What must happen for a reaction to occur, and what factors make a reaction go faster?
Reaction rates and collision theory: use collision theory to explain how concentration, temperature, surface area and a catalyst affect the rate of a reaction.
A focused Regents Chemistry answer on reaction rates and collision theory: why effective collisions need enough energy and the right orientation, and how concentration, temperature, surface area, the nature of the reactants and a catalyst change the rate.
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What this topic is asking
The Core Curriculum asks you to use collision theory to explain reaction rate and how it is changed by concentration, temperature, surface area, the nature of the reactants and a catalyst. The Regents tests this as Part A recall and as Part B-2 "explain in terms of collision theory" questions, and it sets up the energy diagrams on the next page.
Collision theory
The reaction rate depends on how many effective collisions happen per second. Anything that increases the frequency of effective collisions speeds the reaction up. This single idea explains every rate factor the Regents tests, so frame your answers around "more frequent" or "higher-energy" effective collisions.
The factors that change rate
Temperature is the most powerful factor because it acts in two ways at once: more frequent collisions and a higher proportion that are energetic enough. Surface area applies when a solid reacts; a powder reacts faster than a single lump of the same mass.
What a catalyst does
This is a frequent Part A point: a catalyst lowers the activation energy but leaves the heat of reaction () unchanged, a distinction shown clearly on the potential-energy diagram in the next page.
Try this
Q1. State the effect of decreasing the concentration of a reactant on the reaction rate. [1 point]
- Cue. The rate decreases, because fewer particles per unit volume collide less frequently.
Q2. Explain why a catalyst does not change the heat of reaction. [1 point]
- Cue. It only provides a lower-activation-energy pathway; the energies of the reactants and products are unchanged, so is the same.
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (Part A style)1 marksIncreasing the temperature of a reaction increases the reaction rate mainly because the reactant particles (1) collide less often (2) collide more often and with greater energy (3) gain potential energy only (4) form a catalystShow worked answer →
A 1-point Part A item on collision theory. The answer is (2) collide more often and with greater energy.
Raising the temperature gives the particles greater average kinetic energy, so they move faster and collide more frequently, and a larger fraction of collisions has enough energy to react (to exceed the activation energy). Both the increased collision frequency and the greater proportion of effective collisions speed up the reaction.
Markers reward linking higher temperature to more frequent, higher-energy collisions.
Regents (Part B-2 style)3 marksA reaction occurs between a solid metal and an acid. (a) State the effect of increasing the surface area of the metal on the reaction rate. (b) State the effect of adding a catalyst on the reaction rate. (c) Explain, in terms of collision theory, why increasing the acid concentration increases the rate.Show worked answer →
A 3-point constructed-response item on rate factors.
(a) Surface area (1 point): increasing the surface area increases the reaction rate (more of the solid is exposed for collisions).
(b) Catalyst (1 point): adding a catalyst increases the reaction rate (it provides a pathway with lower activation energy).
(c) Concentration explanation (1 point): increasing the acid concentration increases the number of acid particles per unit volume, so collisions between reactant particles happen more frequently, increasing the rate.
Markers reward the correct effects of surface area and a catalyst, and a collision-theory explanation that links higher concentration to more frequent collisions.
Related dot points
- Potential energy diagrams: interpret potential energy diagrams to identify activation energy, the activated complex and the heat of reaction, and show how a catalyst changes the diagram.
A focused Regents Chemistry answer on potential energy diagrams: reading the activation energy, the activated complex and the heat of reaction (delta-H), distinguishing exothermic from endothermic reactions, and how a catalyst lowers the activation energy without changing delta-H.
- Equilibrium and Le Chatelier's principle: describe dynamic equilibrium and predict the shift in a system when concentration, temperature or pressure is changed.
A focused Regents Chemistry answer on dynamic equilibrium and Le Chatelier's principle: equal forward and reverse rates, and how a change in concentration, temperature or pressure shifts the equilibrium to relieve the stress.
- States of matter and kinetic molecular theory: describe the particle arrangement and energy in solids, liquids and gases, and state the assumptions of the kinetic molecular theory of an ideal gas.
A focused Regents Chemistry answer on the three states of matter and kinetic molecular theory: how particle arrangement and motion differ across solids, liquids and gases, the assumptions of an ideal gas, and how real gases deviate from ideal behavior.
- Acids, bases and the pH scale: identify Arrhenius acids and bases, interpret the pH scale, and relate a change in pH to a change in hydrogen ion concentration.
A focused Regents Chemistry answer on Arrhenius acids and bases, the pH scale, and how each pH unit means a tenfold change in hydrogen ion concentration, using Table K and Table L of the Reference Tables.
- Types of chemical reactions: classify reactions as synthesis, decomposition, single replacement, double replacement or combustion, and use Table J and Table F to predict whether a reaction occurs.
A focused Regents Chemistry answer on classifying reactions as synthesis, decomposition, single replacement, double replacement or combustion, and using the Table J activity series and Table F solubility guidelines to predict products and precipitates.
Sources & how we know this
- Physical Setting/Chemistry Core Curriculum — New York State Education Department (2002)
- Reference Tables for Physical Setting/Chemistry, 2011 Edition — New York State Education Department (2011)