Concentration of Reactants

The increase within the concentration of reactants is reflected in the speed of reaction as it will increase. Ions and molecules will react to create a brand new compound by increasing the amount of reactants. As the concentration of reactants decreases, fewer and fewer molecules and ions are present. That will eventually cause the rate of reaction to decrease. 

In gases, if we increase the amount, the concentration will rise, and the rate of reaction will also increase. When the concentration of reactants increases in the reaction, more molecules or ions interact and form newer compounds, and hence a net increase in the rate of reaction is observed.

Definition of Concentration

Concentration is a quantitative measure of the quantity/ amount of matter dissolved in a given quantity of solution. It is generally in terms of mass per unit volume.

Example

The quantity of pollutants in the air depends on the speed of the reaction. SO2 is an acidic chemical compound. It combines with vapour to supply H2SO3. The reaction is as follows:

SO2 + H2O → H2SO4

Then carbonate reacts with sulphuric acid:

 CaCO3 + H2SO3 → CaSO3 + CO2 + H2O

The concentration of reactant (pollutant) is high in impure air. Within the impure air, the CaCO3 degenerates more as compared to pure air.

On the other hand, phosphorus (P) starts to burn at an excellent rate in the presence of pure oxygen (O²) than in air.

Unit of Concentration

Concentration is typically measured in moles per litre.

Rate of reaction and its dependency on the nature of reactants

The speed at which a chemical reaction occurs is called the rate of the reaction. One of the factors guiding the reaction is the concentration of reactants. It is expressed in terms of the amount of product received per unit time. It means the concentration of the reactant is consumed in a unit time.

Rate of reaction = Change in concentration per unit time 

The nature of the reacting substance contributes significantly to the chemical process. Some reactions  might have completely different rates depending on the type of the reactants used. For instance, if small portions of Fe metal and Na are exposed to air, Fe has a minimal effect. However, the Na metallic element reacts quickly with air. H2 gas, along with a base, is formed as an end product of the reaction of active metal Ca with Na. Each reacts with water. Thus, Ca generally reacts at a moderate rate, whereas Na reacts very quickly. Its temperature is also quite high compared to Ca. Because of this, the reaction is explosive.

Chemical kinetics

Chemical kinetics plays an excellent role in physical chemistry as it closely involves the study of the reaction rate. It also deals with the results of several variables, e.g., temperature, the presence of catalysts, concentration of reactants, etc.

Factors affecting reaction speeds

• Concentration of reactants
• Temperature
• Catalysts
• Surface area
• Nature of reactants
•  

Relationship between the speed of reaction and the concentration of reactants

There happens to be a mathematical relationship between the speed of reaction and the concentration of reactants. 

Higher Concentration = Additional Collisions

Effect of concentration of gases

Let’s ignore the pressure of solids and liquids because the pressure in them is negligible. However, we are going to take into consideration the pressure of reaction of gases. There’s an interesting distinction between the pressure of reaction of solid, liquid and gases.

Examples

In the following example, there is one molecule of N2O4 and a couple of molecules of NO2. We notice an increase in the forward reaction by increasing the concentration of reactants. This unbalanced reaction can have an effect on the equilibrium. Sometimes, changes may occur due to the pressure of reactants:

2NO2(g) → N2O4(g)

Here is another example of gaseous reactants. It contains two molecules of a gaseous chemical and two molecules of a gaseous product. By increasing the pressure, the speed of the forward reaction increases, and there are great results on reversing the reaction. Where the reaction rate is equal, the equilibrium doesn’t shift in any direction.

H2(g) + I2 → 2HI(g)

Effects of change in concentration

The following example will depict the direction of the reaction based on the concentration of the reactants and products.

N2 +3H2 → 2NH3

On increasing the concentration of the reactant/ product,

Add N2 (reactant) → Rightward shift (towards products)

Add H2 (reactant) →Rightward shift (towards products)

Add NH3 (reactant) → Leftward shift (towards reactants)

By decreasing the concentration of the reactant or product,

Remove N2 (reactant) → Leftward shift (towards reactants)

Remove H2 (reactant) → Leftward shift (towards reactants)

Remove NH3 (product) → Rightward shift (towards products)

Conclusion

From the given data above, it can be concluded that the more effectively the collisions take place, the quicker reaction rate is. In our hypothesis, we can thus come to the conclusion that increasing the concentration of reactants will increase the rate of the reaction. The rate of a chemical reaction can be understood as the rate at which reactants are being used up, or products are being formed.

Change in pressure brings no difference in the equilibrium of the reaction if the concentration of the reactant gaseous molecules and gaseous product molecules remain equal. So, if the amounts of gas reactant and gas product are the same, hardly any effect is observed on the equilibrium. If the concentrations of reactant molecules of the gas and gaseous product are different, there is a great effect on the equilibrium of the reaction.