Rate Experiments

Question 1

Define initial rate of reaction

Answer 1

The initial rate of reaction is the rate right at the start of the reaction. You can find this from a concentration-time graph by calculating the gradient of the tangent at time = 0

Question 2

How do you use the initial rates method?

Answer 2

1. Repeat an experiment several times using different initial concentrations of the reactants. You should only change one of the concentrations at a time, keeping the rest constant
2. Calculate the initial rate for each experiment
3. See how the initial concentrations affect the initial rates and figure out the order for each reactant

Question 3

What is continuous monitoring?

Answer 3

Instead of working out the initial rate of several reactions, you can follow a reaction all the way to the end by recording the amount of product (or reactant) you have at regular time intervals

Question 4

Give four ways to follow the rate of reaction

Answer 4

1. Gas volume
2. Loss of mass
3. Colour change
4. Change in pH

Question 5

How can you follow rate of reaction using gas volume?

Answer 5

If a gas is given off, you can collect it in a gas syringe and record how much you’ve got at regular intervals. To find the concentration of a reactant at each time point, use the ideal gas equation to work out how many moles of gas you’ve got, then use the molar ratio to work out the concentration of the reactant

Question 6

How can you follow rate of reaction using loss of mass?

Answer 6

If a gas is given off, the system will lower in mass. You can measure this at regular intervals using a balance. Use mole calculations to work out how much gas you’ve lost, and therefore how many moles of reactants are left

Question 7

How can you follow rate of reaction using colour change?

Answer 7

You can track colour change using a colorimeter. A colorimeter measures absorbance. The more concentrated the colour of the solution, the higher the absorbance is.

1. Plot a calibration curve - a graph of known concentrations of the coloured solution plotted against absorbance
2. During the experiment, take a small sample from your reaction solution at regular intervals and read the absorbance
3. Use your calibration curve to convert the absorbance at each time point into a concentration

Question 8

How can you follow rate of reaction using changes in pH?

Answer 8

If the reaction produces or uses up H+ ions, the pH of the solution will change. So you could measure the pH of the solution at regular intervals and calculate the concentration of H+