Notes 

MASS-ACTION EXPRESSION (EQUILIBRIUM CONSTANT)

For the reaction aB + bB <---> cC + dD            K_eq = [C]^c [D]^d / [A]^a [B]^b

Note: Solids and liquids have constant concentration therefore they do not appear in K_eq

Example

BF₃(g) + 3H₂O(l) <---> 3HF(aq) + H₃BO₃(aq)     K_eq = [HF]³ [H₃BO₃] / [BF₃]

The K _eq of a reverse reaction is the reciprocal of the forward reaction.

Example:

K_eq of forward reaction is 1 x 10^-11 than the reverse reaction K_eq is 1 / 1 x 10^-11

Solubility Product Constant (K_sp)

Used for salts.

Example:

PbCl₂(s) ---> Pb²⁺(aq) + 2Cl^-(aq)             Ksp = [Pb²⁺] [Cl^-]²

LAWS OF THERMODYNAMICS

The first Law of Thermodynamics states that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system.

∆E = q − w

q = heat added to the system

w = work done by the system on the surroundings

The Second Law of Thermodynamics states that in any spontaneous process there is always an increase in the entropy of the universe.            

                ∆S _universe = ∆S _system + ∆S _surrounding

The Third Law of Thermodynamics states that a crystal at 0 K has 0 entropy.

GIBBS FREE ENERGY

The function G (free energy) is used to find the spontaneity of a reaction.

            ∆G = ∆H-T∆S     T in Kelvin

When ∆G < 0 the forward reaction is spontaneous therefore reactants form products.

When ∆G > 0 the reverse reaction is spontaneous therefore products form reactants.

When ∆G = 0 the reaction is at equilibrium

Summary

RATE LAWS

Most chemical reaction rates vary with the concentration of the reactants. The rate law of a reaction is only determined by experiment.

                In first order reactions (k[A]¹), as concentration changes the rate changes in the same way.

                In second order reactions (k[A]²), as concentration changes the change in rate is the square of the concentration change.

                In zero order reactions (k[A]⁰ = k), as concentration changes the rate is constant.

k = a proportionality constant

A = a reactant in a reaction

Example

What is the rate law for the reaction 2NO(g) + 2H₂(g) --> N₂(g) + 2H₂O(g) given the following data?

First, write the rate law for the reaction

Rate = k[NO]ⁿ [H₂]^m

Second, calculate “n”

Find the runs where [H₂] remains constant

Comparing run 1 and 3

Third, calculate “m”

Find the runs where [NO] remains constant

Comparing run 1 and 2

Therefore, the rate law for the reaction is

Rate = k[NO]² [H₂]¹

Problem

The following data was obtained for the reaction 2A(g) + B₂(g) --> 2C(g)

What is the rate law for the reaction?

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