Chemistry 12 Solutions Manual provides detailed solutions for various chemistry problems, focusing on potential energy diagrams and reaction mechanisms. It includes step-by-step explanations for calculating activation energies and enthalpy changes for both forward and reverse reactions. Ideal for high school chemistry students preparing for exams, this manual covers critical concepts such as endothermic and exothermic reactions, activation energy, and enthalpy changes. Each section is designed to enhance understanding of chemical reactions and energy transformations.

Key Points

  • Explains potential energy diagrams for chemical reactions, highlighting activation energies and enthalpy changes.
  • Covers endothermic and exothermic reactions with detailed examples and calculations.
  • Includes practice problems with solutions for better comprehension of reaction mechanisms.
  • Designed for high school chemistry students, aligning with curriculum standards.
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124 MHR ● Chemistry 12 Solutions Manual 978-0-07-106042-4
Representing a Reaction with a Potential Energy Diagram
(Student textbook page 371)
11. Complete the following potential energy diagram by adding the following labels: an
appropriate label for the x-axis and y-axis, E
a(fwd)
, E
a(rev)
, ΔH
r
.
a. Is the forward reaction endothermic or exothermic?
b. Which has the higher potential energy, the reactants or the products?
What Is Required?
You need to label the given potential energy diagram, determine if the reaction is
exothermic or endothermic and indicate which has the higher potential energy,
reactants or products.
What Is Given?
You have an unlabelled potential energy diagram.
Plan Your Strategy
Act on Your Strategy
The y-axis is an energy scale, and
the
x- axis follows the reaction
progress. The reactants will be
shown as the first level portion, and
the products will be the second
level portion of the diagram. The
activation energy in the forward
direction,
E
a(fwd)
, is the difference
between the reactant energy and the
transition state at the peak of the
diagram. The activation energy for
the reverse reaction,
E
a(rev)
, is the
difference between the product
energy and transition state at the
peak of the diagram.
ΔH
r
is the
difference between the potential
energy of the reactant and the
potential energy of the product.
a. The reaction is endothermic.
b. The products have a higher potential energy
than the reactants.
Check Your Solution
The potential energy of the reactant is lower than the potential energy of the product.
This corresponds to an endothermic reaction.
Unit 3 Part B ● MHR 125
12. Consider the following reaction:
AB + C → AC + B ΔH
r
= +65 kJ, E
a(rev)
= +34 kJ
Draw and label a potential energy diagram for this reaction. Calculate and label
E
a(fwd)
.
What Is Required?
You need to draw a potential energy diagram labelling the x-axis and y-axis, the
transition state, and E
a(fwd)
.
What Is Given?
You know the balanced chemical equation for the reaction.
You know the activation energy of the reverse reaction: E
a(rev)
= +34 kJ
You know the enthalpy change of the forward reaction ΔH
r
= +65 kJ
Plan Your Strategy
Act on Your Strategy
Use the formula
r a(fwd) a(rev)
H E ED = -
to calculate
E
a(fwd)
.
( )
( )
r a(fwd) a(rev)
a(fwd)
a fwd
65 kJ 34 kJ
99 kJ
H E E
E
E
D = -
+ = - +
= +
Draw and label the potential
energy diagram.
Check Your Solution
Since the sign of ΔH
r
is positive, the reaction is endothermic and the potential energy
diagram shows the reactants at a lower energy than the products.
126 MHR ● Chemistry 12 Solutions Manual 978-0-07-106042-4
13. Consider the reaction below:
C + D → CD E
a(fwd)
= +61 kJ, E
a(rev)
= +150 kJ
Draw and label a potential energy diagram for this reaction. Calculate and label
ΔH
r
.
What Is Required?
You need to draw and label a potential energy diagram for this reaction and
calculate and label ΔH
r
.
What Is Given?
You know the activation energy of the forward reaction: E
a(fwd)
= +61 kJ
You know the activation energy of the reverse reaction: E
a(rev)
= +150 kJ
Plan Your Strategy
Act on Your Strategy
Use the formula
r a(fwd) a(rev)
H E ED = -
to calculate
ΔH
r
.
( )
r a(fwd) a(rev)
61 kJ 150 kJ
89 kJ
H E ED = -
= + - +
= -
Draw and label the potential
energy
diagram.
E
a(rev)
is the energy difference between the
transition state and the products (
+150 kJ). ΔH
r
is
the enthalpy of reaction between the reactants and
the products
(–89 kJ).
Check Your Solution
Since the sign of ΔH
r
is negative, the reaction is exothermic. The diagram correctly
shows the potential energy of the product lower than the potential energy of the
reactant.
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FAQs

What is the significance of activation energy in chemical reactions?
Activation energy is the minimum energy required for a chemical reaction to occur. It determines the rate of the reaction; higher activation energy means a slower reaction rate. Understanding activation energy helps in predicting how temperature changes can influence reaction rates. In potential energy diagrams, it is represented as the energy difference between the reactants and the transition state.

How do potential energy diagrams illustrate reaction types?
Potential energy diagrams visually represent the energy changes during a chemical reaction. They show the energy of reactants, products, and the activated complex. For exothermic reactions, the products have lower energy than the reactants, while for endothermic reactions, the opposite is true. These diagrams help students understand the energy landscape of reactions and the concept of activation energy.

What are the key differences between endothermic and exothermic reactions?
Endothermic reactions absorb energy from their surroundings, resulting in products with higher energy than the reactants. In contrast, exothermic reactions release energy, leading to products with lower energy. The enthalpy change (ΔH) for endothermic reactions is positive, while for exothermic reactions, it is negative. Understanding these differences is crucial for predicting reaction behavior and energy requirements.

What role does enthalpy change play in chemical reactions?
Enthalpy change (ΔH) indicates the heat absorbed or released during a chemical reaction at constant pressure. It is a key factor in determining whether a reaction is endothermic or exothermic. A positive ΔH signifies an endothermic reaction, while a negative ΔH indicates an exothermic reaction. This concept is fundamental in thermodynamics and helps predict the feasibility of reactions.

How can students benefit from using the Chemistry 12 Solutions Manual?
The Chemistry 12 Solutions Manual provides comprehensive solutions to chemistry problems, enhancing students' understanding of complex concepts. It offers step-by-step guidance on calculating activation energies and interpreting potential energy diagrams. By working through the problems, students can improve their problem-solving skills and prepare effectively for exams. This resource aligns with high school chemistry curricula, making it an essential study aid.