chemistry chapter 8: chemical equations and reactions

Chemistry

Chapter 8: Chemical Equations and Reactions

Chemistry Chapter 8 2

Chemical reaction

• The process by which one or more substances are changed into one or more different substances

• Reactants: original substances• They react

• Products: resulting substances• They are produced

Chemistry Chapter 8 3

Indications of a chemical reaction

1. Formation of a precipitate• Solid produced when two liquids are mixed

2. Gas is produced• Foaming or bubbles

3. Energy is released• Heat and light and/or sound are produced

4. Color change

Chemistry Chapter 8 4

Chemical equation

• Represents a chemical reaction with symbols and formulas

• Shows the relative amounts of products and reactants

Chemistry Chapter 8 5

Chemical equations

1. Must represent known facts• All reactants and products must be identified

2. Must contain the correct formulas for reactants and products

• Pure elements are represented with their symbols

• Sulfur is S8 and phosphorus is P4

• Some are diatomic

Chemistry Chapter 8 6

Diatomic molecules

• The magnificent seven• See table 8-1 on page 243• There are 7 of them• On the periodic table, they make a 7 (plus h

ydrogen)

Chemistry Chapter 8 7

Chemical equations

3. Law of conservation of mass must be satisfied

• Same number of atoms of each element on both sides of the equation

• Sometimes coefficients are needed• Small whole number in front of a chemical

formula

Chemistry Chapter 8 8

Word equation

• Zinc + hydrochloric acid hydrogen + zinc chloride

• Qualitative, not quantitative• Read the arrow as “react to yield”, “yield”,

“produce”, or “form”

Chemistry Chapter 8 9

Formula equation

• Represents the reactants and products by their symbols or formulas

• Zn(s) + HCl(aq) H2(g) + ZnCl2(aq)

• (s) means solid• (aq) means aqueous – dissolved in water• (g) means gaseous• (l) means liquid

Chemistry Chapter 8 10

Balancing equations

• We need to have the same number of atoms of each element on both sides of the equation.

• Add coefficients to make this work• Save oxygen and hydrogen until last to

balance• Zn(s) + HCl(aq) H2(g) + ZnCl2(aq)

Chemistry Chapter 8 11

Other symbols used – see page 246

• See table 8-2 on

Chemistry Chapter 8 12

Discuss

• Translate the following equations into sentences:• 2ZnO(s) + C(s) 2Zn(s) + CO2(g)

• Solid zinc oxide and solid carbon react to produce solid zinc metal and carbon dioxide gas

• CS2(l) + 3O2(g) CO2(g) + 2SO2(g)• Liquid carbon disulfide reacts with oxygen gas to

produce carbon dioxide gas and sulfur dioxide gas.• NaCl(aq) + AgNO3(aq) NaNO3(aq) + AgCl(s)

• Aqueous solutions of sodium chloride and silver nitrate react to produce aqueous sodium nitrate and a precipitate of silver chloride.

Chemistry Chapter 8 13

Coefficients in chemical equations

• Indicate relative amounts of reactants and products

• Smallest numbers of atoms, molecules, or ions that satisfy conservation of mass

• Smallest whole number ratio of particles or moles• Can be used to find the relative masses of

reactants and products• Convert number of moles (coefficient) to grams

Chemistry Chapter 8 14

Example

• 2H2O 2H2 + O2

• 2 mol H2O produce 2 mol H2 and 1 mole O2

• 2*18.02 = 36.04 g H2O

• 2*2.02 = 4.04 g H2

• 32.00 g O2

• 36.04 g H2O produce 4.04 g H2 and 32.00 g O2

Chemistry Chapter 8 15

Balancing equations

1. Make sure the formulas in the equation are correct.

2. Balance the elements that appear only once on each side of the equation.

3. Balance polyatomic ions that appear on each side of the equation as single units

4. Balance O atoms and H atoms last5. Count atoms to make sure the equation is

balanced

Chemistry Chapter 8 16

Common errors

• The formulas are incorrect to begin with.• Subscripts get changed in an attempt to

balance.• Coefficients don’t represent the smallest

whole number ratio.

Chemistry Chapter 8 17

Example

• Solid calcium metal reacts with water to form aqueous calcium hydroxide and hydrogen gas.

• Ca(s) + H2O(l) Ca(OH)2(aq) + H2(g)

Chemistry Chapter 8 18

Example

• Nitrogen dioxide gas reacts with water to form aqueous nitric acid and nitrogen monoxide gas.

• NO2(g) + H2O(l) HNO3(aq) + NO(g)

Chemistry Chapter 8 19

You try

• Solid potassium chlorate decomposes to form solid potassium chloride and oxygen gas.

• KClO3(s) KCl(s) + O2(g)

Chemistry Chapter 8 20

You try

• H2SO4(aq)+ BaCl2(aq) HCl(aq) + BaSO4(s)

• Al2O3(s)+ H2SO4(aq) Al2(SO4)3(aq)+ H2O(l)

Chemistry Chapter 8 21

Discuss

• Write the word and balanced chemical equations for the reaction that occurs when hydrogen peroxide in an aqueous solution decomposes to produce oxygen and water.

• Hydrogen peroxide oxygen + water• 2H2O2(aq) O2(g) + 2H2O(l)

Chemistry Chapter 8 22

5 basic types of chemical reactions

• Synthesis• Decomposition• Single replacement• Double replacement• Combustion

Chemistry Chapter 8 23

Synthesis

• AKA composition• Two or more substances combine to form a

new compound• A + X AX• More than one reactant and only one

product

Chemistry Chapter 8 24

Oxides

• Created when an element combines with oxygen in a synthesis reaction

• 2Mg(s) + O2(g) 2MgO(s)

Chemistry Chapter 8 25

Oxides

• Some metals can form two different oxides

• 2Fe(s) + O2(g) 2FeO(s)

• 4Fe(s) + 3O2(g) 2Fe2O3(s)

Chemistry Chapter 8 26

Metals and halogens

• Most metals react with halogens

• 2Na(s) + Cl2(g) 2NaCl(s)

• 2Co(s) + 3F2(g) 2CoF3(s)

Chemistry Chapter 8 27

Synthesis with oxides

• Oxides of active metals react with water to make metal hydroxides, which are bases.

• CaO(s) + H2O(l) Ca(OH)2(s)

• Oxides of nonmetals often react with water to produce acids

• SO3(g) + H2O(l) H2SO4(aq)

Chemistry Chapter 8 28

Decomposition reactions

• A single compound produces two or more simpler substances

• AX A + X• Only 1 reactant and more than 1 product

Chemistry Chapter 8 29

Binary compound

• Decompose into elements that make it up• 2H2O(l) 2H2(g) + O2(g)

• Electrolysis – decomposition by an electric current

electricity

Chemistry Chapter 8 30

Metal Carbonates

• Decompose to produce a metal oxide and carbon dioxide gas

• CaCO3(s) CaO(s) + CO2(g)

Chemistry Chapter 8 31

Metal Hydroxides

• Most decompose to make metal oxides and water

• Ca(OH)2(s) CaO(s) + H2O(l)

Chemistry Chapter 8 32

Metal Chlorates

• Decompose to produce a metal chloride and oxygen

• 2KClO3(s) 2KCl(s) + 3O2(g)

Chemistry Chapter 8 33

Decomposition of Acids

• Some decompose into nonmetal oxides and water

• Carbonic acid decomposes into carbon dioxide and water

• Sulfuric acid decomposes into sulfur trioxide and water

• Sulfurous acid decomposes into sulfur dioxide and water

Chemistry Chapter 8 34

Discuss

• After hearing about the discovery of oxygen in 1774, Antoine Lavoisier began an experiment to study the nature of burning. He slowly heated mercury in air to produce mercury (II) oxide. He then strongly heated the mercury (II) oxide until he had recovered the mercury and the oxygen.

• Write the equations for both the synthesis reaction and the decomposition reaction described above.

Chemistry Chapter 8 35

Single-replacement reactions

• AKA displacement• One element replaces a similar element in a

compound• A + BX AX + B• Y + BX BY + X

Chemistry Chapter 8 36

Metals displacing metals

• A more active metal may replace another metal in a compound

• 2Al(s) +3Pb(NO3)2(aq) 3Pb(s) +2Al(NO3)3(aq)

Chemistry Chapter 8 37

Metals displacing hydrogen in water

• Active metals react with water to form metal hydroxides and hydrogen gas

• 2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)

• Less active metals react with steam to form metal oxides and hydrogen gas

• 3Fe(s) + 2H2O(g) Fe3O2(s) + 2H2(g)

Chemistry Chapter 8 38

Metals displacing hydrogen in acids

• Produce a salt and hydrogen gas

• Zn(s) + 2HCl(aq) H2(g) + ZnCl2(aq)

Chemistry Chapter 8 39

Halogens replacing halogens

• More active ones replace others• Can replace any below it

• Cl2(g) + 2KBr(aq) 2KCl(aq) + Br2(l)

Chemistry Chapter 8 40

Double replacement reactions

• AKA double displacement• The ions of two compounds exchange

places to form two new compounds• The ions switch partners• AX + BY AY + BX

Chemistry Chapter 8 41

Precipitate formation

• 2KI(aq) + Pb(NO3)2(aq) PbI2(s) + 2KNO3(aq)

Chemistry Chapter 8 42

Gas formation

• FeS(s) + 2HCl(aq) H2S(g) + FeCl2(aq)

Chemistry Chapter 8 43

Water formation

• HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

Chemistry Chapter 8 44

Combustion reactions

• Burning by combining with oxygen• Energy released as light and heat• Must have O2 as a reactant

• 2H2(g) + O2(g) 2H2O(g)

Chemistry Chapter 8 45

Burning hydrocarbons

• Hydrocarbons contain only hydrogen and carbon

• When they burn, they always produce carbon dioxide and steam

• C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)

• CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

Chemistry Chapter 8 46

Activity

• The ability of an element to react• For metals – how easily they lose electrons• For nonmetals – how easily they gain

electrons

Chemistry Chapter 8 47

Activity series

• A list of elements arranged according to how easily they undergo certain chemical reactions

• See table 8-3 on page 266• Help determine whether or not a reaction

will actually take place

Chemistry Chapter 8 48

Single displacement reactions

• An element in a series can displace any element below it, but no element above it.

• Often related to position on the periodic table• Can displace elements below, but not above

Chemistry Chapter 8 49

Example

• Using table 8-3, predict whether the following reactions will occur. If they will, write the products and balance the equation.

• MgCl2(aq) + Zn(s) • No reaction, because Zn cannot displace Mg

• Al(s) + H2O(g) • Yes, Al can displace H in steam• 2Al(s) + 3H2O(g) Al2O3(s) + 3H2(g)

Chemistry Chapter 8 50

You try

• Using table 8-3, predict whether the following reactions will occur. If they will, write the products and balance the equation.

• Cd(s) + O2(g) • Cd reacts with oxygen forming an oxide• 2Cd(s) + O2(g) 2CdO(s)

• I2(s) + KF(g) • No reaction, because I cannot replace F

Chemistry Chapter 8 51

You try

• Identify the halogen that will not replace any other halogen in a compound.• Iodine

• Identify the halogen that will not be replaced by any of the other halogens• Fluorine