Chemistry Regents
Mr. Markic
Page 1 of 4
Chapter 18 - Reaction Rates and Equilibrium
Collision Theory
• Atoms, ions, and molecules can react to form products when they collide with one another
• Must have sufficient kinetic energy
• Lacking in kinetic energy, particles bounce apart
unchanged
• Rate – a measure of speed of any change that occurs
within an interval of time
Energy Changes in a Reaction
Activation energy –
 minimum amount of KE that colliding particles need to react
Activated complex –
 temporary intermediate product
 may reform into the reactants or a new product
Potential Energy Diagram
Factors Affecting Reaction Rates
1. Effect of Temperature
• Usually increasing temperature speeds up a reaction
• High kinetic energy
2. Effect of Concentration
• Increasing the number of particles into a fixed volume increases the collision frequency
3. Effect of Particle Size
• Increasing surface area increases the collision frequency
4. Effect of Catalysts
• Increases the rate of reaction without being used up itself in the
reaction
• Lowers the activation energy
Chemistry Regents
Mr. Markic
Page 2 of 4
Reversible Reactions
• Also known as equilibrium reactions
• They can proceed in both directions at the same rate (simultaneously)
• Chemical Equilibrium – when the rates of the forward and reverse reactions are equal
•
•
Forward Reaction: 2SO2(g) + O2(g)  2SO3(g)
Reverse Reaction: 2SO2(g) + O2(g)  2SO3(g)
2SO2(g) + O2(g)  2SO3(g)
Factors Affecting Equilibrium
• Shifting the position of equilibrium
• A chemical shift is when the reaction goes forward (right) or reverse (left)
• This is known as Le Châtlelier’s Principle
1. Changes in Concentration
2. Changes in Pressure
3. Changes in Temperature
Changes in Concentrations of Reactants and Products
At chemical equilibrium, no net change occurs in the actual amounts of the components of the system
The concentrations become constant (Dynamic equilibrium)
• Adding reactants shifts rx → products
• Adding product shifts rx → reactants
• Removing reactants shifts rx → reactants
• Removing product shifts rx → products
1. Changes in Concentration
N2(g) + 3H2(g)  2NH3(g)
What happens (when):
•
N2 is increased
•
NH3 is increased
•
to H2 when N2 is increased
•
NH3 is decreased
2. Effect of Pressure
• Only affects gases
• ↑ pressure rx shifts → fewer # of molecules
• ↓ pressure rx shifts → greater # of molecules
N2(g) + 3H2(g)  2NH3(g)
•
What would happen if pressure is increased on both sides?
Chemistry Regents
Mr. Markic
Page 3 of 4
CO2(g)  C(s) + O2(g)
•
What would happen if pressure is increased on both sides?
3. Changes in Temperature


↑ temp rx shifts → absorbs heat
↓ temp rx shifts → releasing heat
N2(g) + 3H2(g)  2NH3(g) + 22kJ
What would happen if:
•
Temperature was raised
•
Temperature was lowered
Sample Exercise
How will the reaction shift to relieve the “stress” caused by the change?
PCl5(g) + heat  PCl3(g) + Cl2(g)
•
Addition of Cl2
•
Removal of heat
•
Increase in pressure
•
Removal of PCl3 as it is formed
How will the reaction shift to relieve the “stress” caused by the change?
C(s) + H2O(g) + heat  CO(g) + H2(g)
•
Lowering the temperature
•
Addition of a catalyst
•
Increasing the pressure
•
Raising the temperature and decreasing the
pressure
•
•
Removing H2 from the equilibrium mixture
Adding H2O to the equilibrium mixture
Equilibrium Constants
 Ratio of the concentration of products and reactants at equilibrium expressed in moles
 Each concentration is raised to the power of its coefficient
 aA + bB ↔ cC + dD
 Keq = [C]c x [D]d
[A]a x [B]b




per liter
The size of the equilibrium constant shows whether products or reactants are favored at equilibrium
Keq > 1, products are favored
Keq < 1, reactants are favored
Only a change in temperature will affect the value of Keq
Chemistry Regents
Mr. Markic
Page 4 of 4
Sample Problems
The colorless gas dinitrogen tetroxide (N2O4) and the dark brown gas nitrogen dioxide (NO2) exist in
equilibrium with each other.
N2O4(g) ↔ 2NO2(g)
A liter of a gas mixture at equilibrium at 10oC contains 0.0045 mol of N2O4 and 0.030 mol of NO2. Write the
expression for the equilibrium constant and calculate the equilibrium constant (Keq) for the reaction.
The reaction: N2(g) +3H2(g) ↔ 2NH3(g) produces ammonia, a fertilizer. At equilibrium, a 1L flask contains
0.15 mol H2, 0.25 mol N2, and 0.10 mol NH3. Calculate Keq for the reaction.
For the same mixture, under the same conditions described above, calculate Keq for this reaction
2NH3(g) ↔ N2(g) + 3H2(g)
How is the Keq for the forward reaction related to the Keq for the reverse reaction?
Enthalpy (ΔH)
• Measures the amount of heat change
• ΔH = Hproducts - Hreactants
• In nature most reactions change to a state of lower energy
1.
•
•
•
Exothermic
Heat flows out of the system
Heat is lost
ΔH = negative
2.
•
•
•
Endothermic
Heat flows into the system
Heat is gained
ΔH = positive
Entropy (ΔS)
• Measures the disorder of a system (chaos or randomness)
• Entropy increases as temperature increases
• Increasing entropy: Solid < liquid < gas
• In nature most reactions change to a state of greater randomness
Spontaneous Reactions
• Something that happens naturally
• In chemistry, spontaneous reactions naturally favor the formation of products at
specific conditions
• Nonspontaneous reactions do not favor the formation of products at the
specified conditions