Lesson 5 of 11 · 7 min
Zero-order reactions
NCERT §3.3; §3.3.1
On a hot platinum wire, ammonia breaks down into nitrogen and hydrogen. Pump in twice as much ammonia at high pressure and the reaction goes no faster. How can more reactant change nothing?
The lesson in notes
In short
Integrating the rate law gives an equation linking concentration directly to time, which experiments can test; this is how order and k are found in practice.
For zero order, rate = −d[R]/dt = k, a constant. Integration gives [R] = [R]₀ − kt.
A plot of [R] against t is a straight line with slope −k and intercept [R]₀.
So k = ([R]₀ − [R])/t. The reactant is used up entirely at t = [R]₀/k, after which the reaction simply stops.
Zero order is uncommon and appears under special conditions, usually when a surface or an enzyme is saturated.
NH₃ decomposing on a hot platinum surface at 1130 K at high pressure: 2NH₃ → N₂ + 3H₂ with rate = k. The surface is fully covered, so extra gas cannot speed the reaction.
Thermal decomposition of HI on a gold surface is another zero-order example.