What Is Reaction Rate? How Fast a Reaction Goes
Rust takes years. A firework takes milliseconds. Both are chemical reactions, and the only difference your exam actually asks about is how fast. That question has a name.
The short answer: the reaction rate is how quickly reactants are used up or products are formed, measured as a change in concentration per unit time — usually in mol dm⁻³ s⁻¹. A fast reaction has a large rate; a slow one has a small rate.
What reaction rate actually measures
Rate is a speed, and like any speed it's an amount divided by a time:
rate = change in concentration ÷ time taken
Written with symbols, for a reactant that's being consumed:
rate = −Δ[reactant] / Δt
and for a product being made:
rate = +Δ[product] / Δt
The minus sign is bookkeeping, not physics. Reactant concentration falls, so Δ[reactant] is negative; the minus flips it so the rate itself comes out positive. Rates are always quoted as positive numbers.
You don't have to use concentration. In the lab you measure whatever is easiest to watch, so you'll also see rate expressed as cm³ of gas per second, or grams lost per minute. The idea is identical: something measurable, divided by time.
Rate changes as the reaction goes
This is the part students most often miss. The rate is not one number for the whole reaction. It is fastest at the very beginning, when reactant concentration is highest, and it slows continuously as reactants get used up, reaching zero when one of them runs out.
That's why a graph of product against time is a curve that starts steep and flattens into a plateau. Two useful readings come off it:
- The average rate — total amount formed ÷ total time. Easy, but it smears the fast start and the slow finish together.
- The instantaneous rate — the gradient of the tangent drawn at one point on the curve. This is the true rate at that moment. The tangent at t = 0 gives the initial rate, which is the one experiments usually compare, because at t = 0 you know the concentrations exactly.
The five things that change a rate
| Factor | Increase it and… | Why |
|---|---|---|
| Concentration (or pressure, for gases) | rate goes up | More particles per unit volume, so more collisions per second |
| Temperature | rate goes up sharply | Particles move faster and far more of them carry enough energy to react |
| Surface area of a solid | rate goes up | Powder exposes far more particles than a lump |
| Catalyst | rate goes up | Offers a lower-energy route to the products |
| Nature of the reactants | fixed for a given reaction | Ionic reactions in solution are near-instant; reactions that break strong covalent bonds are slow |
A rough rule worth remembering: near room temperature, raising the temperature by 10 °C roughly doubles the rate of many reactions. It's an approximation, not a law, but it shows how disproportionately temperature matters — the other factors don't come close.
How rate is measured in the lab
Pick a property that changes visibly and track it against a clock:
- Gas produced — collect it in a syringe and read the volume each 10 s. Used for Mg + HCl, or the decomposition of hydrogen peroxide.
- Mass lost — stand the flask on a balance; escaping CO₂ makes the reading fall.
- A colour change — a colorimeter tracks how much light gets through as a coloured species appears or disappears.
- A precipitate forming — the classic "disappearing cross": sodium thiosulfate and hydrochloric acid turn cloudy with sulfur, and you time how long a pencil cross under the flask takes to vanish.
- Sampling and titrating — remove a portion at intervals and titrate it to find what's left.
Worked examples
Try each before reading the answer.
1. A reaction produces 48 cm³ of hydrogen in 30 s. What's the average rate?
48 ÷ 30 = 1.6 cm³ s⁻¹.
2. The concentration of a reactant falls from 0.80 mol dm⁻³ to 0.50 mol dm⁻³ in 60 s. What's the average rate of reaction?
Change = 0.80 − 0.50 = 0.30 mol dm⁻³, so rate = 0.30 ÷ 60 = 0.0050 mol dm⁻³ s⁻¹.
3. Magnesium ribbon and magnesium powder are each added to identical acid. Which reacts faster?
The powder — same mass, far more exposed surface, so many more collision sites.
4. A flask of gases reacts at a certain rate. You halve the volume of the flask. What happens?
The concentration of every gas doubles, so the rate increases.
Common mistakes to avoid
- Confusing rate with yield. Rate is how fast; yield is how much. A slow reaction can give a near-perfect yield, and a violently fast one can give almost nothing useful. They are separate questions.
- Treating the rate as constant. It isn't. If a question gives you a curve, take a tangent — don't divide the totals unless it asks for an average.
- Saying temperature works "because particles collide more often". Collisions do get more frequent, but only by a percent or two for a 10 °C rise. Almost all of the speed-up comes from a much larger share of collisions carrying enough energy to actually react.
FAQ
What is the reaction rate in simple terms?
It is how quickly a chemical reaction happens — how much reactant disappears, or product appears, in a given time.
What are the units of reaction rate?
Usually mol dm⁻³ s⁻¹ (concentration per second). In school experiments you'll also see cm³ s⁻¹ for a gas or g s⁻¹ for mass lost.
Why do reactions slow down over time?
Because reactants get used up. Fewer particles remain in the same volume, so there are fewer collisions each second and the rate falls until a reactant runs out.
Does a catalyst change the amount of product?
No. A catalyst changes only how quickly you get there, not how much you end up with.
The takeaway
Reaction rate is simply change ÷ time — how fast reactants turn into products. It is highest at the start, falls as reactants are consumed, and can be raised by increasing concentration, temperature, or surface area, or by adding a catalyst. Every one of those factors works for the same underlying reason, which is the subject of the next post.
Background → What Is a Chemical Reaction? Bonds, Atoms, and Change and What Is Activation Energy? The Barrier Every Reaction Faces. Next → [What Is Collision Theory?] (sibling) explains why these five factors work, and What Is a Catalyst? How Reactions Get a Shortcut covers the shortcut in detail.
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