Lumps against powder
What the calculation showed can be confirmed by experiment. In activity 17.1 exactly one thing is changed — whether the limestone goes in as lumps or as powder.
Activity 17.1
You will need: equal masses of calcium carbonate (CaCO3) powder and lumps, dilute hydrochloric (HCl) acid, a stopwatch, two beakers.
- Put equal volumes of HCl acid into the two beakers.
- Drop the CaCO3 lumps into one beaker and time with the stopwatch how long they take to disappear.
- Add the CaCO3 powder to the other beaker and time how long it takes to disappear in the same way.
- Compare the two times.
Observations
What is seen
In the beaker holding the CaCO3 powder the gas bubbles come off rapidly. The CaCO3 powder disappears quickly. The reaction, in other words, is observably over in a shorter time.
What we used here was method ii — measuring the time taken for a fixed amount of reactant to be used up.
rate of reaction = amount of reactant used up ÷ time taken
The two numerators are equal, since equal masses were used. So the case with the smaller denominator — the powder — gives the greater rate.