Final Review: L17-25

Chem 11 Fall 2006

Balancing Equations

4 ways to understand if a chemical reaction occurred:

1. A gas is detected.

2. A precipitate is formed.

3. A permanent color change is seen.

4. Heat or light is given off.

An exothermic reaction gives off heat and an endothermic reaction absorbs heat.

There are 7 elements that exist as diatomic molecules:

– H2, N2, O2, F2, Cl2, Br2, and I2

When we balance a chemical equation, the number of each type of atom must be the same on both the product and reactant sides of the equation.

We use coefficients in front of compounds to balance chemical reactions.

Balancing Equations

Net Ionic Equations

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

The total ionic equation is:

H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) → Na+(aq) + Cl-(aq) + H2O(l)

Na+ and Cl- appear on both sides of the equation. They are spectator ions. Spectator ions are in the solution, but do not participate in the overall reaction.

Cancel out the spectator ions to get the net ionic equation.

The net ionic equation is:

H+(aq) + OH-(aq) → H2O(l)

Oxidation-Reduction Reactions

Oxidation reduction type of reactions involve transfer of electrons from one substance to another.

Rusting of iron.

Iron metal reacts with oxygen in air to produce ionic iron(III) oxide compound, which is composed of Fe3+ and O2- ions.

4 Fe(s) + 3 O2(g) → 2 Fe2O3(s)

In this reaction:

a) Iron loses electrons and is oxidizedFe → Fe3+ + 3 e-

b) Oxygen gains electrons and is reducedO2 + 4 e- → 2 O2-

Rules for Oxidation Numbers

To determine oxidation numbers apply these seven rules:

1. In the free state, a metal or a nonmetal has 0 oxidation number.

2. A monoatomic ion has an oxidation number equal to its ionic charge.

3. Oxidation number of a hydrogen atom is usually +1.

Rules for Oxidation Numbers

4. Oxidation number of an oxygen atom is usually -2.

5. In a molecular compound, the more electronegative element carries a negative oxidation number equal to its charge as an anion.

6. In an ionic compound, the sum of the oxidation numbers for each of the atoms in the compound is equal to 0.

7. In a polyatomic ion, the sum of the oxidation numbers for each of the atoms in the compound is equal to the ionic charge on the polyatomic ion.

Classifying Chemical Reactions

Chemical reactions can be divided into five categories:

I. Combination Reactions

II. Decomposition Reactions

III. Single-Replacement Reactions

IV.Double-Replacement Reactions

V. Neutralization Reactions

Combination reaction, two simpler substances are combined into a more complex compound.

2 Mg(s) + O2(g) → 2 MgO(s)

Decomposition reaction, a single compound is broken down into simpler substances.

Single-replacement reaction, a more active metal displaces a less active metal in a compound.

Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

Double replacement reaction, two ionic compounds in aqueous solution switch anions and produce two new compounds. BaCl2(aq) + K2CrO4(aq) → BaCrO4(s) + 2 KCl(aq)

Neutralization reaction, is the reaction of an acid and a base. H2SO4(aq) + 2 KOH(aq) → K2SO4(aq) + 2 H2O(l)

Decomposition Reactions

Decomposition reaction, a single compound is broken down into simpler substances.

2 HgO(s) → 2 Hg(l) + O2(g)

Ni(HCO3)2(s) → NiCO3(s) + H2O(l) + CO2(g)

CaCO3(s) → CaO(s) + CO2(g)

Mass Limiting Reactant Problem

How much molten iron is formed from the reaction of 25.0 g FeO and 25.0 g Al?

3 FeO(l) + 2 Al(l) → 3 Fe(l) + Al2O3(s)

19.4 g Fe can be produced if FeO is limiting reactant.

25.0 g FeO ×3 mol Fe

3 mol FeO

1 mol FeO

71.85 g FeO×

1 mol Fe

55.85 g Fe×

= 19.4 g Fe

Cont’d…Mass Limiting Reactant Problem

3 FeO(l) + 2 Al(l) → 3 Fe(l) + Al2O3(s)

77.6 g Fe can be produced if Al is limiting.

25.0 g Al ×3 mol Fe2 mol Al

1 mol Al26.98 g Al

×1 mol Fe

55.85 g Fe×

= 77.6 g Fe

Describing a Gas• Gases are composed of tiny particles

• The particles are small compared to the average space between themAssume the molecules do not have volume

• Molecules constantly and rapidly moving in a straight line until they bump into each other or the wallAverage kinetic energy is proportional to the

temperature

• Assumed that the gas molecules attraction for each other is negligible

Gas LawsBoyle’s Law states that the volume of a gas is inversely proportional to the pressure at constant temperature.

P1V1 = P2V2

Charles Law states that the volume of a gas is directly proportional to the temperature in Kelvin.

V1/T1 = V2/T2

Gay-Lussac discovered that the pressure of a gas is directly proportional to the temperature in Kelvin.

P1/T1 = P2/T2

Combined Gas Law

P1V1/T1 = P2V2/T2

Boyle’s Law is: P1V1 = P2V2

Charles’ Law is:

Gay-Lussac’s Law is:

The combined gas law is:

V1

T1

V2

T2

=

P1

T1

P2

T2

=

P1V1

T1

P2V2

T2

=

Cont’d…Review

Intermolecular Bond Concept

An intermolecular bond is an attraction between molecules.

In the intramolecular bonds, attraction is between the atoms in a molecule.

Attraction strength between molecules determine some of the liquid properties, such as vapor pressure, viscosity, and surface tension.

Intermolecular bonds are much weaker than intramolecular bonds.

Intermolecular Forces

• There are three types of intermolecular bonds:

Dispersion forces

Dipole forces

Hydrogen bonds

• Dispersion forces are the weakest and hydrogen bonds are the strongest.

• These intermolecular attractions affect the physical properties of substances.