Number of molecules Number of atoms Number of moles Molar masses

Types of Chemical Equations

Combination Decomposition Single-Displacement Double-Displacement Combustion

Combination Reactions

Two reactants combine to form one product. A + B  AB

Metal + Oxygen → Metal Oxide 2Ca(s) + O2(g)  2CaO(s) 4Al(s) + 3O2(g)  2Al2O3(s)

Nonmetal + Oxygen → Nonmetal Oxide S(s) + O2(g)  SO2(g) N2(g) + O2(g)  2NO(g)

2K(s) + F2(g)  2KF(s) 2Al(s) + 3Cl2(g)  2AlCl3(s) Metal + Nonmetal → Salt 2K(s) + F2(g)  2KF(s) 2Al(s) + 3Cl2(g)  2AlCl3(s)

Na2O(s) + H2O(l)  2NaOH(aq) Metal Oxide + Water → Metal Hydroxide Na2O(s) + H2O(l)  2NaOH(aq) CaO(s) + 2H2O(l)  2Ca(OH)2(aq)

Nonmetal Oxide + H2O(l) → Oxy-acid SO3(g) + H2O(l)  H2SO4(aq) N2O5(g) + H2O(l)  2HNO3(aq)

Decomposition Reactions

A single substance breaks down to give two or more different substances. AB  A + B

CaCO3(s)  CaO(s) + CO2(g) Carbonate → CO2(g) CaCO3(s)  CaO(s) + CO2(g) Hydrogen Carbonate → CO2(g) 2NaHCO3(s)  Na2CO3(s) + H2O(g) + CO2(g)

2Ag2O(s)  4Ag(s) + O2(g) 2PbO2(s)  2PbO(s) + O2(g) Metal Oxide → Metal + Oxygen 2Ag2O(s)  4Ag(s) + O2(g) Metal Oxide → Metal Oxide + Oxygen 2PbO2(s)  2PbO(s) + O2(g)

2NaNO3(s)  2NaNO2(s) + O2(g) Miscellaneous Reactions 2H2O2(l)  2H2O(l) + O2(g) 2KClO3(s)  2KCl(s) + 3O2(g) 2NaNO3(s)  2NaNO2(s) + O2(g)

Single Displacement Reactions

One element reacts with a compound to replace one the elements of that compound. A + BC  AC + B

Metal + Acid → Hydrogen + Salt Mg(s) + 2HCl(aq)  H2(g) + MgCl2(aq) salt 2Al(s) + 3H2SO4(aq)  3H2(g) + Al2(SO4)3(aq) salt

2Na(s) + 2H2O(l)  H2(g) + 2NaOH(aq) Metal + Water → Hydrogen + Metal Hydroxide 2Na(s) + 2H2O(l)  H2(g) + 2NaOH(aq) metal hydroxide Ca(s) + 2H2O(l)  H2(g) + Ca(OH)2(aq) metal hydroxide

3Fe(s) + 4H2O(g)  4H2(g) + Fe3O4(s) Metal + Water → Hydrogen + Metal Oxide 3Fe(s) + 4H2O(g)  4H2(g) + Fe3O4(s) metal oxide

The Activity Series

Metals K Ca Na Mg Al Zn Fe Ni Sn Pb H Cu Ag Hg An atom of an element in the activity series will displace an atom of an element below it from one of its compounds . Metals K Ca Na Mg Al Zn Fe Ni Sn Pb H Cu Ag Hg Sodium (Na) will displace an atom below it from one of its compounds. increasing activity

Mg(s) + PbS(s)  MgS(s) + Pb(s) Metal Higher in Activity Series Displacing Metal Below It Mg(s) + PbS(s)  MgS(s) + Pb(s) Metals Mg Al Zn Fe Ni Sn Pb Magnesium is above lead in the activity series.

Ag(s) + CuCl2(s)  no reaction Metal Lower in Activity Cannot Displace Metal Above It Ag(s) + CuCl2(s)  no reaction Metals Pb H Cu Ag Hg Silver is below copper in the activity series.

Double Displacement Reactions

Cl2(g) + CaBr2(s)  CaCl2(aq) + Br2(aq) Halogen Higher in Activity Series Displaces Halogen Below It Cl2(g) + CaBr2(s)  CaCl2(aq) + Br2(aq) Halogens F2 Cl2 Br2 I2 Chlorine is above bromine in the activity series.

AB + CD  AD + CB A displaces C and combines with D The reaction can be thought of as an exchange of positive and negative groups. Two compounds exchange partners with each other to produce two different compounds. B displaces D and combines with C AB + CD  AD + CB

The Following Accompany Double Displacement Reactions formation of a precipitate release of gas bubbles release of heat formation of water

Acid Base Neutralization acid + base → salt + water HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l) H2SO4(aq) + 2NaOH(aq)  Na2SO4(aq) + 2H2O(l)

Formation of an Insoluble Precipitate AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq) Pb(NO3)2(aq) + 2KI(aq)  PbI2(s) + 2KNO3(aq)

CaO(s) + 2HCl(aq)  CaCl2(s) + H2O(l) Metal Oxide + Acid metal oxide + acid → salt + water CuO(s) + 2HNO3(aq)  Cu(NO3)2(aq) + H2O(l) CaO(s) + 2HCl(aq)  CaCl2(s) + H2O(l)

indirect gas formation Formation of a Gas H2SO4(aq) + 2NaCN(aq)  Na2SO4(aq) + 2HCN(g) NH4Cl(aq) + NaOH(aq)  NaCl(aq) + NH4OH(aq) indirect gas formation NH4OH(aq)  NH3(g) + H2O(l)

Introduction to Stoichiometry: The Mole-Ratio Method

Stoichiometry: The area of chemistry that deals with the quantitative relationships between reactants and products. Mole Ratio: a ratio between the moles of any two substances involved in a chemical reaction. The coefficients used in mole ratio expressions are derived from the coefficients used in the balanced equation.

N2 + 3H2  2NH3 1 mol 2 mol 3 mol

N2 + 3H2  2NH3 1 mol 2 mol 3 mol

The mole ratio is used to convert the number of moles of one substance to the corresponding number of moles of another substance in a stoichiometry problem. The mole ratio is used in the solution of every type of stoichiometry problem.

Step 1 Determine the number of moles of starting substance. Identify the starting substance from the data given in the problem statement. Convert the quantity of the starting substance to moles, if it is not already in moles.

Step 2 Determine the mole ratio of the desired substance to the starting substance. The number of moles of each substance in the balanced equation is indicated by the coefficient in front of each substance. Use these coefficients to set up the mole ratio.

Step 2 Determine the mole ratio of the desired substance to the starting substance. Multiply the number of moles of starting substance (from Step 1) by the mole ratio to obtain the number of moles of desired substance.

Step 3. Calculate the desired substance in the units specified in the problem. If the answer is to be in moles, the calculation is complete If units other than moles are wanted, multiply the moles of the desired substance (from Step 2) by the appropriate factor to convert moles to the units required.

Step 3. Calculate the desired substance in the units specified in the problem.

Step 3. Calculate the desired substance in the units specified in the problem.

Step 3. Calculate the desired substance in the units specified in the problem.

Limiting-Reactant and Yield Calculations

The limiting reactant limits the amount of product that can be formed. The limiting reactant is one of the reactants in a chemical reaction. It is called the limiting reactant because the amount of it present is insufficient to react with the amounts of other reactants that are present. The limiting reactant limits the amount of product that can be formed.

Steps Used to Determine the Limiting Reactant

Calculate the amount of product (moles or grams, as needed) formed from each reactant. Determine which reactant is limiting. (The reactant that gives the least amount of product is the limiting reactant; the other reactant is in excess. Calculate the amount of the other reactant required to react with the limiting reactant, then subtract this amount from the starting quantity of the reactant. This gives the amount of the that substance that remains unreacted.

Reaction Yield

The quantities of products calculated from equations represent the maximum yield (100%) of product according to the reaction represented by the equation.

Many reactions fail to give a 100% yield of product. This occurs because of side reactions and the fact that many reactions are reversible.

The theoretical yield of a reaction is the calculated amount of product that can be obtained from a given amount of reactant. The actual yield is the amount of product finally obtained from a given amount of reactant.

The percent yield of a reaction is the ratio of the actual yield to the theoretical yield multiplied by 100.