Chapter 18 - Rates of Reactions

Equation to calculate rate

Rate = Change in concentration / Time

Units for rate are usually mol dm^-3 s^-1

What does the square bracket around a reactant mean

This means ‘concentration of reactant’

What does the power next to the concentration mean

This shows the order of reaction for that reactant. The orders are commonly zero, first and second order

Describe zero order

Concentration of reactant has no effect on the rate

Describe first order

If concentration is doubled then the rate increases by a factor of 2

If concentration is tripled then the rate increases by a factor of 3

Describe second order

If concentration is doubled the rate increases by a factor of 4

If concentration is tripled the rate increases by a factor of 9

What is the general rate equation and how do you find the overall order

Rate = k [A] ^m [B] ^n

Overall order = m + n

How do you find units of rate constant k

Rearrange the equation to make k the subject

Substitute units into the expression for k

Cancel common units and show the final units on a single line

Describe what can be deduced from a concentration-time graph

• Gradient of concentration-time graph is rate of reaction

• Zero order reaction - straight line with negative gradient, rate does not change during course of reaction, value of gradient is equal to rate constant k

• First order reaction - downward curve with decreasing gradient, as reaction slows down gradient decreases, time for concentration of reactant to halve is constant (half-life), rate constant can be determined using the half-life value

• Second order reaction - downward curve, steeper at start but trails off more slowly

Define half-life

Half-life is the time taken for half of the reactant to be used up

How can you determine a first order relationship from a concentration-time graph

First order reactions have a constant half-life with concentration halving every half life

This is exponential decay

Measure each successive half-life, if they’re the same, reaction is first order

Equation of rate constant from half-life

k = ln2 / t(1/2)

What shape graph does a zero order reactant produce

Horizontal straight-line with zero gradient

Y-intercept gives the rate constant

Reaction rate does not change with increasing concentration

What shape graph does a first order reactant produce

Straight-line graph through the origin

Rate directly proportional to concentration

Rate is the gradient of the straight line

What shape graph does a second order reactant produce

Upwards curve with increasing gradient

Rate constant can’t be obtained directly from this graph

First you must plot a second graph of rate against concentration squared, the gradient of this straight line is the rate constant

How do you find the initial rate from a concentration-time graph

Initial rate is found by measuring the gradient of a tangent drawn at t=0

What is a clock reaction

This is a way of obtaining initial rate by taking a single measurement

Time from the start of an experiment is measured until a visual change is observed, often a colour or precipitate

Describe the iodine clock reaction

This is where the time taken from the start of the reaction to the point at which the iodine blue-black colour appears is recorded

Describe a multi-step reaction

Some reactions will require many particles to collide simultaneously

These reactions are quite unlikely therefore it is more likely that these reactions will take place in a series of steps

The series of steps that make up an overall reaction is called the reaction mechanism

Define the rate-determining step

The steps in a multi-step reaction will take place at different rates

The slowest step in the sequence is the rate determining step

How can you predict a reaction mechanism and ensure it is correct

Mechanisms can be predicted using knowledge and understanding of chemical principles

How can we know whether a reaction mechanism is correct?

• The rate equation only includes reacting species involved in the rate determining step

• The orders of the rate equation match the number of species involved in the rate determining step

What information can be found from the hydrolysis of haloalkanes

• Overall order of reaction

• Rate equation

• Possible mechanism for reaction

What factors affect the rate constant

• Temperature - Increased temperature increases particle speed so they collide more frequently

• Orientation of particles during collision - Particles will only react if they collide with the correct orientation

Arrhenius equation

Exponential factor - represents the proportion of molecules that exceed activation energy and have sufficient energy for a reaction to take place

Pre-exponential term - takes into account the frequency of collisions with correct orientation, constant over a small range of temperatures, essentially gives the rate if there were no activation energy

What is the logarithmic form of the Arrhenius equation

How do you find activation energy and pre exponential factor (A) from a graph of ln k against 1/T

Gradient = Activation energy / R , therefore, Activation energy = Gradient / R

Intercept = ln A , therefore, A = Inverse ln of intercept