Measuring half-life

In this experiment we look at the kinetics of the following reaction:

CaCO₃(s) + 2HCl(aq) ⇒ CaCl₂(aq) + CO₂(g) + H₂O(l)

The calcium carbonate is a solid, and so cannot change in concentration. So in this reaction the hydrochloric acid is the only reactant which can change in concentration and affect the rate of reaction. From your reaction rate theory you should know that we need to plot a graph of concentration of hydrochloric acid against time. From the shape of this graph we can deduce whether the reaction is zero, first or second order with respect to the hydrochloric acid.

We can use two methods to measure the concentration of the hydrochloric acid as the reaction proceeds. They both rely on the fact that this reaction produces a gas which will be lost from the reaction mixture. We can either measure the volume of this gas in a gas syringe - the faster the increase in volume, the faster the hydrochloric acid is being lost, so the faster the reaction rate is. Alternatively, we can put the whole mixture on a top pan balance and measure the rate at which the mass goes down as the gas escapes.

Procedure 1 - mass loss

In the first experiment we place the reaction mixture on a tared top pan balance and measure the mass loss every 30 seconds. You may do this experiment yourself by data logging. This is where the balance is connected to a computer and suitable software can automatically draw your graph. For this DVD you will be doing the process manually. Watch the video and note down the mass loss every 30 seconds. Also make a note of the final mass loss which was measured after around 40 minutes.It may look as if the mass is going up as the experiment proceeds, but don't worry - you can't see the minus sign in front of the mass! Plot a graph of m_final - m_t on the vertical axis against time on the horizontal access. m_final - m_t is a measure of the concentration of hydrochloric acid - if you cannot see why, check here. From the shape of your graph deduce the order of this reaction with respect to the hydrochloric acid.

Video - procedure 1 (rate by mass loss)

Results - procedure 1

Procedure 2 - gas syringe

Note the times at 10, 20, 30, 40, 50, 60 and 70 cm³, and the final volume of gas. Record your results in the form of a table like that on the student instruction sheet. Add the time for 80 cm³ which was 2 minutes and 17 seconds. Plot a graph of (V_final - V_t) against t, and try to work out the order of reaction with respect to the hydrochloric acid. You can check your answer by clicking on the link below.

Video - procedure 2 (rate by gas syringe)

Results - procedure 2