Rate of Reaction NEET Cheat Sheet: Everything You Need
Rate of ReactionChemical KineticsNEET ChemistryNEET 2025Average RateInstantaneous Rate
Rate of Reaction NEET Cheat Sheet: Everything You Need
Chemical Kinetics·2 min read·NEET 2026
What NEET Asks
Direct conceptual questions on definitions of average and instantaneous rates.
Numerical problems calculating rates using concentration changes and stoichiometry.
Graphical interpretation of rate of reaction.
Key Points
Rate of Reaction: Change in concentration of reactants or products per unit time.
Units: Typically Moles per litre per second (mol L⁻¹ s⁻¹) or atm s⁻¹ for gaseous reactions.
Average Rate: Rate measured over a finite time interval (Δt). Calculated as Δ[conc]/Δt.
Instantaneous Rate: Rate at a specific moment in time (t). Represented as d[conc]/dt, which is the slope of the tangent to the concentration vs. time curve at that point.
Stoichiometry: For a reaction aA + bB → cC + dD, the overall rate is expressed as: Rate = -(1/a) Δ[A]/Δt = -(1/b) Δ[B]/Δt = +(1/c) Δ[C]/Δt = +(1/d) Δ[D]/Δt.
Must-Know Formula / Reaction
Average Rate:r_avg = -Δ[R]/Δt (for reactants) or r_avg = +Δ[P]/Δt (for products).
Δ[R] or Δ[P] represents change in molar concentration of reactant/product.
d[R]/dt or d[P]/dt represents the infinitesimal change in concentration over an infinitesimal time period.
Common Mistakes
Students often forget to include the negative sign for reactants (rate of disappearance) to keep the overall reaction rate positive.
Don't confuse average rate with instantaneous rate – one is over an interval, the other at a point.
Always remember to divide by the stoichiometric coefficient when relating the rate of disappearance/appearance of a specific species to the overall rate of reaction.
Rapid Revision
Rate is concentration change per unit time. Average rate (Δ[conc]/Δt) is for an interval; instantaneous rate (d[conc]/dt) is at a point (tangent slope). Negative sign for reactants, positive for products. Divide by stoichiometric coefficient for overall reaction rate.
Frequently Asked Questions
What is the key difference between average and instantaneous rate of reaction?▾
The average rate of reaction is calculated over a measurable time interval (Δt) and gives an overall change. The instantaneous rate, however, is the rate at a specific moment in time (dt), representing how fast the reaction is proceeding at that precise instant, and is determined by the slope of the tangent to the concentration-time curve.
Why is a negative sign used for the rate of disappearance of reactants?▾
The concentration of reactants decreases over time, so the change in concentration (final - initial) will be negative. To ensure that the rate of reaction is always a positive value, a negative sign is prefixed when expressing the rate in terms of reactants. For products, concentrations increase, so a positive sign is used.
How does stoichiometry affect the rate expression for a general reaction?▾
For a general reaction `aA + bB → cC + dD`, the rates of disappearance of reactants and appearance of products are related by their stoichiometric coefficients. To express the overall rate of reaction, the rate of change of concentration of each species is divided by its respective stoichiometric coefficient, making all terms equal.
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