Rate of a Reaction

The Rate of a Chemical Reaction or Reaction Rate is defined as the speed or the rate at which a chemical reaction takes place. The rate of a Chemical Reaction is directly proportional to the increase in the concentration of a product per unit of time and to the decrease in the concentration of a reactant per unit of time. This can varies drastically. Chemical reactions proceed at extensively different speeds depending on the nature of the reacting substances, the type of chemical transformation, the temperature, and other factors. Let’s understand more about the Rate of a chemical reaction in this article, by understanding important concepts like its Rate of a Chemical Reaction Definition, Formula, Factors affecting it, different types, and FAQs on them.

What is Rate of Reaction?

The rate of reaction is defined as the change in the concentration of any one of the reactants or products per unit of time.

A reaction rate is the speed at which a chemical reaction occurs. A low-rate reaction means that the molecules combine at a slower speed than in a high-rate reaction. Some reactions can take hundreds, if not thousands, of years to occur, whereas others can occur in less than a second. Consider how long it takes plants and ancient fish to become fossils if you want to conceive of an extremely slow reaction (carbonization). The speed of reaction is also affected by the type of molecules combined. The reaction will be slower if an essential element or compound is present in low concentrations.

Rate of Reaction Formula

The rate of reaction is proportional to the increase in the concentration of a product per unit time and to the decrease in the concentration of a reactant per unit time. And can be defined as,

Rate of reaction = Decrease in the concentration of a reactant R / Time interval

or

Rate of reaction = Increase in the concentration of a product P / Time interval

If we consider, [R₁] and [P₁] are the molar concentrations of the reactant and the product respectively at any time t₁ and [R₂] and [P₂] are the concentrations of reactant and product at time t₂, then changes in concentrations of the reactant and product will be d [R] = R₂ – R₁ and d [P] = P₂ – P₁ and time interval is dt = t₂ – t₁  and the rate of reaction in terms of reactant or product is given by

Rate of reaction = -(R₂ – R₂)/ (t₂ – t₁) = + (P₁ – P₂)/(t₂ – t₁)

or

Rate of reaction = – d[R]/dt =+ d[P]/dt.

Significance of Negative and Positive Signs

The sign in the rate of reaction tells about the increase and decrease in the concentration. 

• The negative sign indicates that the concentration of the reactant is decreasing.
• The positive sign indicates that the concentration of the product is increasing.

Unit of Rate of Reaction

The Rate of Reaction in general can be measured as concentration is denoted in moles/liter and time in seconds or minutes. 

So the unit of rate of reaction = mol L^-1 s^-1or mol L^-1 min^-1.

Factors affecting Rate of Reaction

There are various factors that affect the reaction rate which are-

• Reactants concentration- Faster the reaction concentration, the more quickly is the reaction to occur. Also as the concentrations of the reactants decrease, the rate of reaction decreases.
• Temperature- The rate of reaction increases with an increase in the temperature. In many cases, the rate of reaction becomes nearly double for a 10° rise in temperature. There are a few reactions that do not take place at room temperature but take place at high temperatures.
• Presence of light- Some reactions do not take place in the dark but can proceed in the presence of light. For Example- H₂ + Cl₂ ⇢ 2 HCl. This reaction is known as a “photochemical reaction.”
• Presence of Catalyst- The main purpose of the catalyst is to increase the rate of reaction without itself involving in the reactions. So the catalyst increases the reaction rate.
• Surface area- Greater surface area means more collision to take place and hence, greater is the rate of reaction.

Instantaneous Rate of Reaction

The rate of reaction at any instant of time is the rate of change of concentration of any one of the reactants or products at that particular instant of time. Let us suppose that the small change in concentration is given dx in the small interval of time dt. Then the rate of reaction at that instant is given by dx/dt. 

Mathematically, the Instantaneous rate of reaction is given by,

r_inst = dx/dt

r_inst = – d[R]/dt 

= d[R]/dt as dt ⇢ 0

Average Rate of Reaction

From beginning to end the rate of reaction does not remain the same, it can vary from time to time. Therefore, the rate of reaction is defined as the ‘average rate of reaction’.

Mathematically, the Average rate of reaction is given by, 

r_av = – d[R]/dt =+ d[R]/dt

In general, 

r_av = dx/dt

Expressing the Rate of Reaction in terms of different Reactants and Products 

To understand this let us consider a reaction – a A + b B ⇢  x X + y Y

Then the rate of reaction is given by-   

Rate = – 1/a d[A]/dt = – 1/b d[B]/dt = + 1/x d[X]/dt = + 1/y d[Y]/dt

where d[A], d[B] represent small decrease in the concentrations of A and B respectively and d[X] and d[Y] represent small increase in the concentrations of X and Y respectively in the small interval of time dt.

Difference between Rate of Reaction and Rate Constant

Rate of Reaction	Rate Constant
The rate of reaction or the reaction rate is the change in the concentration of reactants or the change in the concentration of products per unit of time.	The rate constant is the proportionality constant related to the rate of a particular reaction.
It depends on the molar concentration of Reactants and Products.	It doesn’t depend on the molar concentration of Reactants and Products.
It indirectly depends on the temperature.	While this directly depends on the temperature
Also, it is time-dependent.	However, this is time-independent.

FAQs on Rate of Reaction

Question 1: Define the Rate of Reaction

Answer:

The rate of reaction is defined as the change in the concentration of any one of the reactants or products per unit of time.

Question 2: How is Activation Energy related to the Rate of Reaction?

Answer:

The activation energy is related to the rate of reaction. The rate of reaction decreases as the activation energy increases. Molecules can only complete the reaction after passing through the activation energy barrier.

Question 3: What is the Average and Instantaneous Rate of Reaction?

Answer:

Instantaneous Rate of Reaction: The rate of reaction at any instant of time is the rate of change of concentration of any one of the reactants or products at that particular instant of time.

Average Rate of Reaction: From beginning to end the rate of reaction does not remain the same, it can vary from time to time. Therefore, the rate of reaction is defined as the ‘average rate of reaction’.

Question 4: Which Factor has no influence on the Rate of Reaction?

Answer:

The Molecularity of any reaction has no influence on the Rate of Reaction.

Question 5: What is the SI Unit of the Rate of Reaction?

Answer:

The unit of rate of reaction = mol L^-1 s^-1or mol L^-1 min^-1.