Chemical Equations Chapter 8 Hein and Arena Eugene Passer Chemistry Department Bronx Community College © John Wiley and Sons, Inc Version 2.0 12th Edition

Chapter Outline 8.1 The Chemical Equation 8.4 Types of Chemical Equations 8.2 Writing and Balancing Chemical Equations 8.5 Heat in Chemical Reactions 8.3 Information in a Chemical Equation

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.

8.1 The Chemical Equation

Chemical reactions always involve change. Atoms, molecules or ions rearrange to form different 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.

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

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

Symbols Used in Chemical Reactions

placed between substances symbol + meaning plus placed between substances location

between reactants and products symbol  meaning yields between reactants and products location

symbol (s) solid meaning location after formula

symbol (l) liquid meaning location after formula

symbol (g) gas meaning location after formula

symbol (aq) aqueous meaning after formula location

symbol  heat meaning written above or below  location

symbol h light energy meaning written above  location

symbol  gas formation meaning after formula location

8.2 Writing and Balancing Equations

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.

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 Hg 1 1 There is one mercury atom on the reactant side and one mercury atom on the product side. Mercury is balanced.

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 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 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

C4H10 (g) + O2 (g) → CO2(g) + H2O(l) Balance the Equation C4H10 (g) + O2 (g) → CO2(g) + H2O(l) 4 Reactant Side Product Side C 4 1 H 10 2 O 2 3 4 9 There are four C on the reactant side and there is one C on the product side. Place a 4 in front of CO2 to balance C.

C4H10 (g) + O2 (g) → CO2(g) + H2O(l) 5 Reactant Side Product Side C 4 4 H 10 2 O 2 9 10 13 Place a 5 in front of H2O to balance H. There are 10 H on the reactant side and there are two H on the product side.

C4H10 (g) + O2 (g) → CO2(g) + H2O(l) 8 C4H10 (g) + O2 (g) → CO2(g) + H2O(l) 4 10 5 5 Reactant Side Product Side C 4 4 H 10 10 O 2 13 20 8 26 There is no whole number coefficient that can be placed in front of O2 to balance O. To balance O, double the coefficients of each substance other than oxygen.

 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.

8.3 Information in a Chemical Equation

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

8.4 Types of Chemical Equations

Combination Decomposition Single-Displacement Double-Displacement

Combination Reactions

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

Examples

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

Examples

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)

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)

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 of 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

Examples Metal Activity Series

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.

Example Halogen Activity Series

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.

Double Displacement Reactions

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

Examples

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)

8.5 Heat in Chemical Reactions

Energy changes always accompany chemical reactions. When this occurs, energy is released to the surroundings. One reason why reactions occur is that the product attains a lower energy state than the reactants.

Exothermic reactions liberate heat. The amounts of substances are expressed in moles. Exothermic reactions liberate heat. H2(g) + Cl2(g) → 2HCl(g) + 185 kJ (exothermic) 1 mol 2 mol Endothermic reactions absorb heat. N2(g) + O2(g) + 185 kJ → 2NO(g) (exothermic) 1 mol 2 mol

For life on Earth the sun is the major provider of energy. The energy for plant photosynthesis is derived from the sun. 6CO2 + 6H2O + 2519 kJ → C6H12O6 + 6O2 glucose glucose

Energy of Activation

A certain amount of energy is always required for a reaction to occur. The energy required to start a reaction is called the energy of activation.

This reaction will not occur unless activation energy is supplied. CH4 + 2O2 → CO2 + 2H2O + 890 kJ This reaction will not occur unless activation energy is supplied. The activation energy can take the form of a spark or a flame.

8.2 8.3

The End