|
Past-paper
questions 4
Francine
Taylor-Campbell, Contributor
We
will examine some more questions as
part of your examination preparation.
NOTE
- Please
remember to read the questions carefully
and answer them as clearly as possible
so that the examiner can understand
what you are trying to say.
- Writing
a lot and being vague will not guarantee
the marks if what is being asked
is not answered.
- Be
relaxed with calculation questions
by practising the use of formulas
and memorising them.
Question
1
1.
The following results were obtained
by burning methane and using the energy
obtained to heat a known mass of water:
Mass
of water = 250 g
Initial
temperature of water = 26 0C
Final
temperature of water = 36 0C
Mass
of methane = 0.2 g
Use
this information to calculate the
heat of combustion in KJ per mole
of methane. Assume that there are
no heat losses and that the 4.2J of
energy are needed to raise the temperature
of 1g of water by 1 0C.
ANSWERS
Q1.
Heat change for rxn = mass of soln
x heat capacity of soln x temperature
change. This is an equation worth
remembering!
H
= 250g x 4.2 Jg-1C-1 x 10 C = 10.5
KJ
This
energy change was given out from the
burning of 0.2g of methane.
1
mole of methane (CH4) has a mass of
(12 + 4) = 16 g
#
mol in 0.2 g = 0.2/16 = 0.0125 mol
Thus
0.0125 mol give out 10.5 KJ of heat
1
mole = 10.5/0.0125 = 840 KJ/mol
Question
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.
| Time
from start of experiment/s |
Total
volume of hydrogen produced/cm3 |
| 0 |
0 |
| 5 |
31 |
| 10 |
54 |
| 15 |
71 |
| 20 |
85 |
| 25 |
94 |
| 30 |
99 |
| 35 |
100 |
| 40 |
100 |
| 45 |
100 |
| 50 |
100 |
(a)
Calculate how many moles of magnesium
and sulphuric acid were used in this
experiment?
(b)
Explain why the reaction stopped.
(c)
1The 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.
GOOD
LUCK IN YOUR EXAMS AND CONTINUE TO
STUDY HARD!!
Francine
Taylor-Campbell is an independent
contributor. Send questions and comments
to kerry-ann.hepburn@gleanerjm.com
|
