Rates of Reactions

Rate of reaction is a straightforward topic once the terminology is known, and in this respect, there are quite a number of technical words to be understood. Know particularly the definition of rate of reaction: the change in the amount (or the concentration) of reactant (or product) per unit time. There are quite a number of allowable units, for example: moles per second, grams per second, cm3 per second, moles per minute, and so on.

Factors Affecting Rate

  • The state of a solid reactant; the more finely divided it is, the faster it reacts.
  • The type of reaction; as a general rule, ionic reactions take place nearly instantaneously whereas covalent reactions are generally slow.
  • The concentration (for a solution) or the pressure (for a gas); as a general rule, rate is proportional to either the concentration or to the pressure.
  • Temperature; rate increases (rapidly) with temperature, for two reasons:
    1. the reacting molecules have greater energy, and
    2. collisions between molecules occur more frequently;
  • Presence of a catalyst; a catalyst (in the usual sense of the word) increases the rate, and a negative catalyst decreases it.


Catalyst Terms

The technical words in the context of catalysts are:

The following catalysed reactions are on the Leaving Certificate syllabus:

Reaction Equation Catalyst
Preparation of oxygen H2O2 —» H2O + ½O2 MnO2
Haber process (NH3 manufacture) N2 + 3H2 —» 2NH3 Iron (Promoter: mixture of Al2O3 and K2O
Oxidation of ammonia 4NH3 + 5O2 —» 4NO + 6H2O Platinum-rhodium gauze
Hydrogenation reactions e.g., C2H4 + H2 —» C2H6 Ni powder
Esterifications acid + alcohol —» ester + water H+ ions
Oxidation of tartrate ions tartrate ions + H2O2 —» CO2 + H2O CO2+ ions
Oxidation of methanol CH3OH + O2 —» HCHO platinum


Frequently-asked items that you need to learn and remember

  • The five variables on which rate of reaction depends
  • How changes in each of these variables alters reaction rate
  • Details of experiments to illustrate the effect of each of these variables
  • Definitions or explanations of the various catalyst terms
  • Details of the two mechanisms by which catalysts catalyse
  • How to plot reaction rate graphs from experimental data
  • How to explain those graphs, and how to extract information (such as Instantaneous reaction rate) from them
  • How to interpret reaction profile diagrams

Rates of reaction is a straightforward topic, once the terminology is known. Think about and understand its definition – the change in the amount (or the concentration) of reactant (or product) per unit time. There are quite a number of different allowable units, e.g., moles per second, grams per second, cm3 per second, moles per minute, etc.

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