Reaction Rates Lech Atelier

Reaction Rates And LeChatelier’s Principle

Reaction Rates • Rates are influences by: – – – –

The nature of the reactants: how reactive the chemicals are. Temperature: Temperature usually increased temperature = increased rate Concentration: usually increased concentration = increased rate (For both temp & concentration, more collisions give faster rates.) – Particle SizeSize usually smaller particles = increased rate (more surface area) • Homogeneous rxns usually react faster than heterogeneous rxns.

– Catalysts: Catalysts catalysts are added to a reaction to speed it up & are then removed unchanged from the reaction. Catalysts often provide a lower energy path for the reactants to take, so lower activation energy. • Homogeneous catalyst = same phase as reactants, • Heterogeneous catalysts = different phase than reactants

Reversible Reactions • Reversible Reactions – reactants form products & products form reactants. (Products can reform reactants.) • There is a forward and a reverse reaction represented by ←→ • Equilibrium occurs when the forward & reverse reactions have equal rates. – EX. 2SO2 + O2 ← → 2SO3

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Arrows can be used to show the “position of equilibrium” – A – A

B B

(products favored) favored (reactants favored) favored

• Catalysts work on both forward & reverse reactions & so have no effect on equilibrium.

Equilibrium Constants

• Equilibrium constants (Keq) can be used to show numerically whether reactants or products are favored. favored • Pure solids & liquids are left out as their concentrations (molarities) cannot change. • Aqueous solutions & gases are common in equilibrium expressions. EX: aA+bB ← → cC+dD Keq = [C]c[D]d [A]a[B]b [ ] = moles/L (M) • Keq is temperature dependant (different at different temperatures) • Keq > 1 products favored@ equilibrium • Keq < 1 reactants favored@ equilibrium

Example Problem • An equilibrium mixture of N2, O2, and NO gases at 1500 K is determined to consist of 6.4 x 10–3 M N2, 1.7 x 10–3 M O2, and 1.1 x 10–5 M NO. What is the equilibrium constant for the system at this temperature? • Equation: N2(g) + O2(g) ← → 2NO(g) Keq = [NO]2 [N2]1[O2]1 = Keq = (1.1 x 10 –5 M)2 1.1 x 10 –5 (6.4 x 10 –3 M)(1.7 x 10 –3 M)

Your Turn! • At equilibrium a mixture of N2, H2, and NH3 gases at 773 K is determined to consist of 0.602 M N2, 0.420 M H2, and 0.113 M NH3. What is the equilibrium constant for the system at this temperature? • Equation: N2(g) + 3H2(g) ← → 2NH3(g) Keq = [NH3]2 [N2]1[H2]3 = Keq = (0.113)2 0.286 (0.602 M)(0.420 M)3

One More! • At equilibrium a container holds 20.0 M H2, 18.0 M CO2, 12.0 M H2O, and 5.9 M CO gases at 500 K What is the equilibrium constant for the system at this temperature? • Equation: H2(g) + CO2(g) ← → CO(g) + H2O(g) Keq = [CO] [H2O] [H2] [CO2] Keq = (5.9 M) (12.0 M) = 0.197 (20.0 M)(18.0 M)

Le Chatelier’s Principle • Le Chatelier’s Principle – if a stress is applied to a system in equlilibrium, it reacts to relieve the stress. • Concentration Changes – equilibria react to make more of what there is less of (react to get back to equilibrium). Ex. when product is added, reaction shifts to make more reactants and vice versa. • Temperature Changes – increased temperature causes the equilibrium to shift in the direction that absorbs heat. Ex. N2 + 3H2 ← → 2NH3 + 92 kJ • Heating this reaction results in a shift to reactants (too many products exist, so reactants need to be made.)

LeChatelier (Continued) • Pressure Changes only affect gaseous reactants & products.

EX: 2NO2 (g) ← → N2O4 (g) + heat what happens if you:

Shifts to the left Shifts to the right 2) remove N2O4 Shifts to the left 3) add N2O4, Shifts to the left 4) heat the reaction Shifts to the right 5) cool the reaction Shifts to the right 6) put pressure on the reaction 1) remove NO2

What is the largest value of change you can have and still not make change for a dollar? 1.19$ (3 quarters, 4 dimes, 4 pennies)