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Reaction Rates and Equilibrium
Ch. 19
Rates of Reaction
19-1
Collision Theory• The rate or speed of a reaction varies
tremendously depending on the reaction
• Rate = measures the speed of any change over time.
• Collision Theory = atoms/ions/molecules react to form products when they collide, if they have enough kinetic energy.
– If they lack enough kinetic energy, will just bounce off each other when they collide
Activation Energy
• Activation Energy (Ea) = minimum amount of energy that particles must have in order to react.
Energy Diagrams – Exothermic Reaction
Ex: 2H2 + O2 2H2O + 286kJ
Energy Diagrams – Endothermic Reaction
Ex: 2H2O + 286kJ 2H2 + O2
DRAW!
R
P
P
R
+ΔH-ΔH
ENDOTHERMIC EXOTHERMIC
REACTION REACTION
EN
ER
GY
EN
ER
GY
Factors Affecting Reaction Rates
1. Temperature Reaction Rate-Inc. temp., inc. kinetic E, inc. # of collisions, inc. rate
2. Concentration Reaction Rate-Inc. # of particles, more collisions, inc. rate
1. Surface Area Reaction Rate-More surface area, inc. reactant exposure to collisions, inc. rate
4. Catalyst = substance that increases rate of reaction without being used up in rxn.
– Catalyst’s lower the activation energy of a rxn.
– Catalyst written above the “yield arrow”
2H2(g) + O2(g) 2H2O(l)
5. Inhibitor = substance that interferes with action of catalyst.
Pt
Reaction without a catalyst
Reaction with a catalyst
Effect of a Catalyst
-H
DRAW!!!!
(Or has an Inhibitor)
Reversible Reactions and Equilibrium
19-2
Reversible Reactions
• Reversible Reactions = reactions which occur simultaneously in both directions.
2SO2(g) + O2(g) 2SO3(g)
• Chemical Equilibrium = forward and reverse reactions take place at the same rate.
Factors affecting Equilibrium:Le Chatelier’s Principle
• Le Chatelier’s Principle = if a stress is applied to equilibrium, the system will change to relieve the stress.
1. Concentration: changing the amount of any reactant or product at equilibrium disturbs EQ.
H2CO3(aq) CO2(aq) + H2O(l)-If add CO2 (product), rxn direction shifts LEFT-If remove CO2 (product), rxn direction shifts RIGHT-If add H2CO3 (reactant), rxn direction shifts RIGHT-If remove H2CO3 (reactant), rxn direction shifts LEFT
2. Temperature = increasing temperature causes equilibrium to shift in direction that absorbs heat.
Exothermic: 2SO2(g) + O2(g) 2SO3(g) + heat
-If add heat (product), rxn shifts LEFT
-If remove heat (product), rxn shifts RIGHT
Endothermic: N2(g) + O2(g) + heat 2NO(g)
-If add heat (reactant), rxn shifts RIGHT
-If remove heat (reactant), rxn shifts LEFT
3. Pressure = change in pressure affects only equilibrium rxn’s with an unequal # of moles of gaseous reactants AND products.
N2(g) + 3H2(g) 2NH3(g)
4 moles reactant to 2 moles product
-If increase pressure, shift RIGHT (move to less moles side)
-If decrease pressure, shift LEFT (move to more moles side)
Another pressure example:
2Ag2O(g) 4Ag(g) + O2(g)
2 moles reactant to 5 moles product
-If increase pressure, shift LEFT (move to less moles side)
-If decrease pressure, shift RIGHT (move to more moles side)
Equilibrium Constants• Equilibrium Constant (Keq) = ratio of product
concentration to reactant concentrations at equilibrium.– aA + bB cC + dD– Keq = [C]c x [D]d
[A]a x [B]b
– Put product concentrations over reactant concentrations.
– Coefficients become exponents of concentrations.– Concentrations are in mol/L or M– Keq will change if temperature changes– If Keq < 1, then reactants are favored (more reactants
than products)– If Keq > 1, then products are favored (more products
than reactants)
• Equilibrium example:– Using the following equation; 4.00 moles/L of Cl2,
1.2 mol/L BrCl, and 4.00 moles/L of Br2. What is the Keq?
• 2BrCl(g) Cl2(g) + Br2(g)
Keq = [Cl2] x [Br2]
[BrCl]2
Keq = 4.00mol/L x 4.00mol/L
(1.2mol/L)2
Keq = 11.1
Keq > 1 Products are favored!!
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