How fast or slow should a reaction be?
A reaction could occur very fast or very slowly depending on a lot of factors like temperature, pressure, concentration etc.
It is pertinent to understand the basic chemistry of Chemical Kinetics
Rate of reaction is defined as the amount of reactants converted to products per unit time.
During a chemical reaction, the amount of reactants decreases while amount of product increases.
There are various parameters that can be used to measure the rate of reactions.
Parameters used in measuring the rate of reaction
| 1. Time to arrive at an observable seen product |
| 2. Amount of products formed per u nit time |
| 3. Amount of reactants converted to products per unit time |
| 4. Amount of precipitates formed per unit time |
| 5. Change in the intensity of color |
When we talk about the speed of a reaction i.e the rate of a reaction, It is important to understand that there are factors that determines or influences how fast or slow a reaction will occur.
While some reactions cold be spontaneous in nature, some other reactions will not occur unless they are quickened by these factors like temperature.
A spontaneous reaction is a reaction that can occur on its own without aid.
There are various factors that influence the rate of reactions.
| 1. Temperature |
| 2. Pressure |
| 3. Concentration |
| 4. Catalyst |
| 5. Presence of light |
| 6.Surface area |
I will briefly talk about the few factors and how they can make a reaction to occur very fast, fast, slow or very slowly, but I need to mention that most organic reactions occur very slowly.
Effect of Temperature
Temperature is one of the main factors that influence the rate of chemical reactions.
This is how temperature increase the rate of chemical reactions; An increase in temperature will first increase the average kinetic energy of reactant particles which will then help the particles overcome the activation energy and then increase the frequency of collisions leading to more successful collisions.
So generally, reactions occur faster at higher temperatures due to the fact that reactant particles have higher energy contents.
Recall that from kinetic theory, temperature is a measure of the average kinetic energy of particles.
Effect of concentration
Another important factor that do make reaction go fast or slow is the effect of concentration.
Concentration could be seen in the form of mass for solids and mol/dm3 for aqueous solutions but the explanation remains the same .
So in concentration, we discovered that the more concentrated (i.e more particles in a smaller space), the more the particles will hit each other thus increasing the frequency of collision and thus increasing the frequency of effective collision and finally rate of reaction.
Generally, a reaction will proceed faster when the concentration increases and will slow down when tyhe concentration decreases.
Effect of surface area
Surface area refers to the amount of solid particles in contact with the liquid or aqueous medium of the other reactant.
This can be evidenced in the reaction of calcium carbonate. with hydrochloric acid
CaCO3 + HCl==CaCl2 + H2O +CO2
if we use lumps of solid calcium carbonate and powdered calcium carbonate, the results wont be the same.
The powdered calcium carbonate will react faster than the lumps of solid calcium carbonate.
The reason is not far-fetched., in the powdered calcium carbonate more particles will be in contact with the aqueous medium and thus increasing the frequency of collision which then make the reaction to occur faster.
While the lump solid calcium carbonate has less number of particles in contact with the medium leading to decreased frequency of collision and thus decreased rate of reaction.
Effect of Pressure
Pressure can affect the rate of gaseous reaction by increasing the frequency of collision when the volume is decreased.
Effect of Catalyst
A catalyst is a substance that when added to a reaction speed up the rate of reaction by decreasing the activation energy.
There is an energy barrier or activation energy that inhibits reactions from occuring.
Activation energy or Energy barrier is the minimum amount of energy that reactant particles must attain or overcome before reactions will take place.
If the activation energy is high, then the particles will struggle in order to equal or overcome. the activation energy and products will be formed.
Also, if the activation energy is low, the particles will easily overcome the activation energy and reaction will occur faster.
Generally, reactions with low activation energy occur faster while reactions with high activation energy occur slowly?
What is the effect of a catalyst on rate of reaction?
A positive catalyst is added to speed up a reaction by decreasing the activation energy since activation energy inhibits the reaction from occurring
Effect of light
The presence of light can also influence a light. A reaction can be very sensitive to light and tends to occur very fast when it is exposed to light.
A photocehemical reaction is a reaction that can only proceed in the presence of light
For example, the halogenation of alkanes is a photochemical reaction and proceed fast in diffused light but very fast in bright light.
Summary of the factors affecting rates of reaction
| Factors | How they affect |
| Temperature | Temperature affect the rate of reaction by increasing average kinetic energy and frequency of collision. |
| Concentration | Increase in concentration increases frequency of collision. |
| Surface area | Large surface area increases the frequency of collision and thus rate of reaction |
| Presence of light | The presence of light increases the rate of reaction for p[hotochemical reaction |
| Catalyst | A catalyst increases the rate of reaction by decreasing the activation energy |
| pressure | Pressure increases the rate of reaction for gaseous reactants |
A reaction could go very fast or slow depending on these factors listed above and it is important to understand how to monitor how to manipulate these rates of reaction
