CHAPTER 3. REACTION RATES 3.4
3.4 Measuring Rates of Reaction
ESCAT
How the rate of a reaction is measured will depend on what the reaction is, and what
product forms. Look back to the reactions thathave been discussed sofar. In each
case, how was the rate of the reaction measured?The following exampleswill give you
some ideas about otherways to measure the rateof a reaction:- Reactions that producehydrogen gas:
When a metal dissolvesin an acid, hydrogen gasis produced. A lit splintcan be
used to test for hydrogen. The ’pop’ sound shows that hydrogen is present. For
example, magnesium reacts with sulphuric acidto produce magnesiumsulphate
and hydrogen.
Mg(s) + H 2 SO 4 → MgSO 4 + H 2- Reactions that producecarbon dioxide:
When a carbonate dissolves in an acid, carbondioxide gas is produced. When
carbon dioxide is passesthrough limewater, it turns the limewater milky.This is
a simple test for the presence of carbon dioxide.For example, calcium carbonate
reacts with hydrochloric acid to produce calcium chloride, water andcarbon
dioxide.
CaCO 3 (s) + 2HCl(aq)→ CaCl 2 (aq) + H 2 O(l) + CO 2 (g)- Reactions that producegases such as oxygen orcarbon dioxide:
Hydrogen peroxide decomposes to produce oxygen. The volume of oxygen pro-
duced can be measuredusing the gas syringe method (figure 3.2). Thegas col-
lects in the syringe, pushing out against the plunger. The volume of gasthat has
been produced can be read from the markings on the syringe. For example, hy-
drogen peroxide decomposes in the presence of a manganese(IV) oxidecatalyst
to produce oxygen andwater.
2H 2 O 2 (aq)→ 2H 2 O(l) + O 2 (g)- Precipitate reactions:
In reactions where a precipitate is formed, the amountof precipitate formed in
a period of time can beused as a measure of the reaction rate. For example,
when sodium thiosulphate reacts with an acid,a yellow precipitate of sulphur is
formed. The reaction isas follows:
Na 2 S 2 O 3 (aq) + 2HCl(aq)→ 2NaCl(aq) + SO 2 (aq) + H 2 O(l) + S(s)One way to estimate the rate of this reaction isto carry out the investigation in
a conical flask and to place a piece of paper with a black cross underneath the
bottom of the flask. Atthe beginning of the reaction, the cross will be clearly
visible when you look into the flask (figure 3.3). However, as the reaction pro-
gresses and more precipitate is formed, the crosswill gradually become less clear
and will eventually disappear altogether. Noting the time that it takes for this to
happen will give an ideaof the reaction rate. Note that it is not possible tocollect
the SO 2 gas that is produced in the reaction, because it is very soluble in water.