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:
- the reacting molecules have greater energy, and
- 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.
The technical words in the context of catalysts are:
- Negative catalyst
- Catalyst poison
- Catalyst promoter
- Homogeneous catalysis
- Heterogeneous catalysis
- Activation energy
- Intermediate compound formation
- Surface adsorption
The following catalysed reactions are on the Leaving Certificate syllabus:
|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.