The increase in the concentration of reactants, the rate of reaction increases. Ions and molecules interact to form a new compound on increasing the concentration. The decrease in the concentration of reactants, fewer molecules and ions are present and the rate of reaction decreases.
In the reaction of gases, if we increase the pressure there will be an increase in concentration and the rate of reaction also increases.
Example
Degenerated reaction of calcium carbonate obtained by the pollutant sulfur dioxide. The amount of sulfur dioxide in the air depends on the rate of reaction. SO2 is an acidic oxide, it combines with water vapor to produce H2SO3. The reaction is as follows:
SO2 + H2O = H2SO3
Then calcium carbonate reacts with sulfurous acid
CaCO3 + H2SO3 = CaSO3 + CO2 + H2O
The concentration of Sulfur dioxide is high in a polluted air. In the polluted air the calcium carbonate more degenerates as compared to less polluted air.
On the other hand, Phosphorus burns at a great rate in a pure oxygen than in air.
Unit
Concentration is usually measured in moles per liter.
Definition of the Reactant
It is a speed at which chemical reaction proceeds. Reactants are converted into products. It is expressed in terms of concentration (amount per unit time) of the product. It means the concentration of the reactant is consumed in a unit of time.
Rate= change in concentration/change in time
Nature of Reactants
Nature of the reacting substance has a major contribution in the chemical reaction. Same condition reactions may have different rates depending on their identity of reactants. For example, the small section of iron metal and sodium is exposed to air, iron have a little effect but the sodium reacts completely with air overnight. The hydrogen gas and a base is formed by the reaction of active metal calcium and sodium. Both react with water. Thus, calcium reacts at the moderate rate whereas, sodium reacts so quickly and its temperature is too high, due to this, the reaction is so much explosive.
Definition of the Concentration
A quantitative measure of the amount of solute dissolved in a given quantity of solution. It is expressed in terms of mass per unit volume.
Chemical Kinetics
Chemical kinetics plays a great role in physical chemistry. It involves the study of the reaction rate. It deals with the effect of different variables e.g. temperature, concentration.
Factors That Affect the Rate of Reaction
- Concentration
- Temperature
- Catalysts
- Surface area
- Nature of reactants
Relationship between the Rate of Reaction and Concentration of Reactants
There is a mathematical relationship between the rate of reaction and the concentration of reactants. The rate of reaction is directly proportional to the number of collisions per second between the reactant molecules. If the concentration decreases with time the number of collisions also decreases. It depends upon the frequency of the total number of collisions. If the frequency increases then the rate of reaction also increases. The composition of this reaction is known as chemical kinetics.
Higher concentration = More collisions
CONCENTRATION AFFECT THE RATE OF REACTION
Reaction involving two particles
According to the collision theory, the reaction is the same whether the collision between two different particles or collisions between the same particles. Particles first collide in any reaction, whether both particles are in solution or one in solution or other one is solid. On the higher concentration, higher will be the collision.
Reaction involving one particle
The orientation of the collision of the molecules is irrelevant. Suppose that at any one time 2 in a million particles have enough energy to equal or exceed the activation energy. If you have 300 million particles then 300 of them would react. By doubling the concentration, double will be the rate of reaction.
CONCENTRATION DOESN’T AFFECT THE RATE OF REACTION
Following are the cases where the concentration doesn’t affect the rate of reaction
Working of catalyst on its maximum capacity
Suppose, a small amount of solid catalyst is using in a reaction and concentration of the reactant in a solution is too high. So, the reacting particles were muddled up on a catalyst surface. If we increase the concentration of reactants then it also doesn’t have any effect because the catalyst is already working on its maximum capacity.
Multi-step reactions
Suppose we react to different variables. Some are of high spend of some of slow. The reaction occurs in a series of small paces. If A and B react together in these different steps of the reaction.
By increasing the concentration of A, how fast reaction dissociates A to make X and Y.
By decreasing the concentration it will reduce the rate of the second reaction.
EFFECT OF CONCENTRATION
By increasing the concentration of reactant or product
Add N2 (reactant) = shift Right (toward products)
Add H2 (reactant) = shift Right (toward products)
Add NH3 (reactant) = shift left (toward reactants)
By decreasing the concentration of reactant or product
Remove N2 (reactant) = shift Left (toward reactants)
Remove H2 (reactant) = shift Left (toward reactants)
Remove NH3 (product) = shift Right (toward products)
How to find the concentration of reactants
To find the concentration reactants, the conversion will still the same throughout the reaction. Suppose a reaction between A and B. the reaction rate is zero concerning B. The rate of reaction is independent of the concentration of reactant B.
Concentration of reactants of solid and liquid
The reaction rate of solids and liquids is increasing by increasing the surface area. If the surface area of solid is increasing then there is a decrease in particle size.
Concentration of reactant of gases
We will ignore the pressure of solids and liquids because there pressure is so small but we will not ignore the pressure of reaction of gases that contain gases. There is a remarkable difference between the pressure of reaction of solid, liquid and gases.
If we write the reactants and products in two sides of the chemical equation then the reaction is carried out in a single container, both reactants and products are in a same container. So, that we cannot assume the increasing pressure of products or reactants. Therefore, the pressure is changed in a reaction and this pressure is caused by the change of volume in a reaction container. Volume of the container is changed but it doesn’t directly change the number of particles in a container. Volume of each unit is changed and it also changes the pressure of the reaction.
Volume increase is number of collisions per unit of surface area decreases lower the pressure
There are less number of molecules of reactant and products per unit volume.
As we know, change in concentration has an effect to an equilibrium. We need to be very careful when we increase the concentration of both reactant and product molecules.
There is 1 molecule of gaseous product and 2 molecules of gaseous reactant. Thus, forward reaction have a greater effect by increasing concentration. This unbalanced on the reaction will affect the equilibrium. Change occurs due to change in pressure. There is another example for gaseous reactants.
It contain 2 molecules of gaseous reactant and 2 molecules of gaseous product. By increasing the pressure, the rate of forward reaction increases and there is a great effect on reverse reaction. In case of both reaction rates are equal then equilibrium is not shifted.
Changing pressure has no effect on equilibrium if the number of gaseous reactant molecules and gaseous product molecules are equal.
So, it is concluded that if the number of gaseous reactant and gaseous product are same then there is no effect on an equilibrium but if the number of molecules of gaseous reactant and gaseous product are different then there is a great effect on an equilibrium.
REFERENCES
- https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_General_Chemistry_(Petrucci_et_al.)/14%3A_Chemical_Kinetics/14.03%3A_Effect_of_Concentration_on_Reaction_Rates%3A_The_Rate_Law
- https://www.chemguide.co.uk/physical/basicrates/concentration.html
- https://www.britannica.com/science/reaction-rate
- https://www.chemguide.co.uk/physical/basicrates/concentration.html
- https://www.chem.tamu.edu/class/fyp/stone/tutorialnotefiles/acid-base/lechatelier.htm#concentration
- http://www.digipac.ca/chemical/mtom/contents/chapter3/chap3_4.htm