Rate of a Chemical Reaction
Defines the speed at which reactants are consumed or products are formed, including average and instantaneous rates and their units.
What is Rate of a Chemical Reaction?
The change in concentration of a reactant or product per unit time.
Key formula / rule: Average Rate of Reaction
Key points
- Define the rate of a chemical reaction.
- Distinguish between average and instantaneous rates.
- Express the rate of reaction in terms of reactants and products using stoichiometric coefficients.
- Identify the units of reaction rate.
Common exam trap
Forgetting the negative sign for reactants.
Definitions
- Term
Rate of Reaction
- Meaning
The change in concentration of a reactant or product per unit time.
- Term
Average Rate
- Meaning
The rate of reaction calculated over a finite time interval.
- Term
Instantaneous Rate
- Meaning
The rate of reaction at a particular instant of time, determined by the slope of the concentration-time graph.
Learning objectives
Define the rate of a chemical reaction.
Distinguish between average and instantaneous rates.
Express the rate of reaction in terms of reactants and products using stoichiometric coefficients.
Identify the units of reaction rate.
Formulae
- Name
Average Rate of Reaction
- Note
Use '-' for reactants, '+' for products. Δt is the time interval.
- Expression
Rate = ± Δ[Concentration] / Δt
- Name
Rate Expression with Stoichiometry
- Note
Ensures rate is independent of the species measured.
- Expression
For aA + bB → cC + dD, Rate = -1/a (Δ[A]/Δt) = -1/b (Δ[B]/Δt) = +1/c (Δ[C]/Δt) = +1/d (Δ[D]/Δt)
Prerequisites
Basic understanding of chemical equations and stoichiometry.
Concept of concentration (molarity).
Understanding of time intervals and rates.
Common mistakes
Forgetting the negative sign for reactants.
Not accounting for stoichiometric coefficients when relating rates of different species.
Confusing average rate with instantaneous rate.
Incorrectly calculating units.
Keywords
Rate of reaction
Chemical kinetics
Average rate
Instantaneous rate
Concentration
Time
Stoichiometry
Practice preview
Which of the following represents the instantaneous rate of reaction for a reaction where reactant R is consumed?…
easy
Consider the reaction 2A + B → C. If the rate of disappearance of A is 2 x 10⁻³ mol L⁻¹ s⁻¹, what is the rate of formation of C?…
medium
For the reaction H₂ + I₂ → 2HI, if the rate of disappearance of H₂ is 5 x 10⁻³ mol L⁻¹ s⁻¹, what is the rate of formation of HI?…
medium