How Fast a Reaction Goes
Introduced| English |
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| reaction rate/rɪˈækʃn reɪt/ |
Some changes race, some crawl
- An explosion finishes in a flash; iron rusts over years.
- The same chemistry can run at wildly different speeds.
- We can measure exactly how quickly reactants turn into products.
- A few knobs let us speed things up or slow them down.
What the rate means
- The reaction rate 反应速率 is how fast concentration changes over time.
- It is measured as concentration used or made per second.
- Reactants fall while products rise.
Reaction rate is usually measured in units of concentration per ____.
Rate is a change in concentration per unit time, e.g. M/s.
What speeds it up
- Higher concentration means more frequent collisions, so a faster rate.
- Higher temperature means faster, harder collisions, so a much faster rate.
- More surface area and a catalyst also speed it up.
Raising the temperature generally makes a reaction...
Hotter particles collide more often and harder, speeding the reaction.
Select all factors that increase reaction rate.
Concentration, temperature, and a catalyst all speed the reaction.
A catalyst is used up during the reaction it speeds up.
A catalyst speeds a reaction without being consumed.
Watching it happen
- Track a reactant's concentration as it drops over time.
- The steeper the curve, the faster the rate.
- The rate is largest at the start and slows as reactants run low.
Speed up the reaction
See how temperature, concentration and a catalyst change how often particles collide and react.
A reaction generally slows down as it proceeds.
Reactants deplete, so collisions become less frequent.
A reactant falls from $1.0\ \text{M}$ to $0.8\ \text{M}$ in $10\ \text{s}$. What is the average rate?
- Rate $=$ change $/$ time $= (1.0 - 0.8) / 10$.
- Rate $= 0.02\ \text{M/s}$.
A reactant drops from $0.9\ \text{M}$ to $0.6\ \text{M}$ in $5\ \text{s}$. The average rate (in M/s)?
Rate $= (0.9 - 0.6)/5 = 0.06\ \text{M/s}$.
The rate usually slows as the reaction proceeds -- reactants get used up, so collisions grow rarer. Temperature has an outsized effect: a small rise can double the rate. And a catalyst speeds a reaction without being consumed.
The reaction rate is how fast concentration changes over time, with reactants falling and products rising. It rises with concentration, temperature, surface area, and a catalyst. The rate is fastest at the start and slows as reactants run out.