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Reaction
rates
Francine
Taylor-Campbell, Contributor
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 the minimum
energy to break the bonds. Most
factors work by increasing the number
of collisions in the reaction.
We
will now examine a few 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 table below)
|
Possible
Catalyst added
|
Time
to collect 50 cm3
of hydrogen/s
|
Other
Observations
|
No
added catalyst
0.1g of copper II sulphate |
65
10 |
Brown
solid coated the zincand a colourless
soln seen |
0.1g
of copper II chloride
|
15 |
Brown
solid coated the zincand a colourless
soln 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 |
(a)
(i) Write the equation for the reaction
between zinc and dilute sulphuric
acid.
(ii)
Describe a chemical test for hydrogen.
(b)
(i) What are two essential features
of
a catalyst?
(ii)
Which of the added substances behaved
as a catalyst?
(c)
(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.
(d)
Explain why the time to collect 50
cm3 of hydrogen was less
when using copper powder rather than
copper lumps.
ANSWERS
(a)
(i) Zn (s) + H2SO4
(aq) === ZnSO4 (aq) + H2
(g)
(ii)
Hydrogen gives a 'pop' sound when
tested with a lighted splint.
(b)
(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.
(c)
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.
(d)
Copper powder provided a greater surface
area than copper lumps. It can, therefore,
behave as a better catalyst.
2.
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.
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 H2SO4
= (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.
d)
Increasing the temperature and using
powdered magnesium can change the
initial rate of production of hydrogen.
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This
Kingston College student enjoys
praising God during Wolmer's
Girls' ISCF concert in the school's
auditorium on April 4.
- Anthony Minott/Freelance
Photographer
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Francine
Taylor-Campbell is an independent
contributor.
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