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Rates of Chemical Reactions

  • When a chemical reaction occurs, the concentrations of the reactants and products change with time
  • The concentration of the reactants decrease as the reactants are used up while the concentrations of the products increase as the products are formed
  • The rate of a chemical reaction is defined as the change in concentration of a reactant or product per unit time. Mathematically,
    rate of reaction = change in concentration ÷ change in time
  • The unit of rate depends on the units of concentration and time. If concentration is expressed in moldm-3 and time is in seconds, the rate of the reaction would be in moldm-3s-1
  • In any reaction, the rates at which reactants are consumed and the products are formed are related by the coefficients of the balanced equation for the reaction
  • In the reaction N2 + 3H2 → 2NH3, 3 moles of hydrogen are consumed for each mole of nitrogen. Therefore the hydrogen is reacting three times faster than nitrogen. Similarly, ammonia is being formed two times faster than nitrogen is reacting
  • Rate of reaction measured with respect to the concentration of a reactant is expressed with a negative sign because concentration of a reactant decreases with time
  • If the rate of consumption of nitrogen in the equation N2 + 3H2 ↔ 2NH3 is -0.30 moldm-3s-1 at a certain temperature, then the rate of consumption of hydrogen is -0.90 moldm-3s-1 and the rate of formation of ammonia is +0.60 moldm-3s-1
  • Chemical reactions proceed at different rates
  • The rate of a reaction depends on the magnitude of its activation energy or the minimum amount of energy which the reactants must acquire to undergo the reaction
  • Fast reactions (combustion of fuel, neutralization reactions, etc.) need very little activation energy to occur and therefore take a very short time to occur
  • If the activation energy of a reaction is high, the reaction is necessarily slow. Examples of slow reaction include rusting of iron, esterification, etc.
Collision Theory of Reaction Rates
  • The basic assumption of the collision theory is that molecules must collide for a chemical reaction to occur
  • However, it is not every collision between the colliding molecules or species that leads to a chemical reaction
  • A collision is effective or capable of resulting in a chemical reaction if the colliding species are energetic enough and properly aligned to provide energy equal to or greater than that required to overcome the energy barrier
Measurement of Reaction Rates
  • The measurement of reaction rates is based on the rate of appearance of a product or the rate of disappearance of a reactant
  • It is usually expressed in terms of change in concentration of the reactant or product per unit time or change in any property of reactant or product that varies with concentration per unit time
  • Thus measurement of reaction rate may involve determination of each of the following which change as concentration changes during a reaction:
    • decrease in the mass of a reaction system which liberates a gaseous product
    • volume of a gaseous product liberated
    • mass of precipitate formed
    • changes in intensity of color
    • changes in pH
    • changes in the total gas pressure
Factors Affecting Reaction Rates
  1. Nature of Reactants: This relates to the types of bonds that must be broken to kick off the reaction. The stronger the bond, the slower the reaction. For this reason, reactions involving the breaking of covalent bonds are slow while reactions between ions are fast
  2. Concentration: In general, the rate of a reaction is directly proportional to the concentrations of the reactants. If the concentration of the reactants is high, the reactant molecules crowded into a small space collide more effectively and change to products in a short time.
    Pressure affects the concentration of gaseous reactants. If the pressure if increased, molecules of gas are pushed closer together and they collide more frequently and react more rapidly.
  3. Temperature:
    • A temperature rise of 10 °C above room temperature usually causes the reaction rate to be doubled.
    • As temperature increases, the average kinetic energy of the reactant particles increases
    • The particles move faster and give rise to more effective collisions and many more particles climb over the energy barrier to form the products in a very short time
  4. Catalyst:
    • A catalyst is a substance which, even in small amounts, increases the rate of a chemical reaction without itself undergoing any permanent chemical change
    • It provides an alternative pathway of lower activation energy without changing the average kinetic energy of the molecules
    • However, reactant molecules with average kinetic energy larger than the new Ea can now cross over the energy barrier and form products
    • The effectiveness of a catalyst increases as its surface area for absorption in the finely divided state increases

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  5. Surface Area:
    • This is particularly relevant to chemical reactions involving solids
    • If a solid in lump form is allowed to react with dilute HCl solution, CO2 is evolved. The reaction is slow because the HCl reacts only with the limited surface of the solid in contact with it layer after layer
    • If the solid is powdered however, the number of contact points increases and the reaction occurs at many sites at the same time. This increases the number of effective collisions and the reaction is fast
  6. Light:
    • This factor is only limited to a few reactions known as photochemical reactions, some examples include, the decomposition of hydrogen peroxide, chlorination of methane, etc.
    • In such reactions, light energy initiates the reaction by energizing the reactant molecules, and this results in a fast reaction
    • The presence of bright sunlight accounts for the explosive reaction between hydrogen and chlorine. This reaction is negligible in darkness or in the absence of light and quite slow in daylight
Arrhenius Equation
  • k = Ae-Ea/RT
  • A is a proportionality constant known as frequency factor
  • e is the base of the natural logarithm system
  • R is the gas constant expressed in energy units (R = 8.314 Jmol-1K-1)
  • T is the absolute temperature
  • A change in temperature alters the rate of a reaction by changing the value of the rate constant, k.
  • If T increases, Ea/RT decreases, -Ea/RT increases, e-Ea/RT increases, the value of k increases and the reaction rate increases
  • Activation energy is constant for a given reaction and so k, as well as reaction rate increases as the temperature increases
  • The logarithmic form of the Arrhenius equation gives
    • lnk = lnA – Ea/RT
    • OR 2.303 log10k = 2.303 log10A – Ea/RT
    • OR log10k = log10A – (EA/2.303R)(1/T)
  • As this logarithmic form gives a straight line equation, a plot of log10k versus 1/T gives a straight line whose slope = -Ea/2.303 which can then be used to determine the activation energy of the reaction
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