Reaction rates: how fast chemistry happens
Learn what a chemical reaction rate is, how rates are measured, and why some reactions finish in seconds while others take centuries.
Quick answer
A reaction rate measures how quickly a reactant is consumed or a product is formed, expressed as the change in concentration divided by the change in time.
Unit: mol·L⁻¹·s⁻¹ (M/s)
Example: If [N₂] falls by 0.20 M over 10 s in N₂ + 3H₂ → 2NH₃, the rate is 0.020 M/s measured on N₂, and NH₃ forms at 0.040 M/s because the equation makes two NH₃ for each N₂ used.
What you will learn
- Define reaction rate in terms of concentration change over time.
- Relate reactant disappearance rates to product appearance rates using stoichiometry.
- Distinguish average rate from instantaneous rate.
- List the main factors that change how fast a reaction runs.
What a reaction rate measures
Thermodynamics tells you whether a reaction can happen; kinetics tells you how fast it happens. Diamond turning into graphite is thermodynamically favorable, yet your jewelry does not crumble — the rate is essentially zero. A reaction rate quantifies speed as concentration change per unit time.
Because concentrations are usually in moles per liter (M) and time in seconds, rates carry units of M/s. For a reactant A disappearing, the rate is written with a negative sign so the rate itself stays positive:
For a product P appearing, no negative sign is needed:
Rates and stoichiometry are linked
Different substances in the same reaction change at different speeds because the balanced equation ties them together. In
three H₂ molecules vanish for every N₂ consumed, and two NH₃ appear. If N₂ disappears at 0.020 M/s, then H₂ disappears three times faster (0.060 M/s) and NH₃ appears twice as fast (0.040 M/s).
The general definition divides each substance's change by its coefficient so everyone agrees on one number:
This coefficient-normalized rate is the single quantity chemists mean when they say "the rate of the reaction."
Average rate versus instantaneous rate
An average rate is measured over a time interval — for example, the concentration change between minute 2 and minute 5. Because most reactions slow down as reactants are used up, the average depends on which interval you pick.
The instantaneous rate is the rate at one exact moment. On a concentration-versus-time graph, it is the slope of the tangent line at that point. The instantaneous rate at the very start, when concentrations are still known exactly, is called the initial rate and is especially useful for figuring out rate laws.
Typical concentration-time curves are steepest at the beginning and flatten as reactants run out, which is why quoting a single "average rate" for a whole reaction can be misleading.
How rates are measured in practice
Any property that changes as the reaction proceeds can be tracked:
- Concentration by titration or spectroscopy (color change over time)
- Gas volume or pressure for reactions that make or consume gas
- Mass loss when a gas escapes from an open flask
- pH or conductivity when ions appear or vanish
- Temperature in some fast exothermic reactions
The key is measuring the same quantity at several known times, then converting the raw readings into concentration versus time.
What changes a reaction rate
Five factors dominate:
- Concentration of reactants — more particles per volume means more collisions per second, so rates usually rise with concentration.
- Temperature — higher temperature means faster-moving particles and more collisions energetic enough to react. Rates often roughly double for a 10 °C rise.
- Surface area — for solids, only exposed surface reacts; powdered zinc dissolves in acid far faster than a zinc strip.
- Catalysts — substances that open a lower-energy pathway, speeding the reaction without being consumed.
- Nature of the reactants — ionic reactions in solution are often nearly instant, while reactions that must break strong covalent bonds can be slow.
Light can also matter: photosynthesis and photographic film depend on it.
Variables and units
| Symbol | Name | Unit | Meaning |
|---|---|---|---|
| Change in concentration | M (mol/L) | Final concentration minus initial concentration of species A over the time interval | |
| Time interval | s | The elapsed time over which the concentration change is measured |
Common mistakes
The equilibrium constant describes how far a reaction goes, not how fast it gets there. Rusting has a huge equilibrium constant but proceeds slowly; the explosive reaction of hydrogen and oxygen is fast but can be held back by a high activation barrier until sparked.
Most rates fall continuously as reactant concentrations drop. That is exactly why chemists distinguish instantaneous rates from average rates, and why rate-versus-time plots curve downward.
The negative sign in the definition only corrects for the fact that reactant concentrations decrease. A rate value itself is positive; the sign is bookkeeping, not a statement about direction.
Real-world applications
Food chemistry: refrigeration slows spoilage reactions, which is why cold storage extends shelf life.
Industrial chemistry: the Haber process for ammonia is run at high temperature and pressure with an iron catalyst to make the rate economically viable.
Medicine: drug design considers how fast a drug breaks down in the body (its metabolic rate).
Safety engineering: airbags rely on a reaction fast enough to inflate the bag in milliseconds.
Environmental chemistry: catalytic converters speed up the breakdown of exhaust pollutants before they leave the tailpipe.
Frequently asked questions
Why are rates expressed per unit of volume (M/s) instead of just moles per second?
Because concentration, not total amount, determines how often particles meet. Doubling the volume at the same concentration does not change collision frequency, so M/s lets chemists compare reactions run at different scales fairly.
Can a reaction rate be zero?
Yes. A rate of zero means no net change is occurring — either the reaction has reached equilibrium, the reactants are gone, or the barrier is so high that nothing measurably happens (like diamond converting to graphite at room temperature).
Why do chemists divide by the stoichiometric coefficient?
Without it, the 'rate' would depend on which substance you watched: H₂ disappears three times faster than N₂ in ammonia synthesis. Dividing by each coefficient gives a single agreed-upon rate for the whole reaction.
Does stirring a solution change the reaction rate?
It can, by bringing fresh reactants together faster — especially in heterogeneous reactions where mixing is the bottleneck. But stirring does not change the intrinsic chemical rate constant; it only removes a physical limitation.