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Energy
changes
Francine
Taylor-Campbell, Contributor
Main
points
- All
reactions involve energy changes.
Energy is absorbed when bonds are
broken and it is released when they
are formed.
- When
energy is released in a reaction
it is said to be exothermic and
DH is negative.
- When
energy is absorbed in a reaction
it is said to be endothermic and
DH is positive.
- The
heat content of a substance is called
its enthalpy (H). The change in
enthalpy or DH is the difference
between the energy content of the
products and the energy content
of the reactants. DH rxn = H products
- H reactants.
- For
a reaction to take place, reactants
must achieve a minimum energy in
order for products to form. This
is called the activation energy
or energy barrier.
- Heat
of neutralisation refers to the
energy change when one mole of water
is formed from the reaction between
an acid and a base.
- Heat
of combustion refers to the energy
change that occurs when one mole
of a substance is burnt completely
in oxygen.
- Heat
of solution is the energy change
that occurs when one mole of a solute
dissolves in a given volume of solvent
until there is no further heat change.
Question
1.
Hydrogen peroxide slowly decomposes
on standing according to the following
equation:
2H2O2
=== 2H2O + O2.
The reaction is catalysed in the presence
of Fe3+ ions. 50 cm3
of a solution of 1.0M hydrogen peroxide
and 10 cm3 of Fe3+
aqueous solution was used.
(a)
The following temperatures were obtained
when the reaction was carried out.
Initial
temperate of soln = 22.5 0c
Highest
temperate obtained upon addition of
catalyst = 40.7 0c.
Calculate
the heat change for the reaction (assume
that the heat capacity of the solution
is 4.2 Jg-1C-1).
(b)
Using your answer in (a), calculate
the heat change for the decomposition
of one mole of hydrogen peroxide.
(c)
Is the decomposition of hydrogen peroxide
an exothermic or endothermic process?
Illustrate by means of a fully labelled
energy profile diagram.
(d)
Would the activation energy for the
reaction be affected by the presence
of the catalyst? Illustrate your answer
on the diagram you have drawn. Label
the catalysed reaction.
Answer
1.
Heat change = mass of solution x heat
capacity of solution x change in temperature.
DH
= (60 g x 4.2 Jg-1C-1 x 18.2 C) =
4586.4J or 4.586 KJ.
(b)
50 cm3 of 1.0 mol dm-3
hydrogen peroxide was used.
1
mol 0f H2O2
is in 1000 cm3 of soln
X
mol is in 50 cm3; X mol
= 50/1000 = 0.05mol
So
for 0.05 mol H2O2,
DH = 4.5864 KJ
For
1mol H2O2, DH = 4.5864/0.05 = 91.73
KJ/mol
(c)
Since heat is given out, this suggests
that it is an exothermic process.
(d)
The catalyst would affect the reaction
by lowering the activation energy,
allowing more to achieve this minimum
energy and hence increase in the reaction
rate.
Question
2.
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 4.2J of energy
is needed to raise the temperature
of 1g of water by 1 0C.
Answer
2.
Heat change for rxn = mass of soln
x heat capacity of soln x temperature
change
- H
= 250g x 4.2 Jg-1C-1 x 10C
= 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
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A
sixth-form student of Wolmer's
Girls' (standing) supports her
friend during a Jamaica Amateur
Softball Association High School's
senior league match against
St Hugh's High at the Wolmer's
playing field on Friday, April
4. Wolmer's won 13-11.
- Anthony Minott/Freelance
Photographer
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Francine
Taylor-Campbell is an independent
contributor.
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