What makes a reaction go faster or slower, and why?
Use collision theory to explain how temperature, concentration, surface area, and catalysts affect the rate of a reaction (MA STE HS-PS1-5, effect of temperature and concentration on reaction rate).
A standard-level answer on reaction rates and collision theory for Massachusetts high school chemistry: how collision theory explains rate, and the effects of temperature, concentration, surface area, and catalysts, grounded in HS-PS1-5.
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What this topic is asking
Standard HS-PS1-5 asks you to explain the effect of changing the temperature or concentration of reacting particles on the rate of a reaction. The explanation comes from collision theory. A Massachusetts high school chemistry course expects you to use collision theory to explain not only those two factors but also surface area and catalysts.
Collision theory
Reactions happen when reactant particles meet. But not every collision works: the particles must hit with at least a minimum energy, the activation energy, and line up the right way. The rate of a reaction depends on how many successful collisions happen each second. So any factor that increases the frequency of collisions or the fraction that are energetic enough will speed up the reaction.
Temperature
This is the factor HS-PS1-5 highlights. Hotter particles move faster (temperature measures average kinetic energy, from states of matter and kinetic molecular theory), so they collide more frequently. More importantly, more of those collisions carry enough energy to overcome the activation energy, so the proportion of successful collisions rises sharply. A small temperature rise can noticeably speed a reaction.
Concentration and surface area
A more concentrated solution has reactant particles packed closer, so they meet more often and the rate rises. For a solid, only the particles at the surface can collide with the other reactant, so breaking the solid into a powder dramatically increases the exposed surface and speeds the reaction. This is why fine powders can react far faster than a single lump of the same substance.
Catalysts
A catalyst does not make collisions more frequent; instead it lowers the bar for success by offering a route with a smaller activation energy, so a larger fraction of the existing collisions have enough energy. Because it is regenerated, a small amount of catalyst can speed a reaction indefinitely. The activation energy a catalyst lowers is pictured in potential energy diagrams and activation energy.
Try this
Q1. Why does milk spoil more slowly in a refrigerator? [1]
- Cue. The lower temperature slows the particles, so fewer energetic collisions occur and the spoiling reactions are slower.
Q2. State one way to speed up a reaction that does not change the temperature. [1]
- Cue. Increase the concentration, increase the surface area, or add a catalyst (any one).
Exam-style practice questions
Practice questions written in the style of MA DESE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
MA Chemistry (style)3 marksUsing collision theory, explain why each change speeds up a reaction. (a) Raising the temperature. (b) Increasing the concentration. (c) Grinding a solid into a powder.Show worked answer →
A 3-point collision-theory item.
(a) 1 point: higher temperature gives particles more kinetic energy, so they collide more often and a greater fraction of collisions have enough energy to react.
(b) 1 point: higher concentration means more particles in the same volume, so collisions happen more frequently.
(c) 1 point: powdering a solid increases its surface area, so more particles are exposed for collisions. Markers reward linking each change to more frequent or more energetic successful collisions.
MA Chemistry (style)2 marksA catalyst is added to a reaction. (a) State its effect on the rate. (b) Explain how it works without being used up.Show worked answer →
A 2-point catalyst item.
(a) 1 point: the catalyst increases the rate of the reaction.
(b) 1 point: it provides an alternative pathway with a lower activation energy, so a greater fraction of collisions succeed; it is regenerated at the end, so it is not used up. Markers reward the lower-activation-energy mechanism and the fact that the catalyst is not consumed.
Related dot points
- Interpret a potential energy diagram to identify activation energy, the energy change of reaction, and the effect of a catalyst, and classify the reaction as exothermic or endothermic (MA STE HS-PS1-4 and HS-PS1-5, energy and rate).
A standard-level answer on potential energy diagrams and activation energy for Massachusetts high school chemistry: reading the reactant and product energy levels, the activation energy barrier, the energy change of reaction, and how a catalyst lowers the barrier, grounded in HS-PS1-4 and HS-PS1-5.
- Classify reactions as exothermic or endothermic, describe energy transfer as heat, and apply the conservation of energy to chemical and physical changes (MA STE HS-PS3-4(MA), thermal energy transfer).
A standard-level answer on energy changes in chemical reactions for Massachusetts high school chemistry: exothermic and endothermic reactions, energy transferred as heat, the conservation of energy, and the link to temperature change, grounded in HS-PS3-4(MA).
- Explain that breaking bonds absorbs energy and forming bonds releases it, and use bond energies to decide whether a reaction is exothermic or endothermic (MA STE HS-PS1-4, energy from changes in total bond energy).
A standard-level answer on bond energy and reaction energy for Massachusetts high school chemistry: why breaking bonds absorbs energy and forming bonds releases it, using bond energies to find the net energy change, and deciding whether a reaction is exothermic or endothermic, grounded in HS-PS1-4.
- Describe dynamic equilibrium in a reversible reaction and use Le Chatelier's principle to predict the effect of changing concentration, temperature, or pressure (MA STE HS-PS1-6(MA), shifting equilibrium to increase product).
A standard-level answer on chemical equilibrium and Le Chatelier's principle for Massachusetts high school chemistry: dynamic equilibrium in a reversible reaction and predicting the shift when concentration, temperature, or pressure changes, grounded in HS-PS1-6(MA).
- Describe the kinetic molecular theory and use it to explain the properties of solids, liquids, and gases and the meaning of temperature (MA STE supporting content, kinetic molecular theory of matter).
A standard-level answer on the states of matter and kinetic molecular theory for Massachusetts high school chemistry: the particle arrangement and motion in solids, liquids, and gases, the assumptions of kinetic molecular theory, and how temperature relates to particle motion, grounded in the framework's matter content.
Sources & how we know this
- Massachusetts Science and Technology/Engineering Curriculum Framework (2016) — Massachusetts Department of Elementary and Secondary Education (2016)
- Science and Technology/Engineering (STE) Test Design and Development — Massachusetts Department of Elementary and Secondary Education (2024)