3.2.2 - Reaction Rates

Question 1

What is the equation sued to calculate rate?

Answer 1

Rate = change in concentration / time

Question 2

What is the unit for rate of reaction?

Answer 2

mol dm-3 s-1

Question 3

What must particles do in order to react?

Answer 3

Collide with sufficient energy (activation energy) and the correct orientation

Question 4

Do most collisions result in a reaction?

Answer 4

No

Question 5

What are the factors that affect rate of reaction

Answer 5

Temperature
Pressure
Concentration
Surface area
Catalyst

Question 6

What is the effect of increasing temperature on rate of reaction? Why?

Answer 6

Increasing the temp, increases the rate of reaction

Much higher proportion of particles have energy greater than the activation energy, so many more successful collisions per second, leading to an increased rate

Question 7

What is the effect of increasing concentration / pressure on rate of reaction and why?

Answer 7

Increased concentration / pressure = increased rate of reaction

There are more particles in a given volume, more frequent successful collisions, increased rate

Question 8

What are the variables in an experiment that can be monitored to calculate the rate of reaction?

Answer 8

Concentration of reactant or product
Gas volume of products
Mass of substances formed

Question 9

How to calculate rate from a concentration time graph?

Answer 9

Draw a tangent

Work out the gradient of the tangent using the equation

Gradient = change in y / change in x

Question 10

What is a catalyst?

Answer 10

A substance which increases the rate of reaction but is not used up in the reaction

Question 11

How do catalysts work and how do they increase the rate of reaction?

Answer 11

They provide an alternate reaction pathway (with a lower activation energy)

Due to lower activation energy, more particles have energy > activation energy so more frequent successful collisions, so increased rate of reaction

Question 12

What does homogenous catalyst mean?

Answer 12

A catalyst that is in the same phase as the reactants

e.g. liquid catalyst mixed with liquid reactants

Question 13

What does heterogenous catalyst mean?

Answer 13

Catalyst used in the reaction is in different phase to the reactants

e.g. gaseous reactants passed over solid catalyst

Question 14

What are catalytic convertors?

Answer 14

They are present in vehicles to reduce toxic emission and prevent photochemical smog

Question 15

Define activation energy

Answer 15

The minimum energy that particles must collide with for a reaction to occur

Question 16

Name some important features of Boltzmann distribution (5)

Answer 16

Area under the curve = total number of molecules

Area under the curve does not change when conditions alter

The curve starts at the origin

Curve down not touch or cross the energy axis

Only molecules with energy greater than activation energy can react

Question 17

What are the axis in a Boltzmann distribution?

Answer 17

X axis - energy

Y axis - number of molecules with a given energy

Question 18

Draw a labelled Boltzmann curve with labels of average energy, activation energy and most probable energy. Draw in a different colour the effect of increasing temperature

Answer 18

T1 < T2

Question 19

Draw a labelled Boltzmann Curve showing the effect of a catalyst on rate of reaction

Answer 19

Actvation energy with a catalyst
A greater proportion of molecules exceeds the lower activation energy. Reaction rate increases

Activation energy without a catalyst
Proportion of molecules exceeding the activation energy without a catalyst is less