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Introduction to speed of reaction
Factors affecting speed of reaction
To measure speed of reaction, we can look at how much time is needed for reactants to be converted to products. The shorter the time needed, the faster the reaction.
In the example of the reaction between magnesium and sulphuric acid, there are 3 ways to measure the speed of reaction.
*Take note that the chemical equation for this reaction is Mg + H2SO4 → MgSO4 + H2
(i) By measuring the volume of hydrogen gas produced at regular time intervals (e.g. every minute).
(ii) By measuring the mass of the reactants (magnesium and sulphuric acid) at regular time intervals (notice that mass of the reactants will decrease over time as hydrogen gas which is produced will escape into the air).
(iii) By measuring the change in pressure of gas at regular time intervals (for reactions which produce gases, pressure of the gas increases as the volume of gas produced increases).
Factors affecting speed of reaction:-
(i) Presence of catalyst or inhibitor.
(ii) Temperature at which reaction takes place.
(iii) Concentration of reactants.
(iv) Pressure of gaseous reactants (note that pressure is applicable to gases only since particles in solids and liquids are already quite close to one another).
(v) Surface area of reactant.
Note that in the factors (iii) to (v), one important understanding is that increase in the factor will result in more frequent collisions allowing the speed of reaction to be faster. For factor (i), the presence of a catalyst lowers the activation energy needed for a chemical reaction to proceed, thus speed of reaction increases. For factor (ii), increase in temperature allow particles to have more energy to react more often when they collide, thus speed of reaction increases. For temperature, the understanding is higher temperature results in more reactant particles having more energy required to break bonds (recall that bond breaking is endothermic process) for the reaction to proceed at faster rate. This is a more accurate understanding of temperature as a factor affecting rate of reaction rather than temperature increasing the frequency of collisions.
In the example of the reaction between magnesium and sulphuric acid, there are 3 ways to measure the speed of reaction.
*Take note that the chemical equation for this reaction is Mg + H2SO4 → MgSO4 + H2
(i) By measuring the volume of hydrogen gas produced at regular time intervals (e.g. every minute).
(ii) By measuring the mass of the reactants (magnesium and sulphuric acid) at regular time intervals (notice that mass of the reactants will decrease over time as hydrogen gas which is produced will escape into the air).
(iii) By measuring the change in pressure of gas at regular time intervals (for reactions which produce gases, pressure of the gas increases as the volume of gas produced increases).
Factors affecting speed of reaction:-
(i) Presence of catalyst or inhibitor.
(ii) Temperature at which reaction takes place.
(iii) Concentration of reactants.
(iv) Pressure of gaseous reactants (note that pressure is applicable to gases only since particles in solids and liquids are already quite close to one another).
(v) Surface area of reactant.
Note that in the factors (iii) to (v), one important understanding is that increase in the factor will result in more frequent collisions allowing the speed of reaction to be faster. For factor (i), the presence of a catalyst lowers the activation energy needed for a chemical reaction to proceed, thus speed of reaction increases. For factor (ii), increase in temperature allow particles to have more energy to react more often when they collide, thus speed of reaction increases. For temperature, the understanding is higher temperature results in more reactant particles having more energy required to break bonds (recall that bond breaking is endothermic process) for the reaction to proceed at faster rate. This is a more accurate understanding of temperature as a factor affecting rate of reaction rather than temperature increasing the frequency of collisions.
Collision theory
Introduction to interpreting graphs on speed of reaction
Kindly ignore the first minute of the video as it is not relevant to this topic. Also ignore the last part of the video which discusses the equilibrium constant k which is not relevant to O level chemistry.
Worked examples on speed of reaction (with interpretation of graphs)
Experiment to show concentration of reactants affecting speed of reaction
Do note that it is more accurate to use the word "particles" instead of "molecules" when describing reactants as not all reactants exist as molecules! Some reactants exist as ions!