13.0 MEANING OF RATE OF REACTION
The rate of chemical reaction can be defined as the number of moles of reactant converted or product formed per unit time. Reaction rates are found by measuring the rate at which the mass or concentration of a:
- Reactant is decreasing.
- Product is increasing.
13.1 FACTORS AFFECTING THE RATE OF REACTION
- TEMPERATURE – Reactions are faster at higher temperatures and slower at lower temperatures. Increasing temperature gives particles more kinetic energy, leading to more frequent and more energetic collisions, thus increasing the rate of the reaction.
CONCENTRATION/PRESSURE – In solutions, an increase in concentration of reactants leads to an increase in the rate of reaction and vice versa. For example, the rate of the reaction between HCl and Na₂S₂O₃ increases as the concentration of HCl increases.
For gases, increasing pressure reduces the volume, increasing the number of collisions and thus increasing the reaction rate. For example, the effect of pressure on reactions involving gases such as in the Haber process:
(N₂ + 3H₂ → 2NH₃).
- SURFACE AREA – When a solid reactant is broken into smaller pieces or powdered, its surface area increases, exposing more particles to collisions. For example, the reaction between marble (CaCO₃) and HCl, comparing the powdered form vs. lumps of the same mass. Powdered substances have a larger surface area than lumps, leading to more frequent collisions at the surface and thus a faster reaction. Therefore, an increase in surface area increases the rate of reaction; while a decrease in surface area decreases the rate of reaction.
CATALYST – A catalyst is a substance which alters the rate of a chemical reaction and remains the same at the end of the reaction (it doesn’t take part in the reaction). The presence of a catalyst provides an alternative reaction pathway with lower the activation energy, increasing reaction rates without being consumed in the reaction. For example, in the decomposition of hydrogen peroxide (H₂O₂) or potassium chlorate (KClO₃), the presence of manganese dioxide (MnO₂) – as a catalyst speeds up the reaction process.
SOME REACTIONS AND THEIR CATALYST
NATURE OF REACTANTS – The chemical nature of reactants determines the rate of reaction as different substances have different energy contents. Some substances react faster than others based on their molecular structure and bond strength.
- LIGHT – Some reactions are influenced by light. For example, the reaction between hydrogen and chlorine. Such reactions are known as photochemical reactions. In these reactions, the reactant molecule becomes activated on absorbing light energy, and reacts rapidly together in a series of chain reactions.
It is important to note that the factors which affects the (a) energy content of reacting particles (b) activation energy of the reaction (c) frequency of effective collision; will also affect the rate of reaction.
13.2 RELATIONSHIP BETWEEN KINETIC THEORY AND RATE OF REACTION
The kinetic theory states that particles are constantly moving and colliding. Reaction rates depend on the frequency and energy of these collisions, which are influenced by factors such as temperature, concentration, and surface area.
COLLISION THEORY – It states that for a chemical reaction to occur, the reacting particles must collide with sufficient energy (equal to or greater than the activation energy) and in the correct orientation. It is a principle of chemistry used to predict the rates of chemical reactions.
ACTIVATION ENERGY – It is defined as the minimum amount of energy which reactant particles must possess for a chemical reaction to occur (i.e. reactants to form products). A higher activation energy means a slower reaction, as fewer particles have enough energy to overcome this barrier. Lowering the activation energy (e.g., using a catalyst) speeds up the reaction.
ARRHENIUS LAW/EQUATION – The Arrhenius law states that the motion of the structural units (atoms, molecules, ions, etc.) should slow down at a slower rate through the glass transition than is experimentally observed. The equation is commonly given in the form of an exponential function; in simpler words, it predicts that a small increase in reaction temperature will produce a marked increase in the magnitude of the reaction-rate constant. The rate of a reaction depends on the temperature and the activation energy (Ea).
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