General Chemistry · §12.3–12.5
Notebook
How concentration sets the speed of a reaction.
§12.1 · What a rate measures
rate = −Δ[A] / Δt
How fast concentration changes with time — scaled so every species reports the same number.
§12.3 · The rate law
rate = k[A]m[B]n
The orders m and n are measured — never read from the coefficients. k sets the intrinsic speed.
§12.3 · What the order means
Zero order ignores [A]; first order tracks it one-for-one; second order rises with [A]2.
§12.3 · Method of initial rates
rate2 / rate1 = ([A]2 / [A]1)m
Change one concentration at a time; the rate ratio hands you the exponent.
§12.3 · Worked example
2m = 4 → m = 2
Double [A] at fixed [B] and the rate quadruples — so the reaction is second order in A.
§12.3 · The rate constant
k = rate / [A][B]
For rate = k[A][B], one trial gives k = 0.30 M−1s−1 — the units follow from the overall order.
§12.4 · Half-life
t1/2 = ln 2 / k
A first-order half-life holds no [A]0 — the reaction takes the same time to halve, again and again.
§12.4 · Read the order off the curve
The shape of rate against [A] names the order at a glance — flat, linear, or upward-curving.
§12.3–12.5 · conclusion
Read each order from how the rate answers a change in concentration — the units, half-life, and decay curve all follow.