Revise: Chemical Kinetics: Rates of Reaction
How fast reactions happen, collision theory and activation energy, the Maxwell–Boltzmann distribution, and the factors — concentration, temperature, surface area, catalysts — that speed reactions up or slow them down.
rate = Δ[concentration] / Δt.
0.80→0.50 mol/L over 60 s → rate = 0.005 mol/(L·s).
Reactions need collisions with enough energy AND correct orientation.
Most collisions between gas molecules fail to react.
Ea is the minimum energy needed for a collision to succeed — even exothermic reactions have one.
A catalyst lowers Ea from 75 to 50 kJ/mol, speeding up the reaction.
Rate roughly doubles for every 10°C rise (rule of thumb).
0.010 mol/(L·s) at 20°C → ≈0.040 mol/(L·s) at 40°C.
More surface area or a catalyst → faster rate, without changing the products.
A car's catalytic converter speeds up exhaust reactions using platinum/palladium.
Rate can be measured from any reactant or product, always reported as positive.
0→0.40 mol/L over 20 s → rate = 0.02 mol/(L·s).
Average rate = secant line slope; instantaneous rate = tangent line slope.
Reaction rate is fastest at the start, slows as reactants are used up.
Every rate-affecting factor works by changing collision frequency or collision success rate.
Pure O₂ reacts faster than air because collisions are more frequent.
Ea and ΔH are independent — a reaction can be exothermic but still need a spark to start.
Wood combustion is exothermic overall but needs an initial flame.
Only particles beyond Ea (the tail of the distribution) can react.
A small temperature rise disproportionately grows the fraction beyond Ea.
Higher concentration/pressure → more frequent collisions → faster rate.
Compressed gas reacts faster than the same gas at low pressure.
Temperature increases BOTH collision frequency and the fraction of successful collisions.
0.005 mol/(L·s) at 10°C → ≈0.02 mol/(L·s) at 30°C.
A catalyst provides a lower-Ea pathway and is regenerated, never consumed.
Powdered magnesium reacts far faster with acid than a magnesium strip.