| Reaction
rates
Francine
Taylor-Campbell, Contributor
 |
| The
Convent of Mercy Alpha Academy Key Club donates clothes and a cheque to the Salvation
Army, located at 53 Lyndhurst Road, St. Andrew, on Friday, December 8, 2006. The
Salvation Army office was broken into and cash and clothes were stolen. - Contributed |
Main points:
A change
in the amount of reactant or product with time describes the reaction rate. Several
factors such as concentration, pressure, temperature, particle size and catalysts
affect reaction rates. The
collision theory states that particles must collide in order to react, and they
must do so with a minimum energy to break the bonds. Most factors work by increasing
the number of collisions in the reaction. Questions:
1.
The apparatus shown in the diagram was used to study the catalytic effect of certain
substances on the reaction between zinc and dilute sulphuric acid. 
Several
experiments were carried out. In each experiment, 1.0g of zinc powder, 50 cm³
of 1.0 mol/dm³
sulphuric acid and 0.1g of a possible catalyst were used. The time taken to collect
50 cm³
of hydrogen was recorded. (see figure A) (i)
Write the equation for the reaction between zinc and dilute sulphuric acid. (ii)
Describe a chemical test for hydrogen. (i)
What are two essential features of a catalyst? (ii)
Which of the added substances behaved as a catalyst? (i)
Explain why it was important in this investigation to carry out an experiment
with just zinc and dilute sulphuric acid. (ii)
Identify one additional factor which must be controlled. Explain
why the time to collect 50 cm 3 of hydrogen was less when using copper powder
rather than copper lumps. | Possible
catalyst added | time
to collect 50 cm³
of hydrogen/s | Other
Observations | | No
added catalyst | 65 | - | | 0.1g
of copper II sulphate | 10 | Brown
solid coated the zinc and a colourless solution seen | | 0.1g
of copper II chloride | 15 | Brown
solid coated the zinc and a colourless solution seen | | 0.1g
of copper powder | 19 | Brown
solid remained | | 0.1g
of copper lumps | 56 | Brown
solid remained | | 0.1g
of sodium chloride | 65 | Colourless
solution formed | (figure
A) |
| Time
from start of experiment/s | Total
volume of hydrogen produced /cm³ | 0 | 0 | 10 | 54
| 15 | 71 | 20 | 85 | 25 | 94 | 30 | 99 | 35 | 100 | 40 | 100 | 45 | 100 | 50 | 100 |
(figure B) |
ANSWERS:
(i)
Zn (s) + H2 SO4 (aq) === ZnSO4
(aq) + H2 (g) (ii)
Hydrogen gives a 'pop' sound when tested with a lighted splint. (i)
A catalyst speeds up a reaction without being chemically changed at the end of
the reaction. (ii)
Copper II sulphate, copper II chloride and copper are behaving as catalysts as
they have reduced the time taken to collect 50 cm³
of hydrogen. To
be able to compare the times of the catalysed and uncatalysed reactions the reactants
zinc and sulphuric had to remain the same. (ii)
Temperature is the additional factor that must be controlled as this affects the
reaction rate. Copper
powder provided a greater surface area than copper lumps. It can, therefore, behave
as a better catalyst. Q2.
0.10g of magnesium ribbon and 25 cm³
of 2.0 mol/dm³
sulphuric acid were mixed and the total volume of hydrogen was measured.
(see
figure B) a)
Calculate how many moles of magnesium and of sulphuric acid were used in this
experiment? b)
Explain why the reaction stopped. c)
The experiment was repeated using the same mass of magnesium, but 25 cm³
of 3.0 mol/dm³
sulphuric acid. How will the initial rate of formation of hydrogen and the total
volume of hydrogen collected compare with the original experiment? d)
Give two other methods of changing the initial rate of reaction. ANSWERS
a)
# mol of Mg = 0.1/24 = 4.17x10 -3 mol #
mol of H 2 SO 4 = (25 x 2)/1000 = 5 x 10 -2 mol b)
Acid is in excess. The reaction ends when all the magnesium has reacted. c)
If 3.0 mol/dm³
sulphuric acid is used the initial rate of formation of hydrogen will increase.
As concentration increases the reaction becomes faster. The
total volume of hydrogen will remain the same, as the mass of magnesium is the
same. Increasing
the temperature and using powdered magnesium can change the initial rate of production
of hydrogen. Francine
Taylor-Campbell is an independent contributor. | 
