Download Section 5.4

Section 5.4
Enthalpies of Reaction
Objectives
• Continue examining enthalpy as
the quantity of heat energy
gained or lost by a system in a
process under a constant
pressure.
Key Terms
• Enthalpy
• Enthalpy of reaction
Enthalpies of Reaction
The change in enthalpy,
H, is the enthalpy
of the products
minus the enthalpy of
the reactants:
H = Hproducts − Hreactants
Enthalpies of Reaction
This quantity, H, is called the
enthalpy of reaction, or the heat of
reaction.
Enthalpy Guidelines
1. Enthalpy is an extensive property.
2. H for a reaction in the forward
direction is equal in magnitude,
but opposite in sign, to H for the
reverse reaction.
3. H for a reaction depends on the
state of the products and the state
of the reactants.
What is the value of the
unknown DH in the diagram?
1.
2.
3.
4.
5.
+329.5 kJ
-329.5 kJ
+285.8 kJ
-241.8 kJ
+241.8 kJ
H2 (g) + 1/2 O2 (g)
H = -285.8 kJ
H = ?
H2O (g)
H = -44.0 kJ
H2O (l)
What is the value of the
unknown ΔH in the diagram?
1.
2.
3.
4.
5.
+329.5 kJ
-329.5 kJ
+285.8 kJ
-241.8 kJ
+241.8 kJ
H2 (g) + 1/2 O2 (g)
H = -285.8 kJ
H = ?
H2O (g)
H = -44.0 kJ
H2O (l)
The enthalpy of reaction for:
2 C(s) + O2(g)  2 CO(g) is _____.
Hf° (CO) = 110.5 kJ
1. 110.5 kJ
3. 221.0 kJ
2. +110.5 kJ
4. +221.0 kJ
Correct Answer:
1. 110.5 kJ
3. 221.0 kJ
2. +110.5 kJ
4. +221.0 kJ
1. The coefficients indicate the numbers of
moles of reactants and products that give
rise to the stated enthalpy change.
2. The coefficients indicate the whole number
mass ratios with which the reactants and
products give rise to the stated enthalpy
change.
3. The coefficients indicate the relative
amounts of heat absorbed or given off by each
reactant and product in the reaction.
4. The coefficients indicate the number of bonds
which must be broken or formed in each
reactant and product in the reaction.
1. The coefficients indicate the numbers of
moles of reactants and products that give
rise to the stated enthalpy change.
2. The coefficients indicate the whole number
mass ratios with which the reactants and
products give rise to the stated enthalpy
change.
3. The coefficients indicate the relative
amounts of heat absorbed or given off by each
reactant and product in the reaction.
4. The coefficients indicate the number of bonds
which must be broken or formed in each
reactant and product in the reaction.
1. The energy changes of a system and its
surroundings are not related.
2. The energy lost by a system is
proportional to the energy lost by the
surroundings.
3. The energy lost by a system is equal to
the energy lost by the surroundings.
qsystem = qsurroundings
4. The energy lost by a system is gained by
its surroundings. qsystem = -qsurroundings
1. The energy changes of a system and its
surroundings are not related.
2. The energy lost by a system is
proportional to the energy lost by the
surroundings.
3. The energy lost by a system is equal to
the energy lost by the surroundings.
qsystem = qsurroundings
4. The energy lost by a system is gained by
its surroundings. qsystem = -qsurroundings
Give it Some Thought
What effect does reversing a reaction have
on H for the reaction?
1. Hnew = –Hold.
2. Hnew = Hold

K
3. Hnew = Hold
4. Hnew = 1/Hold.
Give it Some Thought
What effect does reversing a reaction have
on H for the reaction?
1. Hnew = –Hold.
2. Hnew = Hold

K
3. Hnew = Hold
4. Hnew = 1/Hold.