What is the rate of reaction?
The change in the amount of reactants or products per unit time.
Describe how you could use a colourimeter to follow the rate of a reaction with one coloured reactant.
- A colourimeter can measure the absorbance of the solution.
- Once the absorbance data is collected, the concentration (of the coloured substance) can be calculated…
- …using a calibration curve.
How can a concentration-time graph be used to work out the rate of a reaction at a particular time?
The rate of a reaction can be worked out by the gradient at a particular time on the graph.
The rate of the acid-catalysed reaction between bromine and methanoic acid was investigated.
Br₂ + HCOOH → 2H⁺ +2Br⁻ +CO₂
Describe one method that could be used to follow the reaction rate.
[2 marks]
- Measure the volume of CO₂(g) produced…
- …using a gas syringe.
OR - Measure how the absorbance/colour of the solution changes over time…
- …using a colourimeter.
OR - Measure the decrease in mass (since CO₂ is given off)…
- …using a mass balance.
If the concentration of a certain reactant was recorded at set intervals over the course of the reaction, outline how the rate of reaction, at any particular time, could be determined.
[3 marks]
- Plot a graph of concentration of the reactant against time.
- Draw a tangent to the curve at a particular time.
- Calculate the gradient of the tangent.
State two assumptions you need to make when carrying out an initial rates experiment.
Any of the two:
- The concentration of the other reactants isn’t changing significantly.
- The temperature stays constant.
- The reaction has not proceeded too far a measurement was taken.
What happens at the end point of a clock reaction?
Clock reactions tend to have an easily observable endpoint, which is usually a colour change.
What does the order of a reaction with respect to a particular reactant tell us?
The order of reaction with respect to that particular reactant tells us how the reactant’s concentration affects the rate.
If the rate of a reaction doubles when you double the concentration of reactant X, what is the order of reaction with respect to reactant X?
It’s the first order with respect to reactant X.
How do you work out the overall order of a reaction from a rate equation?
The sum of the orders of all the different reactants will give you the overall order.
What’s the half-life of a reaction?
The half-life of a reaction is the time it takes for half of the reactant to be used up.
