Milk sours in hours on a warm porch. In a fridge it lasts two weeks.
Milk left on a warm porch sours in hours. Sealed in a 4 °C refrigerator, it stays drinkable for two weeks. Your body does the reverse when it fights an infection. It heats itself to 39 °C, and its enzymes work faster. Temperature, concentration and surface area all change how fast a reaction runs. This page shows what makes a collision lead to a reaction.
Core Claims
- Collision theory. A reaction needs a collision with enough kinetic energy (KE ≥ Ea) and the correct orientation.
- Temperature. Hotter particles move faster. They collide slightly more often, and a much larger share of the collisions have enough energy.
- Concentration and pressure. More particles in the same volume means more collisions each second. Doubling one reactant doubles the rate.
- Surface area. A powder exposes more reactant atoms to collisions than a lump of the same mass.
- The rate. It depends on how often particles collide, how many of those collisions have enough energy, and how many have the correct orientation.
Collision requirements
Retrieval Checklist
- State the two conditions a collision needs to react.
- Explain why raising the temperature can speed up a reaction more than raising the concentration.
- Say how a rise in temperature changes the share of collisions with enough energy.
- Predict how surface area changes the rate of a reaction with a solid.
Why 99.999% of collisions produce nothing at all.
At room temperature and atmospheric pressure, one molecule in the air collides roughly 10 billion times every second. If every collision between methane and oxygen set off a reaction, your room would explode. Almost all collisions are gentle, glancing blows. The molecules bounce apart unchanged.
Collision theory says a collision must pass two tests before it makes a reaction.
1. Enough kinetic energy (KE ≥ Ea)
Molecules are surrounded by clouds of negative electrons that repel each other. To push through that repulsion and break bonds, the particles must hit with at least the activation energy (Ea). A collision with less energy bounces apart with every bond intact.
2. Correct molecular orientation
Atoms are not smooth spheres. Each molecule reacts at a particular place. In NO + O3 → NO2 + O2, the nitrogen atom of NO must hit an oxygen atom of the ozone. If the oxygen end of NO hits the ozone, the molecules bounce apart. A faster collision does not fix that.
Thinking every collision makes a reaction. Most collisions fail one test. The particles are too gentle, or they meet at the wrong angle.
Why a 10 °C warm-up can double the reaction rate.
For many reactions, raising the temperature by just 10 °C doubles or triples the rate. Yet a rise from 20 °C (293 K) to 30 °C (303 K) increases the average kinetic energy by only about 3%. How does a 3% rise in energy produce a 100% to 200% rise in rate?
Temperature changes the rate in two ways.
- More collisions, a little. Faster particles cross the vessel sooner and collide slightly more often. The rise is about 2%.
- Harder collisions, a lot. At a higher temperature a much larger share of the collisions carry at least Ea. That share is what makes the rate jump.
Cold slows spoilage almost to a stop. A fever speeds up the proteins that fight infection.
Thinking a hotter mixture reacts faster only because particles collide more often. Collisions rise by about 2%. The share of collisions with enough energy rises far more.
EnrichmentThe spread of particle energies
Extra depth. The rest of the lesson works without it.
At one temperature the particles do not all have the same energy. The curve in Figure 4.1 shows how many particles have each energy. Chemists call it the Maxwell-Boltzmann distribution. Raising the temperature flattens the curve and moves it to the right. The area to the right of the Ea line is the share of particles with enough energy. It grows far faster than the average energy does.
Count the reactions: temperature, amount of A, and form of A.
Set up a run, press Start run, and count the reactions in 10 seconds. A hard, well-aimed collision between A and B makes the product AB. Each time that happens, a fresh A and B enter the vessel. The amounts stay steady, so the slope of the graph shows the rate.
What makes a collision lead to a reaction?
The explanation appears after you predict.
- Reactant A (it vibrates in place in a lump)
- Reactant B
- Product AB, shown where a reaction happens
- Reactions so far
- Compare 15 °C with 80 °C. Load 15 °C, press Start run, then press Record. Load 80 °C, press Start run, then press Record. Look for: about 480 collisions in both runs, but 23 reactions at 15 °C against 102 at 80 °C.
- Double the particles of A. Load 50 particles at 15 °C, press Start run, then press Record. Look for: 971 collisions and 44 reactions, about twice as many of each as your 15 °C row.
- Pack A into a lump. Load the lump at 15 °C, press Start run, then press Record. Look for: only 49 collisions and 2 reactions, because only the surface of the lump meets B.
Use your evidence
Crowding particles together: concentration, pressure, and surface area.
Temperature changes the share of collisions with enough energy. Concentration, pressure and surface area change something else. They change how often particles collide.
Concentration & pressure
In a dilute solution or a low-pressure gas, molecules travel far before they meet. Double the concentration of a reactant and you double the particles in each cubic centimeter. Collisions happen twice as often, so the rate doubles.
Surface area of a solid
Drop a solid iron nail into hydrochloric acid. Only the iron atoms on the outside can collide with H+ ions. The atoms inside the metal are shielded. Grind the same nail into fine iron powder and the surface area grows thousands of times. The rate rises with it.
Thinking a big lump of solid reacts faster because it holds more reactant. Only the surface meets the other reactant. The same mass as a powder exposes far more surface.
A pile of flour on a kitchen counter is hard to ignite. A match only singes it. Blow the same flour into the air of a grain silo and oxygen surrounds every starch particle. One spark can set off an explosion that wrecks reinforced concrete.
Fill the blanks from memory.
Type each blank from memory, then press Check. Press Show answers after your first check if you are stuck.
1. Under collision theory, particles must collide with enough and with the correct orientation.
2. A rise in temperature speeds up a reaction mainly because a larger share of collisions has at least the .
3. Hotter particles move , so they collide slightly more often.
4. Doubling the concentration of a reactant raises the rate mainly by raising the collision .
5. Grinding a solid into a fine powder increases its , so more atoms meet the other reactant.
A student runs two experiments.
“Experiment 1: Doubling the concentration of hydrochloric acid doubled the reaction rate. Experiment 2: Raising the temperature of the acid by only 25 °C raised the rate by more than 500%.”
Explain, in 3 to 4 sentences, why the modest temperature rise had a much larger effect than doubling the concentration. Use collision theory.
Compare how often the particles collide with the share of collisions that have enough energy. Write your answer first, then compare it with the model answer.
Doubling the concentration puts twice as many particles in the same volume. Collisions happen twice as often. The share of collisions with enough energy stays the same, so the rate doubles.
Raising the temperature does more. The particles collide only slightly more often. But a much larger share of the collisions have at least Ea. A 25 °C rise multiplies the number of reacting collisions several times, so the rate jumps by more than 500%.
Write your answer first. Then grade yourself.
Give yourself 1 mark for each idea you wrote down.
A student studies the reaction between solid zinc and aqueous hydrochloric acid: Zn(s) + 2 HCl(aq) → ZnCl2(aq) + H2(g)
The student runs three trials and measures the volume of hydrogen gas made each second.
• Trial 1 (baseline): 2.0 g zinc strip, 50 mL of 1.0 M HCl at 20 °C → 2.4 mL H2 per second.
• Trial 2: 2.0 g zinc strip, 50 mL of 2.0 M HCl at 20 °C → 4.8 mL H2 per second.
• Trial 3: 2.0 g zinc strip, 50 mL of 1.0 M HCl at 40 °C → 9.6 mL H2 per second.
(a) Use collision theory to explain why doubling the HCl concentration doubled the rate in Trial 2. [1 mark]
(b) Explain why 40 °C gave a four-fold rate in Trial 3. Say which factor is mainly responsible: how often particles collide, or the share of collisions with enough energy. [2 marks]
(c) Propose one other change that would raise the rate. Do not change temperature or concentration. Give the particle reason. [1 mark]
- Concentration (1 mark): You say twice the H+ ions per volume means twice as many collisions per second.
- Temperature (1 mark): You say a higher temperature means faster particles, so more collisions carry at least Ea.
- Main factor (1 mark): You say the share with enough energy matters most. Collision frequency rises only slightly.
- Modification (1 mark): You propose zinc powder or a catalyst. You say why more collisions react.
Many mRNA vaccines are fragile molecules. They break down in slow reactions with water. These reactions slow down a lot when the temperature drops, so makers ship vaccines in ultracold chains at −80 °C. At that temperature few collisions have enough energy to damage the lipid particles. The vaccine stays good for months.
Adaptive Competency Practice: HS-PS1-5
Test your understanding of collision theory and reaction rates one question at a time. The adaptive engine covers collision frequency, orientation, the spread of particle energies, concentration and surface area.