C8: Rates of Reaction

How is the mean rate of a chemical reaction calculated?

• Mean rate of reaction = quantity of reactant used / time taken

• Mean rate of reaction = quantity of product formed / time taken

• Units can be grams per second g/s, centimetres cubed per second cm3/s, or moles per second mol/s.

How do you calculate the rate of reaction at a specific time using a graph?

• Draw a straight tangent line touching the curve at that specific time point.

• Calculate the gradient of the tangent using change in y / change in x.

What does the gradient of a line on a reaction graph tell you about the rate?

• A steeper line indicates a faster rate of reaction.

• A flat horizontal line indicates the reaction has stopped because one of the reactants has been completely used up.

What are three common laboratory methods to measure the rate of a reaction?

• Measuring the volume of gas produced over time using a gas syringe or inverted measuring cylinder.

• Measuring the loss of mass of a reaction mixture over time as gas escapes using a mass balance.

• Measuring the time taken for a solution to change colour or turn cloudy (disappearing cross method).

What is collision theory?

Chemical reactions can only occur when reacting particles collide with each other with sufficient energy equal to or greater than the activation energy.

What is activation energy?

The minimum amount of energy that colliding particles must possess in order to react successfully.

How does increasing concentration affect the rate of reaction?

• Increases the number of reactant particles in a given volume of solution.

• Particles are closer together, so they collide more frequently.

• Increases the frequency of successful collisions per second.

How does increasing pressure of a gas affect the rate of reaction?

• Forces gas particles into a smaller volume.

• Particles are packed closer together, leading to more frequent collisions per second.

• Increases the frequency of successful collisions.

How does increasing surface area to volume ratio affect the rate of reaction?

• Exposes more reactant particles at the surface to collide with other particles.

• Increases the collision frequency.

• Increases the frequency of successful collisions per second.

How does increasing temperature affect the rate of reaction?

• Particles gain kinetic energy and move much faster, increasing the overall collision frequency.

• A significantly larger proportion of colliding particles have energy equal to or greater than the activation energy.

• Results in a higher frequency of successful collisions per second.

Why does a small increase in temperature cause a large increase in reaction rate?

It exponentially increases the fraction of particles that possess energy greater than or equal to the activation energy required for a successful collision.

What is a catalyst?

A substance that increases the rate of a chemical reaction without being used up or chemically changed at the end of the reaction.

How does a catalyst increase the rate of a reaction?

It provides an alternative reaction pathway with a lower activation energy, meaning a higher proportion of collisions have sufficient energy to react successfully.

How does a catalyst affect a reaction profile diagram?

It lowers the activation energy peak curve between reactants and products without changing the overall energy levels of reactants or produ