Rate law cannot be determined form balanced chemical equation if …….. .

Options

A. reverse reaction is involved
B. it is an elementary reaction
C. it is a sequence of elementary reactions
D. any of the reactants is in excess

Explanation

To solve the question "Rate law cannot be determined from balanced chemical equation if...", we need to analyze the conditions under which the rate law can or cannot be derived from a balanced equation.

### Step-by-Step Solution:

1. **Understanding Rate Law**:
The rate law expresses the relationship between the rate of a chemical reaction and the concentration of its reactants. It is typically determined experimentally.

2. **Elementary Reactions**:
- An elementary reaction is a single step reaction where reactants convert to products in one step. The rate law for an elementary reaction can be directly derived from its stoichiometry.
- Therefore, if the reaction is elementary, we can determine the rate law from the balanced equation.

3. **Complex Reactions**:
- For complex reactions, which involve multiple steps (elementary steps), the overall rate law cannot be determined solely from the balanced equation. The rate law must be derived from the rate-determining step or experimental data.

4. **Equilibrium Reactions**:
- In reactions that reach equilibrium, the forward and reverse reactions occur simultaneously. The rate law for the overall reaction cannot be determined just from the balanced equation because it depends on the concentrations of both products and reactants.

5. **Conclusion**:
- The rate law cannot be determined from a balanced chemical equation if the reaction is complex (not elementary) or if it involves equilibrium. Thus, the correct answer to the question is that the rate law cannot be determined from a balanced equation if the reaction is not elementary or if it is an equilibrium reaction.

### Final Answer:
Rate law cannot be determined from a balanced chemical equation if the reaction is complex or involves equilibrium.

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