Everything Science Grade 12

3.5 CHAPTER 3. REACTION RATES

Beaker Solution A (cm^3 ) Water (cm^3 ) Solution B (cm^3 ) Time (s)

1 60 20 10

2 50 30 10

3 40 40 10

4 30 50 10

5 20 60 10

6 10 70 10

The equation for the reaction between sodium thiosulphate and hydrochlo-

ric acid is:

Na 2 S 2 O 3 (aq) + 2HCl(aq)→ 2NaCl(aq) + SO 2 (aq) + H 2 O(l) + S(s)

Results:

• Calculate the reactionrate in each beaker. This can be done using the
  following equation:

Rate of reaction =

1

time

• Represent your results on a graph. Concentration will be on the x-axis and
  reaction rate on the y-axis. Note thatthe original volume of Na 2 S 2 O 3 can
  be used as a measure ofconcentration.


• Why was it important to keep the volume of HCl constant?


• Describe the relationship between concentration and reaction rate.
  Conclusions:

The rate of the reactionis fastest when the concentration of the reactants

was the highest.

3.5 Mechanism of reaction and catalysis

ESCAU

Earlier it was mentionedthat it is the collision of particles that causes reactions to occur

and that only some ofthese collisions are ’successful’. This is because the reactant

particles have a wide range of kinetic energy, and only a small fraction of the particles

will have enough energy to actually break bonds so that a chemical reaction can take

place. The minimum energy that is needed for a reaction to take place is called the

activation energy. For more information on the energy of reactions, refer to Grade 11.

DEFINITION: Activation energy

The energy that is needed to break the bonds inreactant molecules so

that a chemical reactioncan proceed.