Chapter 10

Chemical Reactions

Evidence of a Chemical Reaction

• Energy release (heat or light)

Evidence of a Chemical Reaction

• Color change

Evidence of a Chemical Reaction

• Odor

• gas bubbles

• smoke formation

Evidence of a Chemical Reaction

• Precipitation

Symbols used in Equations

+ separates two reactants or two products

→ “Yields” separated reactants from products

(s) Designates a reactant in the solid state

(t) Designates a reactant in the liquid state

(g) Designates a reactant in the gaseous state

(aq) Designates a reactant in the aqueous state

→ → indicates that heat is supplied to a reaction

→ Indicates that a catalyst is used (here, platinum)

Representing Chemical Equations

reactant 1 + reactant 2 → product 1 + product 2

produces

Product(s) must be different than the reactants

Representing Chemical Equations

Simply a “recipe”

+ + →

Eggs and butter and flour produce a cake

Word equations name the reactants and products.

What is missing from this recipe? AMOUNTS!!!

• + →

• Iron (II) and oxygen produce iron oxide• Fe (s) + O2 (g) → FeO (s)

• What is missing?• Amounts! • How do you figure out the amounts needed and

produced?

Representing Chemical Equations

Balancing Equations

• Based on the Law of Conservation of Mass

• “Matter is not created or destroyed in a reaction, just rearranged.”

• Fe + O2 → Fe O

• What is wrong here?

• an oxygen atom is missing

Balancing Equations

• Atom inventory Fe + O2 → Fe O

• Reactant side Product side

Fe O Fe O

1 2 1 1

Fe + O2 → Fe O

Final equation: Fe + O2 → Fe O

Balancing Equations

Steps:

• 1. Write the skeleton equation (can’t escape formula writing)

• 2. Count atoms in reactants

• 3. Count atoms in products

• 4. Change coefficients to make atom counts match. Never change subscripts.

• 5. Reduce coefficients if necessary.

Practice Problems

____ AlBr3 + ____ K→____ KBr + ____ Al

____ AlBr3 + 3 K → 3 KBr + ____ Al

____ P4 + ____ Br2 → ____ PBr3

____ P4 + 6 Br2 → 4 PBr3

C6H8O7(aq) + 3NaHCO3(aq) → 3H2O(l) + 3CO2(g) + Na3C6H5O7(aq)

citric acid + baking soda → water + carbon dioxide + sodium citrate

1. Calculate the molar mass for each compound in the equation.

2. If there is a coefficient, multiply the molar mass by that number.

3. Add the compounds on the left of the arrow.

4. Add the compounds on the right of the arrow.

5. How are the two sums related?

Symbols used in Equations#1 separates two reactants or two products

#2 “Yields” separated reactants from products

#3 Designates a reactant in the solid state

(l) Designates a reactant in the # 4 state

(g) Designates a reactant in the # 5 state

#6 Designates a reactant in the aqueous state

# 7 indicates that heat is supplied to a reaction

→ Indicates that a # 8 is used (here, platinum)

# 9 name the diatomic elements

Classifying Chemical Reactions

• Five general types• Synthesis one new product• Solid sodium and chlorine gas form solid sodium chloride

2Na(s) + Cl2(g) → 2NaCl(s)

+ →

http://jchemed.chem.wisc.edu/jcesoft/cca/cca0/Movies/NACL1.html

http://genchem.chem.wisc.edu/demonstrations/Gen_Chem_Pages/04chemrxnpage/chemicalreactions.htm

Classifying Chemical Reactions

Combustion Must have O2 as a reactant

2 Mg + O2 → 2 MgO

+ →

Combustion reactions are exothermic; energy is produced.

Classifying Chemical Reactions

• Combustion of hydrocarbons (compounds containing hydrogen and carbon)

• CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

• Most fuels are hydrocarbons

Classifying Chemical Reactions

• Decomposition Only one reactant• Hydrogen peroxide decomposes to release

oxygen gas and leave water on your skin

H2O2(aq) → O2 (g) + H2O(l)

2 H2O2(aq) → O2 (g) + 2 H2O(l)

Classifying Chemical Reactions

• Single replacement reactions One reactant is an element

• Copper reacts with silver nitrate to produce silver and copper nitrate.

• Cu(s) + AgNO3(aq) → Ag(s) + Cu(NO3)2(aq)

Single replacement reactions

• Activity Series of Metals

Lithium Iron

Rubidium Nickel

Potassium Tin

Calcium Lead

Sodium Copper

Magnesium Silver

Aluminum Platinum

Manganese Gold

Zinc

Single replacement reactions

• Halogens

FluorineChlorine BromineIodine

Classifying Chemical Reactions

• Double replacement reactions both reactants are ionic compounds

• A gas, liquid, or solid must be producedSilver nitrate and sodium chloride react to produce sodium nitrate and silver chloride

AgNO3 (aq) + Na Cl (aq) → NaNO3 (aq) + AgCl (s)

Ag+

Cl-

Na+ NO3- AgCl

Na+ NO3-

Summary Reaction Classification

Class of Reaction Reactants Probable products

Synthesis Two or more substances

One compound

Combustion Some substance and oxygen

An oxide

H20 and CO2 for hydrocarbon combustion

Decomposition One compound Two or more (elements or compounds)

Single-replacement An element and a compound

A new compound and the replaced element

Double-replacement Two compounds Two different compounds including a s, l, g

Reactions in Aqueous Solutions

• Review:

• Solution – homogeneous mixture

• Solute – substance dissolved

• Solvent – most plentiful substance in the solution

• Water is the “universal solvent”

• Other substances can also be solvents

Reactions in Aqueous Solutions

• How does water dissolve ionic compounds? Water is a polar molecule.

• http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/molvie1.swf

http://programs.northlandcollege.edu/biology/Biology1111/animations/dissolve.html

Reactions in Aqueous Solutions Reactions that form solids

• Lead nitrate and potassium iodide

• Two clear solutions – mix – precipitate

forms

Pb +2

NO3 -

K+

I-

Reactions in Aqueous Solutions Reactions that form solids

• What happened on an atomic level?

• + →

• Lead and iodide form a new compound• Nitrate and potassium are still dissolved ions –

they did nothing but “watch” – spectator ions

NO3 –

Pb +2

K+

I-

K+

NO3 –

PbI2

Reactions in Aqueous Solutions Reactions that form solids

• Complete ionic equation• Pb +2 + NO3

- + K+ + I- → PbI2 + NO3 - + K+

• Spectator ions

• Net ionic equation• Pb +2 + I- → PbI2

Reactions in Aqueous Solutions Reactions that form liquids

• HBr (aq) + NaOH(aq) → H2O (l) + NaBr(aq)

Complete ionic equation .H+

(aq) + Br-(aq) + Na+

(aq) + OH -(aq) → H2O(l) + Na+

(aq) + Br-(aq)

Net ionic equation H+

(aq) + OH -(aq) → H2O(l)

Reactions in Aqueous Solutions Reactions that form gases

2HI (aq) + Li2S (aq) → H2S (g) + 2LiI (aq)

Complete ionic equation2H+

(aq) + 2I- (aq) + 2Li+

(aq) + S2-(aq) → H2S(g) + 2Li+

(aq) + 2I- (aq)

Net ionic equation2H+

(aq) + S2-(aq) → H2S(g)