Enthalpy Worksheet
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1
2Chemistry II
Enthalpy Worksheet
Name _____________________
As we have studied heat capacity and specific heat capacity, we’ve learned how heat can
be exchanged between objects. Heat can also be exchanged during a chemical reaction. In an
exothermic chemical reaction, heat is a product; while in an endothermic chemical reaction, heat
is a reactant. The heat of a reaction, which is the change in energy of the products and reactants,
is referred to as Enthalpy. For example, in the reaction of calcium oxide and water heat is
released from the reaction and calcium hydroxide is formed, as shown in the equation below:
O(l) → Ca(OH)
CaO(s) + H
(s) + 65.2kJ
2
2
In this reaction, the release of energy indicates that the total energy of the reactants is greater than
the energy of the product. The amount of heat that is released is generally referred to as the
change in enthalpy (ΔH) between the reactants and products. ΔH is equal to H
– H
.
products
reactants
Because enthalpy (H) is, in a sense, a measure of the internal energy of a substance, it is
impossible to calculate, but we can use changes in heat to measure changes in H. We when refer
to the change in enthalpy, the chemical reaction can be written as follows:
O(l) → Ca(OH)
ΔH = −65.2kJ
CaO(s) + H
(s)
2
2
Notice that the change in enthalpy is negative, which means that heat from the reactants (system)
was given off to the surroundings. This is an exothermic reaction, which is indicated by a
negative change in enthalpy.
There are two ways for us to use enthalpy – we can use it to predict what ΔH will be for a
given reaction, and, once we know that, we can use it to predict ΔH depending on the amounts of
reactants (this is where stoichiometry re-enters the picture ). Chemistry problems involving
enthalpy changes are similar to stoichiometry problems. The amount of heat released or absorbed
during a reaction depends on the number of moles of the reactants involved. The reaction of a
mole of CaO with water released 65.2 kJ of heat, so the reaction of 2 moles of CaO with water
would release twice as much heat, or 130.4 kJ
P
P
:
RACTICE
ROBLEMS
1) The combustion of methane, CH
, releases 890.4 kJ/mol. That is, when one mole of methane
4
is burned, 890.4 kJ are given off to the surroundings. This means that the products have
890.4 kJ less than the reactants. Thus, ΔH for the reaction = - 890.4 kJ. A negative symbol for
Δ H indicates an exothermic reaction.
(g) → CO
O (l); ΔH = - 890.4 kJ
CH
(g) + 2 O
(g) + 2 H
4
2
2
2
a) How much energy is given off when 2.00 mol of CH
are burned? [-1780 kJ]
4
b) How much energy is released when 22.4g of CH
are burned? [-1240 kJ]
4
c) If you were to attempt to make 45.0g of methane from CO
and H
O (with O
also being
2
2
2
3
made), how much energy would be required? [2.50 x 10
kJ]
2) What is the change in enthalpy when 9.75 g of aluminum reacts with excess ammonium
nitrate (NH
NO
) according to the equation: [-367 kJ]
4
3
ΔH = -2030kJ
2Al + 3NH
NO
3N
+ 6 H
O + Al
O
4
3
2
2
2
3
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