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Factors Affecting Rate of Reaction

The collision theory that effectively describes the reaction mode and rate could also utilized to understand the factors that affect the overall reaction rates. The temperature is found to be very unique factor as far as the relationship goes for temperature and kinetic energy. Kinetic energy although invariably depends upon the mass and velocity is also a measure of temperature. Similarly the concentration and rest of factors that help understand the rate of reaction gives us ample idea for the order of reaction, and rate law.

It was Van’t Hoff in 19th century who gave a lot of specific rules with regard to the reaction mechanism and factors. For every category of factors there are specific designs to understand the process of reaction rate.

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Factors that Affect the Rate of Reaction

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The five factors which help in understanding the reaction rate of a chemical change are:
(a) nature of the reactants, (b) the concentration of the reactants (c) the surface area of the reacting substances, (d) the temperature of the reaction system, and finally the catalysts used for the reaction.

Nature of reactants:

Some of the chemical compounds are found to be more reactive than the rest and since the change that take place between reactants are carried out in ionic forms hence the chemical reactive state is very important to determine the rate of such reactions.
In general the chemical reactions that takes place between the ions in aqueous solutions are quite rapid as compared to the slow reaction between the covalent molecules.

Concentration of reactants:

The reaction state of any chemical change increases many fold as we increase either or both of the reactant molecules concentrations. This is very much true for collision theory as we have more molecules in a given space and we could observe more collision rate amongst the particles and hence more chances of product formation.

Hence if we double the concentration of molecules of one reactant, it will definitely collide in every second twice as many times as before with the next reacting molecule. Since the rate of reaction depends upon the effective collision number per second the rate would definitely go up.
$CH_{4}$ + $2 O_{2} \rightarrow CO_{2}$ + $2 H_{2}O$ + $\Delta heat$

$2 CH_{4} + 4 O_{2} \rightarrow 2 CO_{2} + 4 H_{2}O + \Delta heat$

Surface area of the reactants:

In case the reactants are found to be a combination of solid and liquid or gas then the other reactant gets to collide only with the surface of the solid reactant. Hence the surface area of the solid is considered as its concentration. So by increasing the surface area of this solid reactant we could also increase the rate of reaction.

The surface area of the solid could be increased by grinding the solid into powder. As the grinded solid will have more surface area for the other reactant to cling on, hence the increase in reaction rate.

Temperature of the reacting system:

By increasing the temperature of the system the reactants average molecular kinetic energy goes up which makes them more energetic than before. This also signifies that more molecules will have the energy required more or equal to the activation energy.
This finally leads to increase in reaction rate with the increase in temperature.
The thumb rule is that for every rise of 10 degree C the rate of reaction goes up by twice and applies to many reactions.

Catalysts of the reacting system:

Catalysts are substances which helps in raising the rate of reaction but without being consumed during the process. The catalysts help in increasing the rate of reaction by lowering the activation energy very much essential for the reaction getting started.
Hence the presence of catalysts help in getting more molecules across the energy barrier than without it. The catalysts that help in getting the chemical reactions carried out within our body are called enzymes.

Kinetics - Factors Affecting Rate of Reaction Experiment

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The chemical kinetics is the reaction rate study where all the factors which influence the reaction are measured against. The rate of reaction of a chemical change is the amount of reaction that occurs in unit time. These are measured against the changes that we carry out one by one in temperature, concentration, catalyst and nature of the substance. When we bring about the changes in reacting system the molecules of the system starts colliding but not every collision will bring about the chemical change.

The molecules which possess enough energy to cross over the threshold value will react in a collision and finally produce products. This threshold energy is called the activation energy which is considered as a basic characteristic of a reaction. 

Threshold Energy

Experiment for Concentration

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Putting up an iodine clock reaction is a convenient reaction to observe the concentration effects and the reaction between potassium iodate and sodium bisulphite gives a net ionic reaction. The overall reaction rate could be observed by the disappearance of the bisulphite and this could be carried out by adding more of iodate than bisulphite at the beginning of reaction.

Once all the bisulphites are over, the remaining iodate slowly turn into iodide and finally into Iodine. The using up of bisulphite and appearance of Iodine at the end could be monitored with the help of a stop watch and hence the name iodine clock.

Experiment for Nature of Substance

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A certain volume of HCl of a known pH is put into three test tube. The test tubes are numbered and polished pieces of magnesium, zinc and copper are put inside the test tubes respectively. The reaction in all three test tubes are monitored to understand the importance of nature as far chemical reaction is concerned. Magnesium will react the fastest with HCl and will displace H from HCl followed by Zinc with HCl and finally the least reactive would be the copper strip.

Experiment for Temperature of the System

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Reaction between HCl and sodium thiosulphate $Na_{2}S_{2}O_{3}$ is recorded with a timer both at low temperature and higher temperature. Since the rate of reaction is directly proportional to temperature hence the following relation would be applicable.

Rate of reaction $\propto$ $\frac{1}{time}$.

As soon the temperature is raised the time for the completion is reduced as well.

Factors Affecting Rate of Reaction Nature of Reactants

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The nature of reactants as far the reaction rate is concerned is very important as the chemical substances are categorised according to their position in reactivity series.

Higher the position of the elements in reactivity series, higher would be the rate of reaction. Hence a substance like Aluminium would react faster and produce products faster than metals like Zinc, as Aluminium is higher in order of reactivity.

There are reactivity series for both metals and non-metals and hence elements higher on either of these reactivity series would react faster and produce more products faster and easily.

Factors Affecting Rate of Reaction Surface Area

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The surface area of substance is very important as it’s a kind of concentration for solids if one of the reactant in reacting medium is a solid. If the surface area of the solid is less then the other reactant get less surface to react with. Finally the reaction might slow down or cease. As soon the solid is grinded and added to the other reacting substance the reaction rate goes higher and produce products faster.

Solid (un-grinded) + second reactant $\rightarrow$ slow reaction
Solid (grinded) + second reactant $\rightarrow$ fast reaction
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