Collision Theory
What two things must particles have (according to collision theory) when they collide for a reaction to occur?
Sufficient kinetic energy and correct orientation.
Name four factors that commonly affect reaction rate.
Temperature, concentration, surface area, presence of catalyst
What is a catalyst?
Substance that speeds reaction by providing alternative pathway without being consumed.
Paraphrase Le Châtelier's Principle in one sentence.
A system at equilibrium will shift to oppose applied changes (concentration, pressure, temperature)
If Keq = 1.0, which side is favored at equilibrium: reactants, products, or neither?
Keq = 1: neither side favored, means the reaction is at equilibrium
Explain why a powder reacts faster than a single large block of the same material.
Powder has much greater exposed surface area → more collisions in the same amount of time.
How does increasing concentration of reactants affect collision frequency and rate?
Increases collision frequency → more effective collisions → rate increases
Do catalysts change the activation energy, and if so, how?
Yes — lowers activation energy
For the exothermic equilibrium N2 + 3H2 ⇌ 2NH3, predict the shift when NH3 is added.
Shift left (to consume added NH3)
Given Keq = 0.0045, which side is favored? Explain briefly.
Keq < 1: reactants favored
Define activation energy in one sentence
The minimum energy required for reactants to convert to products (activation energy)
Explain why increasing temperature generally increases reaction rate (refer to particle energy).
Raises average kinetic energy so more particles exceed activation energy → rate increases
True or False: A catalyst is consumed permanently in the reaction.
False — catalysts are not consumed
For the same equilibrium, predict the shift when volume of container decreases (pressure increases)
Decrease volume (increase pressure) favors side with fewer gas moles: here NH3 side (2 mol) vs reactants (4 mol) → shifts right.
Write the equilibrium expression (Keq) for CS2(g) + 2H2(g) ⇌ CH4(g) + S2(g)
Keq = [CH4][S2] / ([CS2][H2]^2)
Describe how collision orientation affects reaction outcomes.
Only collisions with proper relative orientation form bonds leading to products.
How does surface area affect reaction rate?
More surface area → more particles exposed → reaction speed increases
Describe one real-world example where a catalyst is used and explain how it increases reaction rate.
Varies based on answer
For N2 + 3H2 ⇌ 2NH3 (exothermic), predict the shift if pressure increased by adding argon gas — give brief explanation
Adding argon increases total pressure but not partial pressures of reacting gases → no shift.
For 2 O3(g) ⇌ 3 O2(g), at equilibrium [O2] = 0.21 M and [O3] = 0.07 M. Write the equilibrium expression with values plugged in
Keq = [O2]^3 / [O3]^2 = (0.21)^3 / (0.07)^2