In simple terms
A friendly intro before the formal notes — no formulas yet.
The Speed of Chemistry
Reaction rate is simply how fast reactants are converted into products. This speed is governed by how often and how energetically particles collide with each other.
Imagine you're in a quiet library. You're unlikely to bump into anyone. Now, picture a crowded concert. People are everywhere, moving quickly and bumping into each other constantly. Chemical reactions are similar: the more particles (concentration) and the faster they move (temperature), the more 'collisions' happen, and the faster the reaction proceeds.
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Rate depends on the frequency of collisions between particles and the energy they possess when they collide.
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The limiting reagent is the reactant that gets completely used up first, determining the maximum amount of product that can be formed.
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Factors like surface area, concentration, temperature, and the presence of a catalyst can all be adjusted to change the rate of reaction.
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We can measure reaction rates by tracking a noticeable change over time, such as the loss of mass, the volume of gas produced, or a change in colour.
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Key formulas
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$Rate of reaction = \frac{\text{change in quantity of reactant or product}}{\text{time taken}}$
Full topic notes
Formal explanation with the rigour you need for the exam.
Collision Theory: The Basis of Reaction Rates
For a reaction to occur, reactant particles must physically interact. Collision theory is a model that explains how these interactions lead to a chemical change. It states that for a collision to be 'successful' or 'effective'—that is, to result in the formation of products—two conditions must be met simultaneously.
The rate of a reaction, therefore, depends on the frequency of successful collisions. Anything that increases the frequency of total collisions or, more importantly, the proportion of collisions that are successful, will increase the reaction rate.
Energy Condition: The colliding particles must have a combined kinetic energy that is equal to or greater than a minimum value called the activation energy (). This energy is needed to break existing bonds and initiate the reaction.
Orientation Condition: The particles must collide in the correct spatial orientation, allowing the relevant atoms to come into contact and form new bonds.
Factors Affecting the Rate of Reaction
We can manipulate several factors to alter the rate of a reaction. Each factor influences the rate by affecting the frequency or energy of particle collisions.
Concentration (for solutions) & Pressure (for gases): Increasing the concentration or pressure means there are more reactant particles in a given volume. This leads to more frequent collisions between particles, and thus a higher frequency of successful collisions, increasing the reaction rate.
Temperature: Increasing the temperature gives particles more kinetic energy. This has a dual effect: particles move faster, leading to more frequent collisions, and a significantly larger proportion of particles possess energy greater than or equal to the activation energy. This second effect is the primary reason for the substantial increase in rate with temperature.
Surface Area: For reactions involving a solid, breaking it into smaller pieces (e.g., from a lump to a powder) increases its surface area. This exposes more reactant particles to collisions, increasing the frequency of collisions and thus the reaction rate.
Catalyst: A catalyst is a substance that increases the rate of a reaction without being consumed itself. It does this by providing an alternative reaction pathway with a lower activation energy. This means that at the same temperature, a much larger fraction of collisions will be successful.
When explaining the effect of temperature on rate, a top-level answer must mention both points: increased collision frequency AND a greater proportion of particles having energy ≥ . The second point carries more marks as it's the more significant factor.
Measuring Reaction Rates Experimentally
To determine the rate of a reaction, we must measure a change in a measurable quantity over a period of time. The choice of method depends on the specific reaction and the states of the reactants and products.
$Rate of reaction = \frac{\text{change in quantity of reactant or product}}{\text{time taken}}$
Change in Gas Volume: If a gas is produced, its volume can be collected and measured at regular time intervals using a gas syringe or by displacing water in an inverted measuring cylinder.
Change in Mass: If a gas is produced and allowed to escape from an open flask, the total mass of the flask and its contents will decrease. This mass loss can be monitored over time using a digital balance.
Change in Concentration: The concentration of a reactant or product can be measured at intervals. This is often done by taking small samples ('aliquots') and performing a titration ('quenching' the reaction first), or by using instrumental methods like colorimetry if a coloured substance is involved.
Worked examples
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The reaction between magnesium ribbon and excess hydrochloric acid produces hydrogen gas. The volume of hydrogen collected was recorded over time. A tangent drawn to the curve at t = 0 s had a gradient of +3.6 cm³ s⁻¹. Calculate the initial rate of reaction in mol s⁻¹. (Assume the experiment was conducted at room temperature and pressure, where the molar volume of a gas is 24,000 cm³ mol⁻¹).
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Identify the given rate: The gradient of the volume-time graph gives the rate in cm³ s⁻¹. Initial rate = 3.6 cm³ s⁻¹.
In the decomposition of hydrogen peroxide, , the concentration of was measured. At t = 60 s, the concentration was 0.500 mol dm⁻³. At t = 180 s, the concentration had fallen to 0.200 mol dm⁻³. Calculate the average rate of reaction over this interval.
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State the formula for rate:
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Glossary
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What is the formal definition of 'rate of reaction'?
The change in concentration of a reactant or product per unit time. The standard units are mol dm⁻³ s⁻¹.
Key takeaways
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Energy Condition: The colliding particles must have a combined kinetic energy that is equal to or greater than a minimum value called the activation energy (). This energy is needed to break existing bonds and initiate the reaction.
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Orientation Condition: The particles must collide in the correct spatial orientation, allowing the relevant atoms to come into contact and form new bonds.
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Test Your Knowledge on Reaction Rates
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