Chemical Reactions: Energy,
Rates and Equilibrium
Chapter 7
Heat Changes During Chemical
Reactions
• Bond Dissociation Energy- The amount of energy
that must be supplied to break a bond and separate
the atoms in the gaseous state
• Bond breaking requires energy
• Triple bond > double bond > single bond
• Bond making releases energy
• The difference between the bond breaking and
bond making energies is called the heat of reaction
or the enthalpy change, denoted by: ΔH
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Exothermic and Endothermic Reactions
• Exothermic Reaction - A chemical reaction in which
energy is released as the reaction occurs
– The products have lower energy than the reactants
– The chemical bonds that are broken in the reactants are weaker
than the chemical bonds that are formed in the products
• Endothermic Reaction - A chemical reaction in which
energy must be supplied overall to get the reaction to occur
– The products have a higher energy than the reactants
– The chemical bonds that are broken in the reactants are stronger
than the chemical bonds that are formed in the products
Examples of Exothermic and
Endothermic Reactions
• Exothermic: ΔH is a negative value
• Endothermic: ΔH is a positive value
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Why Chemical Reactions Occur?
• Two factors determine if a process proceeds on its
own without any external influence
– Enthalpy, ΔH
– Entropy, ΔS
• Entropy: A measure of the amount of disorder or
randomness in a system
– Entropy increases as physical state changes from solid
to liquid to gas
– Entropy increases when the amount (moles) of products
increases compared to the reactants
Free Energy, ΔG
• Free Energy, ΔG, takes into account the contributions by ΔH
and ΔS
• If ΔG is negative, free energy is released and the process is
spontaneous, exergonic
• If ΔG is positive, free energy is absorbed and the process is non
spontaneous, endergonic
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Evaluating ΔG
•
ΔG = ΔH - TΔS
• A process will always be spontaneous (ΔG<0) if Δ
H<0 (favorable) and ΔS>0(favorable)
• A process will always be nonspontaneous (ΔG>0)
if ΔH>0 (unfavorable) and ΔS<0 (unfavorable)
• A process may be spontaneous (ΔG<0) if ΔH<0
(favorable) and ΔS<0 (unfavorable) or ΔH>0
(unfavorable) and ΔS>0 (favorable) depending on
the temperature, T due to - TΔS term
Collision Theory and Chemical
Reactions
• Collision Theory
– For a chemical reaction to occur, the reactant molecules
must collide with each other
– Not all collisions lead to a chemical reaction
• The molecules must have a proper orientation for the reaction to
occur
• The collision must occur with sufficient impact to overcome the
activation energy (Eact) barrier
• Activation Energy- The minimum amount of
kinetic energy the reacting molecules must possess
for their collision to produce a chemical reaction
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Orientation of Reactants
Exergonic and Endergonic
Energy Diagrams
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Factors Influencing the Rate of a Reaction
• The nature of the reactants
– If strong bonds have to be broken, the reaction rate is slower
– The physical state of the reactants
• The concentration of the reactants
– Higher concentration leads to more collisions per second
• The temperature
– At higher temperatures, the average kinetic energy of the reactants is
greater
– A larger percentage of the collisions have sufficient energy to overcome
the activation energy barrier
• The presence of a catalyst
– Catalyst- A substance that increases the rate of a reaction without
appearing in the overall balanced equation
– Lowers the activation energy barrier
Effect of a Catalyst
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Chemical Equilibrium
• Chemical Equilibrium - a situation in which
two opposing chemical reactions occur at the
same rate.
– The chemical reaction must be reversible
– The chemical reaction must not go to completion
– Often occurs for chemical reactions in the gas phase
in a closed container or for reactions in solution
– A double arrow is used to indicate chemical
equilibrium
Reaction Rates at Equilibrium
• Forward and reverse reactions DO NOT stop
• Forward and reverse reaction rates become equal
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Equilibrium Constants
• aA + bB + … <===> mM + nN +…
• Equilibrium constant relates the relative amounts
of the products and reactants
Magnitude of the Equilibrium Constant
• The value of the equilibrium constant indicates
which reaction (forward or reverse) is favored
• K >1 favors products, K<1 favors reactants
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Le Chatelier’s Principle
• Le Chatelier’s Principle- If a stress is
applied to a system at equilibrium, the
equilibrium will shift in such a way as to
partially remove the stress
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–
–
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Change in concentration
Change in temperature
Change in pressure (if gases are involved)
Addition of a catalyst
Effect by Change in Concentration
• Consider the following reaction:
CO (g) + 2 H2 (g) <===> CH3OH (g)
What happens if more CO is added after
equilibrium has been achieved?
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Forcing a Reaction to Completion
• By continuously removing product as it is made the
reaction continues in the forward direction until all of
the reactants are used
Effects by Changing Temperature
• Reversible reactions are endothermic in one
direction and exothermic in the other
direction
• Endothermic (heat absorbing) reactions will
be favored by increasing the temperature
• Exothermic (heat releasing) reactions will
be favored by decreasing the temperature
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Effects by Changing Pressure
• Increasing pressure for reactions involving gases
favors the side with the fewer number of moles of
gas
Effect by Adding a Catalyst
• Adding a catalyst only speeds up how fast
equilibrium is achieved
• Adding a catalyst DOES NOT change the
equilibrium constant therefore the
concentrations at equilibrium will be
unchanged
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