| Energy
changes
Francine
Taylor-Campbell, Contributor
 |
| Children
enjoy the PALS Peace Concert at the Urban Development Corporation car park, in
downtown Kingston, on Tuesday, March 6. - Colin Hamilton/Freelance Photographer |
Main points
All
reactions involve energy changes - energy is absorbed when bonds are broken and
released when bonds are formed. When
energy is released in a reaction, it is said to be exothermic and 
H is negative. When
energy is absorbed in a reaction, it is said to be endothermic and
H is positive. The
heat content of a substance is called its enthalpy (H). The change in enthalpy
or H is the difference between the energy content of the products and the energy
content of the reactants.
H 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. Q1.
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 1.0 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 1
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. Answers
Q1.
Heat change = mass of solution x heat capacity of solution x change in temperature.
H = (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,
H = 4.5864 KJ For
1mol H2O2, H = 4.5864/0.05
= 91.73 KJ/mol (c)
Since heat is given out, this suggests that it is an exothermic process. 
Rxn
pathway (d)
The catalyst would affect the reaction by lowering the activation energy allowing
more to achieve this minimum energy and hence increasing the reaction rate. 
Q2.
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
Q2.
Heat change for rxn = mass of soln x heat capacity of soln x temperature change
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 Francine
Taylor-Campbell is an independent contributor. | 
