5A Rates of Reactions

Explain by reference to Collision theory, how the rate of reaction changes when the concentration of a reagent changes

• the higher the concentration, the faster the rate of reaction

• at higher concentrations there are more reactant particles per unit volume

• So collisions are more frequent

Explain how increasing the temperature increases the rate of reaction

• at a higher temperature more molecules have energy above the activation energy

• Molecules also have higher kinetic energy, so they move faster and frequency of collisions also increase

Write down the rate equation for A+B→C with orders m and n

rate = k[A]m[B]n

Define order of reaction

The power to which the concentration of a reactant is raised in the rate equation

What is the overall order of a reaction

The same of the individual orders of reactants in the rate equation

Sketch the rate-conc graphs for 0, 1st, and 2nd order reactions

What change does a 0 order reactant have on the rate

The rate is independent to the concentration of the reactant

What change does a 1st order reactant have on the rate

Rate is directionally proportional to the concentration of the reactant

What change does a 2nd order reactant have on the rate

Rate is directionally proportional to the squared of the reactant concentration. E.g. if conc x2, rate x4

What does ‘initial rate’ mean

The instantaneous rate at the start of the reaction (i.e. when t=0)

Sketch a conc-time graph for 0, 1st, and 2nd order reactants

What’s the definition of ‘half-life’

Half-life is the time taken for the concentration of a reactant to decrease by half

What happens to the half-life of a 0, 1st, and 2nd order reactant

• 0 order - decreases

• 1st order - stays the same

• 2nd order - increases

How can a colorimeter be used to moniter the concentration of a colourful reactant during a reaction

1. Produce a calibration curve by measuring the absorbance of a series of known concentration solutions and plotting absorbance against conc

2. Measure the absorbance measured by the colorimeter during the reaction

3. Plot a graph

Which equation describes how the value of the rate equation constant (k) changes with temperature

• The Arrhenius Equation

• 𝒌 = 𝑨𝒆−𝑬𝒂/𝑹𝑻 (Present −𝑬𝒂/𝑹𝑻 is in superscript)

How does the value of the rate constant of a catalysed reaction differ from that of the uncatalysed reaction

• K for the catalysed reaction is increased

• This is because the activation energy for the catalysed route is lower, so the 𝒆−𝑬𝒂/𝑹𝑻 part of the Arrhenius equation increases

What is the formula that links the relationship between k and half-life

K = ln(2) / 0.5t

How can the rate of reaction be found from a conc-time graph

• rate = change in conc / change in time

• So rate is equal to the gradient

Give 4 examples of continuous monitoring methods

• colorimeter- measure increase in absorbance of light

• conductivity probe - measure the decrease in conductivity

• gas syringe - measure volume of a gas produced

• balance - measure mass loss

Describe what a ‘clock method’ is

Method where a significant change (like colour change) is suddenly detected after a little delay

Why can we use 1/time to measure initial rate in the ‘clock’ method

• rate = change in conc / change in time

• As we measure the same amount of reactant each time change in conc is constant, so initial rate is proportional to 1/time

Compare the advantages and disadvantages of the continuous monitoring method vs the clock method

Continuous monitoring

• more accurate :)

• more work to monitor several reactions over some time :(

• takes longer to process results :(

Clock method

• quick and simple to carry out :)

• easy to process results :)

• only gives an approximation of the initial rate :(

According to collision theory, what conditions must be satisfied for a reaction to occur

Reactant particles must collide:

• in the correct orientation

• with sufficient energy to overcome the activation energy barrier

What is the ‘rate determining step’

The slowest step in the reaction mechanism of a multi-step reaction