Rate of Reaction NEET Cheat Sheet: Everything You Need

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.
  • Δt represents the time interval.
• Instantaneous Rate: r_inst = -d[R]/dt (for reactants) or r_inst = +d[P]/dt (for products).
  • 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.