9.2 Flashcards
- 2 The rate of reaction between calcium carbonate and hydrochloric acid - To use appropriate measurements to
- monitor the rate of reaction using mass lost, and gas collection.
- To use appropriate software to process data
You will investigate the rate of the reaction between calcium carbonate and hydrochloric acid. You will do this by carrying out two different methods.
Equation: CaCO3(s) + 2HCI(aq) CaCl2(aq) + CO2(g) + H2O(l)
The calcium carbonate is in the form of fairly large pieces of marble so that the particle size does not change much during the reaction. This ensures that the calcium carbonate has no effect on the rate.
The volume and concentration of the hydrochloric acid used are chosen so that the acid is virtually all used up (the CaCO3 is in excess). We can determine the order with respect to the acid.
Method 1 – Monitoring the rate of reaction by mass loss
In this Part, you will monitor the rate of this reaction by measuring the mass of the reaction mixture during the course of the reaction.
- Choose 6 or 7 lumps of marble weighing about 10 g in total and place them in a 100 cm3 conical flask which has a loose plug of cotton wool in the mouth.
- Measure 20 cm3 of 2.0 mol dm–3 hydrochloric acid into a 25 cm3 measuring cylinder.
- Place the conical flask on the balance pan.
- Pour the acid into the conical flask and replace the cotton wool plug.
- Now record the mass and start the stop-clock (at the same time!)
- Record the mass every 10 seconds in initially and every 30 seconds when the reaction slows down. Continue until the mass remains constant.
Analysis of resultsMethod 1
- Plot a graph of mass against time, choosing a sensible scale so that the graph fills the as much paper as much as possible.
- From your graph, work out three successive half lives, based on the starting mass and the final mass.
- At any time, t, during the experiment, [HCl(aq)] (Mass at time t – Final mass)
What conclusions can you draw about the order with respect to HCl and the rate equation?
Method 2 – Monitoring the rate of reaction by gas collection
In this Part, you will monitor the rate of this reaction by measuring the volume of gas produced during the course of the reaction.
1. Choose 6 or 7 lumps of marble weighing about 10 g in total and place them in a Buchner flask, with a glass syringe and bung fitted.
2. Measure 20 cm3 of 0.500 mol dm–3 hydrochloric acid into a 25 cm3 measuring cylinder.
3. Pour the acid into the Buchner flask.
4. Wait about 10 seconds for the liquid to become saturated with gas.
Then insert the bung in the Buchner flask, record the volume of gas in the syringe and start the stop-clock (all at the same time!)
5. Record the volume of gas in the syringe every 10 seconds in initally and every 30 seconds when the reaction slows down. Continue until the volume remains constant.
Method 2 –Analysis of results
- Plot a graph of volume against time.
- At any time, t, during the experiment, [HCl(aq)] (Final volume – Volume at time t)
What conclusions can you draw about the order with respect to HCl and the rate equation?
- Neither reaction goes to completion
(a) Use the results to both experiments to show that this statement is true.
(a) n(HCl) used = 0.500 20/1000 = 0.01 mol.
From equation 2 mol HCl 1 mol CO2
So n(CO2(g)) from complete reaction = 0.005 mol.
CO2 lost is responsible for both mass loss and volume measured.
A complete reaction would give:
• a total mass loss of 0.005 44.0 = 0.22 g
• a total volume of 0.005 24000 = 120 cm3
(b) Suggest why the experiments do not go to completion in the time available for the experiment.
The concentration of HCl decreases so much that the reaction slows right down, so much that the reaction seems to stop. If the reaction were left longer, it would go to completion.
(c) If the reaction has gone to completion, suggest one essential modification of Method 2.
) The size of the syringe (100 cm3) would need to be increased to hold 120 cm3 of gas.
- For each experiment, draw tangents at t = 0 s and after one half life.
(a) Calculate the gradients of each tangent.
Gradients will depend on results of each experiment.
(b) Compare the gradients and explain any relationship between them.
b) Gradient rate. Gradient after one half life is less than gradient at t = 0 s reflecting the decrease in rate as the HCl concentration decreases during the reaction
note
The volume and concentration of the hydrochloric acid used are chosen so that the acid is virtually all used up (the CaCO3 is in excess). We can determine the order with respect to the acid.The calcium carbonate is in the form of fairly large pieces of marble so that the particle size does not change much during the reaction. This ensures that the calcium carbonate has no effect on the rate.
Marble chips • Marble chips (calcium carbonate), CaCO3(aq).
No current hazard
2 mol dm–3 HCl(aq)
harmful
0.500 mol dm–3 HCl(aq)
Currently no hazard classification at this concentration
Equipment
Each student or group will require: • Balance to 2 decimal places • 100 cm3 conical flask • 25 cm3 measuring cylinder • Cotton wool plug • stopwatch • Buchner flask with appropriate bung • Glass syringe
