5.1. Rates, Equilibrium and pH: Module 5: Physical Chemistry & Transition Elements: Chemistry OCR A A Level

Rates and Rate Equations

The rate of a reaction shows how fast reactants are converted into products. It depends on the concentrations of the reactants and the rate constant.

Rate Constant (k)

The rate constant for a reaction is constant when the reaction temperature is constant. The rate constant relates the concentrations of the species that affect the rate of a reaction to the overall rate of reaction. The rate constant, k, can be calculated by rearranging the rate equation for that reaction.

Initial Rates

Using the initial rate of reactions is one way the order of a reaction can be determined. This involves varying the concentrations of reactants and measuring the initial rate of the reaction.

Zero Order

No change to the initial rate.

First Order

Initial rate doubles.

Second Order

Initial rate quadruples (2^2).

Mass Change

If a gas is produced by a reaction, then the mass of the reaction mixture will decrease as the reaction proceeds. Plotting a mass-time graph and drawing a tangent to the curve can be used to find the rate of reaction.

Volume of Gas Evolved

If a gas is produced by a reaction, the rate of reaction can be found by measuring the volume of gas produced over the course of the reaction, and plotting a graph of volume evolved against time. A gas syringe or an underwater upside down measuring cylinder can be used to collect the gas.

Titration

Small samples of a reaction mixture can be removed at regular intervals throughout a reaction. These samples can then be titrated to determine the concentration of a given reactant or product at that time. A concentration-time graph can then be plotted.

Colorimetry

Colorimetry can be used to determine the rate of reaction for a reaction that involves the formation or depletion of a coloured species. A colorimeter is a device that measures the amount of light that is absorbed by a solution. The amount of light absorbed by the solution is proportional to the concentration of the coloured species.

Mole Fraction

A mole fraction shows the proportion that a molecule accounts for of the total moles present. It is calculated by dividing moles of substance A by the total moles present.

K_c

K_c is the equilibrium constant of a reversible reaction. Kc is equal to the concentration of the products divided by the concentration of the reactants at equilibrium. The concentration terms are raised to a power of the same value as the number of moles of that substance.

Gaseous Equilibrium Constant (Kp)

Kp is the equilibrium constant used for gaseous equilibria. Kp is calculated from gaseous reactants and products. If all reactants and products are in the gaseous state, the system is said to be homogeneous.

Brønsted-Lowry Acid

A Brønsted-Lowry acid is a proton donor.

For example, ammonium ions (NH4+).

Brønsted-Lowry Base

A Brønsted-Lowry base is a proton acceptor.

For example, hydroxide ions (OH-).

Conjugate Acid

A conjugate acid is the species formed when a base accepts a proton.

Conjugate Base

A conjugate base is the species formed when an acid donates a proton.

Spectator Ions

Spectator ions are ions that do not change in the reaction and are left out of the ionic equation.

pH

pH is a measure of acidity and alkalinity.

Diluting Acids

If you dilute a strong acid 10 times its pH will increase by one unit, because pH is a logarithmic scale. Diluting it 100 times and 1000 times would, therefore, increase the pH by two units and three units, respectively.

Two types of Buffer Calculations.

1. Acid + Base

- Find the number of moles of each species.

- Calculate their concentration when at equilibrium using the total volume.

- Use Ka to find [H+] and therefore pH.

2. Acid + Salt

- Find the moles of the salt.

- Use Ka to find pH.