Rate of Reaction

Explain the term rate of reaction

The change in concentration of a reactant or product per unit time.[cite: 21]

State and explain how the rate of reaction changes if the pressure is increased for N2 + 3H2 ⇌ 2NH3

Increased rate of reaction; increased [N2] and [H2]; increased frequency of collisions.[cite: 21]

State and explain how the rate of reaction changes if the concentration of HCl is increased for Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g)

Increased rate of reaction; increased [H+]; increased frequency of collisions.[cite: 21]

Explain how the observations differ when 25.0 cm³ of 0.5 mol dm⁻³ ethanoic acid is used instead of HCl with excess Mg

HCl is a strong acid whereas ethanoic acid is a weak acid; [H+] is lower for ethanoic acid; less frequent collisions / lower rate of reaction; solid dissolves more slowly / less vigorous effervescence.[cite: 21]

Using a Boltzmann Distribution describe and explain the effect of increasing temperature on the rate of reaction

Rate of reaction increases because a greater proportion of particles have energy greater than activation energy (Ea).[cite: 21]

Describe an experiment to measure the rate of reaction of hydrochloric acid with limestone chips (CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g))

Use gas syringe to measure volume of gas collected at set intervals of time; plot graph of [HCl] against time; gradient of tangent to graph at time t gives rate of reaction at time t.[cite: 29]

Explain the term rate equation

For mA + nB -> products: rate = k[A]^m [B]^n.[cite: 29]

Explain the term rate determining step

The slowest step in the reaction mechanism of a multi-step reaction.[cite: 29]

Explain the term order of reactant

The order with respect to a reactant is the power to which the concentration of the reactant is raised in the rate equation.[cite: 29]

Explain the term total order

The sum of all orders for all species in the rate equation.[cite: 29]

Explain why a large excess of methanol is used to determine the order of reaction with respect to ethanoic acid (CH3COOH + CH3OH ⇌ CH3COOCH3 + H2O)

To keep [CH3OH] constant so that reaction is zero order with respect to CH3OH (allows position of equilibrium to move far to the right).[cite: 29]

Explain the term rate constant, k

The constant that links the rate of reaction with the concentrations of the reactants raised to the powers of their orders in the rate equation.[cite: 28]

Explain the term half-life

The time taken for the concentration of the reactant to reduce by half.[cite: 28]

Explain from the overall equation and rate equation why NO2(g) + CO(g) -> NO(g) + CO2(g) (rate = k[NO2]²) must proceed via a multi-step reaction mechanism

NO2 has power 2 in the rate equation but a stoichiometry of 1 in the overall equation.[cite: 28]