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RATES OF REACTION
(Cambridge IGCSE CHEMISTRY 0620)

NOTE: Please look at the figures at the end!

➢ Rates of Reaction
✓ The rate of a reaction refers to the speed at which reactants are converted into
products
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✓ Mathematically, it can be represented as:

➢ Factors Affecting the Rate of Reaction
➢ Several factors influence the rate at which chemical reactions proceed, including:
✓ SURFACE AREA OF SOLID REACTANTS:
✓ Increasing the surface area of a solid reactant increases the rate of reaction
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✓ For example, powdered calcium carbonate reacts faster with hydrochloric acid
than larger chunks because the powder has a greater surface area
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✓ A higher concentration means more particles per unit volume, which leads to an
increased likelihood of collisions
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✓ PRESSURE (FOR REACTIONS INVOLVING GASES):
✓ Increasing the pressure of gaseous reactants effectively increases their
concentration, leading to more frequent collisions and a higher rate of reaction
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✓ TEMPERATURE:
✓ Raising the temperature increases the kinetic energy of particles, resulting in more
frequent and more energetic collisions
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✓ For example, heating a solution of sodium thiosulfate and hydrochloric acid
speeds up the reaction that produces a sulfur precipitate
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They provide an alternative reaction pathway with a lower activation energy
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➢ Catalysts and Enzymes
✓ Catalysts are substances that increase the rate of a chemical reaction without
undergoing permanent changes themselves
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✓ Both catalysts and enzymes operate by lowering the activation energy required for
a reaction to proceed
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✓ It is the energy barrier that must be overcome for reactants to be transformed into
products
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➢ Collision Theory
✓ Collision theory explains how chemical reactions occur and why reaction rates
vary
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Reactant particles must collide
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The collisions must have sufficient energy to overcome the activation energy
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The particles must collide with the correct orientation
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✓ Increasing Concentration: More particles in a given volume increase the likelihood
of collisions
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✓ Increasing Temperature: Higher temperatures increase the kinetic energy of
particles, leading to more frequent and energetic collisions, which can surpass the
activation energy more easily
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Formation of a Precipitate: Indicates the formation of an insoluble product
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Temperature Change: Exothermic reactions release heat, while endothermic
reactions absorb heat
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✓
✓
✓
✓
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➢ Adsorption
✓ Adsorption is the process by which atoms, ions, or molecules adhere to a surface
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✓ During adsorption, reactant molecules are held on the surface of the catalyst,
where they can more easily react due to the increased local concentration and
favorable orientation
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Catalysts, including enzymes, function by lowering the activation energy,
thereby increasing the reaction rate
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Indicators of chemical reactions include changes in color, temperature, gas
production, and the formation of precipitates
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