1. Studying Rates
C2.2 Rates of Reaction
09 November 2018

2. Starter
Complete the start exam question on measuring the rate of a chemical reaction.

3. How can we measure rates?
There are six main ways of measuring rates of reaction.
These often depend on the state of the reactants and products in the reaction.

4. Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g)
Change in gas volume
This method is used when one of the products of the reaction is a gas.
A gas syringe is the preferred method for colleting the gas produced as it is more precise than an inverted measuring cylinder/counting bubbles
An example of a reaction where this could be used is:
Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g)

5. Change in gas pressure
This is the method used where the reactants and products are gases.
These reactions usually involve a change in the number of moles of the gas so change in pressure can be measured using a manometer.
A typical reaction of this type is phosphorus pentachloride decomposing.
PCl5(g)  PCl3(g) + Cl2(g)

6. CaCO3(s) + 2HCl(aq)  CaCl2(aq) + H2O(l) + CO2(g)
Change in mass
If a reaction causes a gas to form and this gas is allowed to escape, the reaction mass will change.
The change in mass can be measured at various intervals, thus indicating the rate of the reaction as it progresses.
A typical example of this would be calcium carbonate reacting with hydrochloric acid.
CaCO3(s) + 2HCl(aq)  CaCl2(aq) + H2O(l) + CO2(g)

7. AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)
Rate of precipitation
Measuring the amount of precipitate (insoluble solid) that is formed over time.
AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)
Start
t = 10secs
t = 20secs
As the reaction proceeds more precipitate is formed and the solution becomes cloudier until the cross cannot be seen.
t = 30secs
t = 40secs
t = 50secs

8. CH3COCH3(aq) + I2(aq)  CH3COCH2I(aq) + HI(aq)
Colorimetry
Over the course of some reactions, they change colour.
This change in colour can be measured using something called a colorimeter which measures the light intensity which can be passed through a sample of the reaction mixture.
As the colour changes the light intensity will change and this will indicate the rate of the reaction.
An example of a reaction where this could be used is:
CH3COCH3(aq) + I2(aq)  CH3COCH2I(aq) + HI(aq)

9. Sampling & Titration
In this instance, the reaction mixture would be sampled at various intervals and the concentration of one of the reactants or products calculated via titration.
Usually the reaction is slowed down (by cooling etc.) so that readings can be taken easily.
(This method is only used as a last resort because it is very inefficient)

10. Knowledge Check
Describe a method for measuring the rate of the following reactions. You should also include what you would expect to observed and how you could make the investigation a fair test. 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) AgNO3(aq) + NaBr(aq) → AgBr(s) + NaNO3(aq) CuCO3(s) → CuO(s) + CO2(g) Ca(s) + O2(g) → CaO(s) CuO(s) + 2HCl(aq) → CuCl2(aq) + H2O(l)

11. Investigation of rates by gas collection
Follow the method laid out in the booklet to complete the practical.

12. Investigation of rates by colour change (Iodine Clock)
Follow the method laid out in the booklet to complete the practical.