Zero-Order Reaction
Definition and meaning of Zero-Order Reaction in chemistry.
A zero-order reaction is a chemical process where the rate of the reaction is completely independent of the concentration of the reactants. As long as some reactant is present, the reaction will proceed at a constant, steady speed until all the starting material is completely exhausted.
In more detail
Zero-order reactions are unique in the realm of chemical kinetics because changing the amount of reactant in the reaction vessel has absolutely no effect on how fast the products are formed. In a typical chemical reaction, adding more starting material speeds up the process because more molecular collisions occur.
However, in a zero-order reaction, the rate is fixed by an external physical limiting factor, not by how many reactant molecules are bouncing around. Because the rate remains perfectly constant, a graph of the reactant concentration versus time yields a straight, downward-sloping diagonal line until the material is entirely gone.
The most common scenarios that produce zero-order kinetics involve a physical bottleneck that strictly limits the reaction speed. A classic example is a reaction catalyzed by a solid metal surface. Once every active site on the metal surface is occupied by a reactant molecule, the surface is saturated.
Adding more reactant molecules to the liquid or gas mixture will not increase the overall rate, because the extra molecules must wait in line for a space to open up on the catalyst. Thus, the reaction proceeds at a steady, maximum velocity dictated entirely by the available surface area of the solid catalyst.
Similarly to second-order reactions, the half-life of a zero-order reaction is not a constant value. Because a fixed, unchanging amount of material is converted into product every second, it takes less time to consume half of a small sample than it does to consume half of a massive sample.
Therefore, the half-life of a zero-order reaction gets progressively shorter as the reaction proceeds and the overall concentration dwindles. Aside from solid catalysts, zero-order kinetics are also frequently observed in biological systems when a specific enzyme is fully saturated with its substrate.
Key facts
| Field | Physical Chemistry |
|---|---|
| Rate Law | Rate = k |
| Concentration Effect | Changing reactant concentration does not change the rate |
| Linear Graph | Plotting [Reactant] vs time yields a straight line |
| Half-Life | Decreases as the concentration decreases |
| Common Cause | Saturated catalyst or enzyme surface |
The biological elimination of alcohol from the human bloodstream is a zero-order process, processing at a constant hourly rate regardless of how much alcohol was consumed.
Frequently asked questions
Why doesn't adding more reactant speed up a zero-order reaction?
Because the reaction is limited by something else, like the available surface area of a catalyst. The extra molecules just have to wait their turn.
What does a graph of concentration versus time look like for a zero-order reaction?
It is a perfectly straight, downward-sloping line, showing that the reactant is being consumed at a steady, constant speed.
How does the half-life of a zero-order reaction change over time?
The half-life gets progressively shorter as the amount of reactant decreases, because a constant rate clears out a smaller amount of material faster.