Kinetics & Equilibrium and Compounds & Stoichiometry

MCAT Prep: General Chemistry Part 3

Experimental determination of rate law

the values of k, x, and y in the rate law equation rate must be determined experimentally for a given reaction at a given temperature. The rate is usually measured as a function of the initial concentrations of the reactants, A and B

rate = k [A]^x [B]^y

rate law equation

Kc = ([C]^c [D]^d) / ([A]^a [B]^b)

equilibrium constant equation

Keq

characteristic of a given system at a given temperature

Keq » 1

an equilibrium mixture of reactants and products will contain very little of the reactants compared to the products

Keq « 1

an equilibrium mixture of reactants and products will contain very little of the products compared to the reactants

Keq close to 1

an equilibrium mixture of products and reactants will contain approximately equal amounts of the two

Le Chatelier’s principle

used to determine the direction in which a reaction at equilibrium will proceed when subjected to a stress, such as a change in concentration, pressure, volume, or temperature. The key is to remember that a system to which these kinds of stresses are applied tends to change so as to relieve the applied stress

right

Shift to the _______

A + B ⇌ C + heat

1. if more A or B added

2. if C taken away

3. if pressure applied or volume reduced (assuming A, B, and C are gases)

4. if temperature reduced

left

Shift to the _______

A + B ⇌ C + heat

1. if more C added

2. if A or B taken away

3. if pressure reduced or volume increased (assuming A, B, and C are gases)

4. if temperature increased

Mole

the amount of a substance that contains the same number of particles that are found in a 12.000 g sample of carbon-12

Molecular or formula weight

measured in amu per molecule (or formula unit)

Molar mass

measured in grams per molecule

Combustion reactions

a fuel, such as a hydrocarbon, is reacted with an oxidant, such as oxygen, to produce an oxide and water

CH₄ (g) + 2 O₂ (g) → CO₂ (g) + 2 H₂O (g)

Combination reactions

two or more reactants form one product

S (s) + O₂ (g) → SO₂ (g)

Decomposition reactions

a compound breaks down into two or more substances, usually as a result of heating or electrolysis

2 HgO (s) → 2 Hg (l) + O₂ (g)

Single-displacement reactions

an atom (or ion) of one compound is replaced by an atom of another element

Zn (s) + CuSO₄ (aq) → Cu (s) + ZnSO₄ (aq)

Double-displacement reactions

also called metathesis reactions; elements from two different compounds displace each other to form two new compounds

CaCl₂ (aq) + 2 AgNO₃ (aq) → Ca(NO₃)₂ (aq) + 2 AgCl (s)

Net ionic equations

these types of equations are written showing only the species that actually participate in the reaction. Consider the following equations:

Zn (s) + Cu²⁺ (aq) + SO₄^2- (aq) → Cu (s) + Zn²⁺ (aq) + SO₄^2- (aq)

The spectator ion (SO₄^2-) does not take part in the overall reaction, but simply remains in solution throughout. This would be:

Zn (s) + Cu²⁺ (aq) → Cu (s) + Zn²⁺ (aq)

Neutralization reactions

these are specific double-displacement reactions that occur when an acid reacts with a base to produce a solution of a salt (and, usually, water):

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

Factors affecting reactions: reactant concentrations, temperature, medium, catalysts

Catalysts

unique substances that increase reaction rate without being consumed; they do this by lowering the activation energy