Chapter 21Chemical Reactions (Rxns)

Sec. 1 Chemical Changes

• Chemical Reaction—a change in which 1 or more substances are converted into new substances.

• Reactants—substances that react or the starting substances.

• Products—new substances produced.• Ways to tell if a chemical reaction has

occurred:• Odor, color change, change in heat, bubbles

Conservation of Mass

• Law of Conservation of Mass—mass cannot be created nor destroyed.

• The mass of reactants is always equal to the mass of products.• Ex. 0.7g O2 + 9.3g Hg 10.0g HgO2

Chemical Equations• Chemical equation—a way to

describe a chemical rxn using chemical formulas and other symbols.

• Reactants are written on the left, products on the right.• Reactants Products• The is read as “yields” or “produces”

States of Matter & Coefficients• The physical state of the reactants and

products is written after the formula in parentheses.• Solid (s)• Liquid (l)• Gas (g)• Aqueous (aq): a substance dissolved in

water.• Coefficients (#s to the left of formulas)

tell the # of units of each substance in the rxn.

Writing Equations• A chemical reaction can be written in

words• Ex. One unit of solid zinc plus 2 units of

hydrochloric acid dissolved in water produce one unit of hydrogen gas plus one unit of zinc chloride.

• You can use chemical formulas & symbols in the chemical equation.• Ex. Zn(s) + 2HCl(aq) H2(g) + ZnCl2(aq)

Balancing Chemical Equations1. Determine the formulas for all reactants

and products. (this may be given)2. Write reactants on the left and products

on the right of the arrow.3. Count the # of each element in reactants

& productsEx. CH4 + O2 CO2 + H2O

C = 1 C = 1 H = 4 H = 2O = 2 O = 3

Balancing Chemical Equations4. Balance each element by using

coefficients—number placed in front of the symbol.

5. Make sure all elements are balanced.6. Make sure all coefficients are in the

lowest ratio.Ex. CH4 + O2 CO2 + H2O

C = 1 C = 1 H = 4 H = 2O = 2 O = 3

____ CH4 + ____ O2 ____ CO2 + ____ H2O2 2

Balancing Hints

• It is usually easiest to start by balancing oxygen

• Try making odd #’s even with a coefficient of 2

• (*) Balance oxygen last on combustion reactions

Types of Chemical

Rxns

Combination/Synthesis Rxn

• 2 or more reactants combine to form a single product.

• Generalized Formula• A + B AB

• Examples:• 2K(s) + Cl2(g) 2KCl(s)

• CaO(s) + H2O(l) Ca(OH)2(aq)

Decomposition Rxn• A single compound is broken

down into 2 or more products.• Generalized Formula

• AB A + B

• Examples:• PbO2(s) Pb(l) + O2(g)

• CaCO3(s) CaO(s) + CO2(g)

Single-Replacement Rxn

• One element replaces a second element in a compound.

• Generalized Formula• A + BC AC + B

• Example:• Mg(s) + 2AgNO3(aq)Mg(NO3)2(aq) + 2Ag(s)

Double-Replacement Rxn• Involve an exchange of positive ions

between 2 ionic compounds. Generalized Formula• AB + CD AD + CB

• Examples:• Na2S + Cd(NO3)2 CdS + 2NaNO3

• 2NaCN + H2SO4 2HCN + Na2SO4

Combustion Rxn• An element or compound reacts with

oxygen (O2) often producing CO2 & H2O

• Heat & light are also often given off• Example:

• 2CH3OH(l) + 3O2(g)2CO2(g) + 4H2O(g)

Romance analogy for the four types of reaction

• Combination Falling in love

• Decomposition The Break-up

• Single Replacement The player

• Double Replacement The swingers

Completing Chem Rxns• Some steps for completing reactions

1. Identify the type of reaction2. Predict the product(s) 3. Balance the equation using COEFFICIENTS

• Remember: some elements are diatomic (exist as 2 atoms)• Hydrogen: H2, Nitrogen: N2, Oxygen: O2,

Fluorine: F2, Chlorine: Cl2, Bromine: Br2, Iodine: I2

When Predicting Products

• When predicting products, remember to write the formulas correctly—DON’T FORGET TO BALANCE OUT THE CHARGES!• Ex. Li + CuSO4 • Type of Rxn: Single Replacement• Li replaces Cu & bonds with SO4

• Look at charges of ions: Li+1 SO4-2

• Products are…

Li2SO4 + Cu

Activity Series for Metals

• In Single-Replacement, not all metals elements can replace other metals.

• The single metal element must be more reactive than the one in the compound.

• Look on P. 643 in textbook for the activity series

WRITE THIS DOWN!!!

State the type of reaction, Predict the products & Balance the equation.

Type of Rxn Reactants Products

1. _____________ __BaCl2 + __H2SO4

2. _____________ __C6H12 + __O2

3. _____________ __Zn + __CuSO4

4. _____________ __Cs + __Br2

5. _____________ __FeCO3

__BaSO4 + __HCl

__CO2 + __H2O

__ZnSO4 + __Cu

__CsBr

__FeO + __CO2

Double-Replacement

Combustion

Single-Replacement

Combination

Decomposition

2

6 69

Balanced!!!

22

Balanced!!!

Sec. 4 Chem Rxns & Energy

• Some chemical rxns require heat in order to happen

• Some rxns give off heat.• Thermochemistry—heat

changes that occur during chemical reactions.

Exo & Endo• Exothermic—a rxn where energy

(heat) is given off.• Ex: burning wood, exploding dynamite.• Exothermic rxns feel warm.

• Endothermic—a rxn where energy (heat) is absorbed.• Ex. Cold packs• Endothermic rxns feel cold.

Catalysts & Inhibitors• Catalyst—a substance that

speeds up a chem rxn without being used in the rxn.• Written above the arrow in an

equation.

• Used in making plastics

2 H2O + O2MnO22H2O2

• Inhibitors—prevent certain rxns from occurring. • Used in food preservatives to prevent spoiling.

Heat as a catalyst