Topic 6a - Rate of Reaction

What 3 things must particles do in order to react

• Collide

• Must have at least the minimum required energy to react (activation energy)

• Collide with the correct orientation

What happens in this reaction progress graph

1) Initial rate, fast, lots of reacting, particles = lots of collisions

2) Rate slows, successful collisions occur less frequently

3) Rate = 0, one or more of the reactants have been used up

4 factors that affect the rate of reaction

• Concentration/pressure

• Temperature

• Surface area

• Catalyst

How does concentration affect the rate of reaction

• Increased concentration means more particles in the same space therefore more collisions

• Therefore more frequent successful collisions

• Therefore the rate of reaction increases

What happens to the rate of reaction when the concentration is doubled

If we double the concentration, we double the number of collisions and therefore the rate of reaction doubles

How does temperature affect the rate of reaction

• Increase temp = increase rate of reaction

• Particles have more kinetic energy so collide more frequently (and with more energy)

• More particles have at least the minimum required energy to react

• Increased frequency of successful collisions

How does surface area affect the rate of reaction

• Increasing SA provides particles with more space to collide

• Increases number of collisions

• Increases frequency of successful collisions

• Rate increases

Using collision theory, what happens to the rate of reaction when the volume the reaction takes place in is increased

• Increasing the volume decreases the pressure

• This causes particles to collide less frequently (as they are further apart)

• Therefore less frequent successful collisions

• Rate of reaction decreases

What is a catalyst

A catalyst is a chemical that speeds up a reaction by offering an alternate reaction pathway with a lower activation energy requirement without being changed or used up in the reaction

How does catalysts affect the rate of reaction

• Lower the minimum energy required to react

• More particles now have at least minimum required energy

• Increased frequency of successful collisions

• Rate increases

A reaction profile for a catalysed reaction

Mean rate of reaction equation

• The quantity of reactant or product can be measured by the mass in grams or by a volume in cm³

• The units of rate of reaction may be given as g/s or cm³ /s

• The amount of product or reactant can be given in moles, so the units of rate could also be mol/s

Why is mean rate called mean rate

It’s the average

How to find rate of reaction from a graph:

Calculate the mean rate

The mean rate after 20 seconds is:

15/20 = 0.75

The mean rate after 40 seconds is:

19/40 = 0.48

The mean rate between two times eg 20 & 40 seconds is:

Volume at 40 seconds - Volume at 20 seconds/ difference in time

19-15/40-20 = 4/20 = 0.2

Calculate rate of reaction from a tangent

How to do it:

• Draw a straight line that touches the curve at the point you are asked about

• The middle of your straight line must touch the curve

• Draw a triangle

• Gradient = change in y / change in x

For example for this graph:

= (2.2 - 1.3) / (80-40)

= 0.90 / 40 = 0.023

Why the control variable had to be controlled in the experiments

So its accurate and so you know what causes the changes

Example questions for experiments -

Describe what the results tell us how changing the concentration affects the rate of reaction

When the rate of concentration increases the rate of reaction increases too. When the rate is … the concentration is … and when the rate is … the concentration is …