What NEET Asks
- Direct definition of rate of reaction and its units.
- Writing rate expressions for given balanced chemical equations.
- Relating rates of disappearance of reactants to rates of formation of products using stoichiometry.
- Distinguishing between average and instantaneous rates.
Key Points
- Rate of Reaction: Change in concentration of reactants or products per unit time.
- Units: Typically mol L⁻¹ s⁻¹ (or M s⁻¹). For gaseous reactions, atm s⁻¹ can also be used.
- Always Positive: The rate of a reaction is conventionally expressed as a positive quantity.
- Reactants vs. Products: Rate of disappearance for reactants (negative change), rate of appearance for products (positive change).
- Stoichiometry: The rate expression for a general reaction
aA + bB → cC + dDinvolves dividing by stoichiometric coefficients.
Must-Know Formula / Reaction
For a general reaction: aA + bB → cC + dD
Rate of Reaction = - (1/a) d[A]/dt = - (1/b) d[B]/dt = + (1/c) d[C]/dt = + (1/d) d[D]/dt
d[X]/dt: Change in concentration of species X over time.a, b, c, d: Stoichiometric coefficients of A, B, C, D respectively.- Negative sign: Indicates decrease in concentration of reactants.
- Positive sign: Indicates increase in concentration of products.
Common Mistakes
- Students often forget to divide by the stoichiometric coefficient when writing the overall rate expression.
- Don't confuse the rate of disappearance/appearance of a specific species with the overall rate of reaction (they are related by stoichiometry).
- Forgetting to apply the negative sign for reactants, or incorrectly applying it for products.
Rapid Revision
Rate is always positive, defined as change in conc. per time. For aA → bB, Rate = -1/a d[A]/dt = +1/b d[B]/dt. Units are typically mol L⁻¹ s⁻¹. Use stoichiometry carefully for interconversion of rates.