Factors That Affect The Rates of Chemical Reactions

FACTORS AFFECTING THE RATES OF CHEMICAL REACTION

Chemical reactions go at different rates (speed) due to there chemical properties/makeup. But these rates can be changed inorder to attain the desired outcome of a reaction by applying different factors.

1. The nature of reactants: Particles of matter are usually in the form of molecules, atoms, and ions, but in order of increasing reactivity (molecules < atoms < ions), ions react more fast than the others. Ionic compounds when reacting, break apart to form free ions, thus, there is no bond breaking which makes the reaction rate faster. But in covalent compounds, atoms/molecules needs to break bonds first before reacting, hence there reactions are slower. In terms of physical states of matter (solid, liquid, and gas), gases react very fast than liquids, and liquids to solids. Particles of a solid substance, only vibrate against each other when in motion. In liquids, the particles slide past each other. But in gases, they move very fast randomly which increase the frequency of effective collisions/reaction rates.
2. Concentration of reactants: The amount of a particular substance in a known quantity of another substance (solvent/solution) determines the concentration of the solution. When there are more (greater) reactant particles per unit volume of a solution, the solution is said to be concentrated, but when there are less reactant particles per unit volume of a solution, the solution is said to be dilute. An increase in the concentration of reactants makes the particles close enough to one another, which increases the collision between particles and hence, increases reaction rate. Also, the faster the rate, the shorter the time taken, and vice versa.
3. Temperature: Increase in the temperature of a system (reaction), increases the frequency of collision between reactants particles, and in turn, increases the rate (speed) of the reaction. When cooking rice for example, we lower the heat from the gas cooker to slow down the speed of the rice getting cooked, and we increase the heat, to get the rice done, fast. In terms of activation energy, at a higher temperature, reactant particles attain the activation energy which increases effective collision.
4. Surface area of reactants: Surface area is the amount of particles of a substance that is well exposed for any physical or chemical attack. Reactants with larger surface area would react faster than the ones with small surface area. When a substance like iron nail is ground to its powdered form, iron filing, it makes the particles of the iron well exposed to any reaction, than when it is still in its nail form, where it would take time for any reaction to get all through the particles of the nail. - Powdered CaCO3(s) will react readily with HCl(aq) than when CaCO3 is in its lump or chips form. - Zinc dust in any reaction will react readily than when it is in its rod, granules or foil form.
5. Presence of a catalyst: A catalyst is a substance which when used in a reaction, changes the reaction rate (speed) without being used up i.e, reacting with the reactants in the reaction. A catalyst either increases the reaction rate, if it is a promoter, or decreases the reaction rate, if it is an inhibitor. Catalysts generally have an effect on the activation energy, EA of a reaction. Normally, reacting particles in a reaction attain the activation energy inorder to acquire effective collisions, but when a catalyst (promoter) is used in a reaction, it increases the rate of the reaction by creating a new reaction path which has a lower activation energy than the supposed original path of the reaction. So if a catalyst (inhibitor) is used in a reaction, it decreases the reaction rate by creating a new reaction path which has a higher activation energy than the supposed original path of the reaction.
6. Pressure: Pressure effect relates to gases. According to Boyle's law, V = 1/P at constant temperature, an increase in pressure, P on a given mass of gas leads to a decrease in volume, V. So then, when the volume of a solution is reduced, of course, the concentration of the reactants in the solution would increase i.e there will be more reactant particles per unit volume. And increase in concentration of reactants leads to the closeness between particles thereby particles, having more frequent collision, which in turn, increases the rate of reaction. Conversely, a decrease in pressure on a gaseous system decreases reaction rate.
7. Light or radiation: Photochemical/photocatalytic reactions are reactions that take place only in the presence of sunlight. This sunlight provides the required energy to make the reactant particles collide frequently, leading to a successful collision, and hence, increases the rate of reaction.

- In photosynthesis, glucose is formed by plants in the presence of sunlight.

- Hydrogen and chlorine form hydrogen chloride readily on exposure to sunlight, or when radiation is absorbed.