Kinetics

How to determine reaction rate from a graph (reactant/product with time)

Draw a tangent at that point and calculate gradient

Typical methods to measure the rate of a reaction

• Measure volume of gad produced

• Measure loss of mass

• Measure production of solid

How to measure the volume of gas produced

• Use a gas syringe at timed intervals

• Measure water displacement in an upturned measuring cylinder

Equation to calculate reaction rate

Change in concentration of product/reactant over time

What does collision theory state?

For a reaction to occur, particles must collide with each other with sufficient energy

What is the minimum kinetic energy needed for a reaction to occur called?

Activation energy (E_a)

What is the Maxwell-Boltzmann distribution?

The distribution of energy amongst gaseous molecules at different temperatures.

When temperature increases, what happens to the Maxwell-Boltzmann distribution curve?

It shifts to the right and is lower than before.

What part of the Maxwell-Boltzmann distribution curve always stays the same and what does this represent?

The area under the curve always stays the same and it corresponds to the total number of particles.

Effect of adding a catalyst on the Maxwell-Boltzmann distribution curve

Lowers the required activation energy, resulting in a greater proportion of molecules having energy equal to or more than the required E_a

What does a rate equation show?

How changes in the concentrations of reactants affect the rate of a reaction.

What factor affects a rate constant’s value?

Change in temperature

How can you tell the order of a reactant?

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

What is the overall order of a reaction?

The sum of all the individual orders

Unit for zero order rate constant

mol dm^-3 s^-1

Unit for first order rate constant

s^-1

Unit for second order rate constant

mol^-1 dm³ s^-1

Unit for third order rate constant

mol^-2dm⁶s^-1

Which step of a (multi-step) determines the overall rate of a reaction?

The slowest step

What is the slowest step of a multi-step reaction called?

The rate determining step

What are the two methods to determine orders of reaction

• Continuous

• Initial-rate

How do you carry out the continuous method?

One reaction mixture is made up ad samples of the reaction mixture are withdrawn at regular time intervals and their concentration tested.

The reaction is stopped by quenching (adding a known volume of cold water)

How can the concentration of the samples during the continuous method be measured?

Titration

Steps to determine order of reaction after continuous method

1. Draw a concentration (y) time (x) graph

2. Find out the half life for the reaction at different concentrations

What is the half-life of a reaction?

The time taken for the concentration of the reactant to fall to half of the initial value

What order is a reactant with a concentration-graph with a straight line with a negative gradient?

Zero order

How can you work out the rate constant of a zero order reaction from a conc-time graph?

Calculate the gradient of the line

What order is a reactant with a constant half-life?

First order

What order is a reactant with an increasing half-life?

Second order

How can you calculate the rate of a reaction from a concentration/volume-time graph?

Drawing a tangent to the curve at the given time and calculating the gradient of the tangent.

How to calculate initial rate from a volume-time graph?

Calculate the gradient of a tangent at time = 0.

How do you carry out the initial-rate method?

Several reaction mixtures are made up and the initial rate is measures.

Initial rate is found by measuring the time taken for a small, fixed amount of reactant to be used up/ product to be formed.

From these times, is it possible to calculate the relationship between the rate of the reaction and the concentration of the reactant.

What does a zero order reactant look like on a rate-concentration graph?

Straight, horizontal line

What does a first order reactant look like on a rate-concentration graph?

Straight, diagonal line

What does a second order reactant look like on a rate-concentration graph?

Curved line (plot rate against A² and a straight line should pass through the origin)

Why might the stoichiometry of a reaction equation not match the rate equation?

The reaction proceeds through a series of interconnected reactions that are collectively called the mechanism for the reaction.

Describe the rate of reactions with a large activation energy

Slow but the rate increases rapidly with an increase in temperature

Describe the rate of reactions with a small activation energy

Fast but the rate does not increase as rapidly with an increase in temperature

Describe the values of activation energy for catalysed reactions

Small

Define activation energy

The minimum amount of energy required for reactant particles to collide successfully and start a chemical reaction

Define what is meant by a catalyst

A substance that increases the rate of a chemical reaction without being changed in its chemical composition or amount

What are the two reasons why an increase in temperature increases the rate of reaction?

1. An increase in the fraction of molecules with energy equal to or greater than the activation energy for the reaction

2. An overall increase in the frequency of successful collisions between the reacting molecules

What data is plotted in an Arrhenius graph?

1/T (x)

lnK (y)

Describe the gradient of an Arrhenius graph

Negative

How can you calculate the value of activation energy from an Arrhenius graph?

E_a = R x -gradient