3.2.2 Reaction Rates: Module 3: Periodic Table and Energy: Chemistry OCR A A Level

What is the equation used to calculate rate?

Rate = Change in concentration/Time

What is the unit for rate of reaction?

mol • dm^-3 • s^-1

What must particles do in order to react?

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

Do most collisions result in a reaction?

No.

What are the factors that affect rate of reaction?

● Temperature

● Pressure

● Concentration

● Surface area

● Catalyst

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

● Increasing temperature → Increased rate of reaction

● Much higher proportion of particles have energy

greater than the activation energy → Many more

● Successful collisions per second → Increased rate

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

Increased concentration/pressure → increased rate of reaction

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

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

● Concentration of reactant or product

● Gas volume of products

● Mass of substances formed

How to calculate rate from a concentration time graph?

● Draw a tangent.

● Work out the gradient of the tangent using the equation.

● Gradient = change in y/change in x.

What is a catalyst?

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

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

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 reaction rate.

What does homogeneous catalyst mean?

A catalyst that is in the same phase as the reactants.

Eg. liquid catalyst mixed with liquid reactants

What does heterogeneous catalyst mean?

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

Eg. gaseous reactants passed over solid catalyst.

What are catalytic convertors?

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

Define activation energy.

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

Name some important features of Boltzmann distribution.

● 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 does not touch or cross the energy axis.

● Only the molecules with energy greater than activation energy can react.

What are the axis in a Boltzmann distribution?

X axis - energy

Y axis - number of molecules with a given energy