The rate of a reaction, which may also be called its velocity or speed, can be defined with relation to the concentration of any of the reacting substances, or to that of any product of the reaction. If the species chosen is a reactant which has a concentration c at time t the rate is - dc/dt, while the rate with reference to a product having a concentration x at time t is dx/dt. Any concentration units may be used for expressing the rate, thus, if moles per liter are employed for concentration and seconds for the time, the units for the rate are moles `"liter"^(-1) sec^(-1)`. For gas reactions pressure units are sometimes used in place of concentrations, so that legitimate units for the rate would be (mm. Hg) `sec^(-1)` and atm. `sec^(-1)`
The order of a reaction concerns the dependence of the rate upon the concentrations of reacting substances, thus, if the rate is found experimentally to be proportional to the `alpha^(th)` power of the concentration of one of the reactants A, to the `beta^(th)` power of the concentration of a second reactant B, and so forth, via.,
rate `kC_(A)^(alpha) C_(B)^(beta)" " (1)`
the over-all order of the reaction is simply
`n = alpha +beta + ` ---------(2)
Such a reaction is said to be of the `alpha^(th)` order with respect to the substance A, the `beta^(th)` order with respect to B and so on...
(Laidler, K. J., & Glasstone, S. (1948). Rate, order and molecularity in chemical kinetics. Journal of Chemical Education, 25(7), 383.)
In the following questions, a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices on the basis of the above passage.
Assertion: Rate of reaction is a measure of change in concentration of reactant with respect to time.
Reason: Rate of reaction is a measure of change in concentration of product with respect to time.

Options

A. Assertion and reason both are correct statements and reason is correct explanation for assertion.
B. Assertion and reason both are correct statements but reason is not correct explanation for assertion.
C. Assertion is correct statement but reason is wrong statement.
D. Assertion is wrong statement but reason is correct statement.

Answer

To solve the question, we need to analyze the assertion and the reason provided in the context of the rate of a chemical reaction.

**Step 1: Understand the Assertion**
The assertion states: "Rate of reaction is a measure of change in concentration of reactant with respect to time."
- This is true because the rate of a reaction can indeed be defined as the change in concentration of a reactant over time. Mathematically, this can be expressed as:
\[ \text{Rate} = -\frac{dc}{dt} \]
where \(c\) is the concentration of the reactant.

**Step 2: Understand the Reason**
The reason states: "Rate of reaction is a measure of change in concentration of product with respect to time."
- This is also true because the rate of a reaction can be expressed in terms of the change in concentration of a product over time. This can be mathematically expressed as:
\[ \text{Rate} = \frac{dx}{dt} \]
where \(x\) is the concentration of the product.

**Step 3: Analyze the Relationship Between Assertion and Reason**
Both the assertion and the reason are true statements about the rate of reaction. However, the assertion focuses on reactants while the reason focuses on products. They both describe different aspects of the same concept (the rate of reaction).

**Step 4: Determine if the Assertion is the Correct Explanation for the Reason**
While both statements are true, the assertion does not explain the reason. The assertion talks about reactants, and the reason talks about products. Therefore, the assertion is not the correct explanation of the reason.

**Conclusion: Choose the Correct Option**
Based on the analysis:
- Both the assertion and the reason are true.
- The assertion is not the correct explanation of the reason.

Thus, the correct answer is: **Option 2**.

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