13.1
Reaction Rate
The rate of reaction is the change in the amount of a reactant or product per unit time. Reaction rates are therefore determined by measuring the time…
A chemical reaction involves the chemical transformation of reactants into products. As the reaction progresses, the concentration of reactants decreases while the concentration of products increases. This variance in concentrations of reactants and products can be plotted in a graph as a function of time.
The speed at which a reaction progresses is called the reaction rate. It measures the rate of reactants’ disappearance or the rate of products’ appearance and is expressed in units of molarity-per-second.
The average reaction rate can be calculated from the change in concentrations of reactants or products during a specific period of time. The molar concentration value is denoted in brackets, t stands for the period of time, and the delta symbol represents ‘change in’.
Since reactants are depleted during a chemical reaction, the value of concentration change of reactants is always negative. Therefore, a reaction rate calculated in terms of reactants is supplemented by a negative sign, to make the overall value positive.
The reaction rate is not uniform throughout a reaction. The rate at the start of the reaction, at ‘time-zero’, is called the initial reaction rate. As the concentrations of reactants reduce, the reaction rate decreases, or the reaction slows down.
The reaction rate at a given time point, or the instantaneous reaction rate, is measured by calculating the slope of a tangent drawn to the reaction curve, for either reactants or products, at the time of interest.
The slope value or the instantaneous rate at a specific time-point is equal for all reactants and products.
The reaction rate of a chemical reaction also reflects the actual stoichiometric coefficients of reactants and products.
So, for any balanced reaction, where a moles of A reacts with b moles of B, producing c moles of C and d moles of D, the reaction rate can be expressed and calculated using the generic formula: −1/a × Δ[A]/Δt = −1/b × Δ[B]/Δt. These values are the same as +1/c × Δ[C]/Δt, which is the same as +1/d × Δ[D]/Δt.
Determining reaction rates is fundamental to studying chemical kinetics, which helps to understand how quickly a drug, catalyst, or a synthetic reaction works so that it can be better controlled and optimized in its function.
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Q1: How is reaction rate defined and measured in chemistry?
Reaction rate measures the speed at which reactants disappear or products appear during a chemical reaction, expressed in molarity-per-second. It is calculated from the change in concentration of reactants or products over a specific time period. The average reaction rate can be determined by dividing the change in molar concentration by the change in time, using the formula: −Δ[concentration]/Δt for reactants or +Δ[concentration]/Δt for products.
Q2: Why is a negative sign used when calculating reaction rate from reactant concentration?
A negative sign is applied to reactant concentration changes because reactant concentrations decrease as a reaction progresses, making the change value negative. By convention, reaction rates are expressed as positive quantities. Multiplying the negative concentration change by −1 converts the rate to a positive value, ensuring consistent representation across all reaction rate calculations.
Q3: What is the difference between initial reaction rate and instantaneous reaction rate?
The initial reaction rate is the rate at time-zero, when the reaction begins. The instantaneous reaction rate is the rate at any specific moment during the reaction, calculated by finding the slope of a tangent line drawn to the concentration-versus-time curve at that point. As reactant concentrations decrease, the instantaneous reaction rate also decreases, showing that reactions slow down over time.
Q4: How does stoichiometry relate to reaction rate expressions?
Reaction rates reflect the stoichiometric coefficients of reactants and products in a balanced equation. For a reaction where a moles of A react with b moles of B to produce c moles of C, the rate expressions −1/a × Δ[A]/Δt, −1/b × Δ[B]/Δt, +1/c × Δ[C]/Δt, and +1/d × Δ[D]/Δt are all equal. This relationship ensures that rate values are consistent regardless of which reactant or product is used to express the rate.
Q5: How can you determine the instantaneous rate from a concentration-time graph?
The instantaneous rate at any time point is determined by calculating the slope of a tangent line drawn to the concentration-versus-time curve at that specific moment. The slope value represents the rate of change in concentration at that instant. This graphical method allows you to find the instantaneous rate at any time, including the initial rate at t = 0 and rates at later time points as the reaction proceeds.
Q6: Why does reaction rate change as a chemical reaction progresses?
Reaction rate is not uniform throughout a reaction because it depends on reactant concentration. As reactants are consumed, their concentrations decrease, which reduces the frequency of molecular collisions and slows the reaction. This is why the initial reaction rate is fastest and the rate decreases over time. Understanding this concentration dependence is essential for studying chemical kinetics and optimizing reaction conditions.
Q7: How can average reaction rate be calculated from experimental data?
Average reaction rate is calculated by measuring molar concentrations at the beginning and end of a time interval, then dividing the change in concentration by the change in time. This method provides a single rate value for the entire period. For more precise rate information at specific moments, the instantaneous rate can be determined using the integrated rate law the dependence of concentration on time, which relates concentration changes to specific time points.