Calculating Rates of Reaction

Calculating Rates of Reaction

  • A reaction rate is defined as the speed at which reactants are transformed into products during a chemical reaction.

  • It is generally quantified in terms of how much reactant is consumed or how much product is produced over a given time period. The units are usually moles per second (mol/s).

  • It’s crucial to understand that not all chemical reactions occur at the same rate. Some are nearly instantaneous, while others could require years or even centuries to complete.

  • The rate can be calculated from the gradient of a plot of product against time or reactant against time, depending on which data is easier to collect.

  • There are essentially two primary ways to determine the rate of a reaction: average rate of reaction and instantaneous rate of reaction.

  • The average rate of reaction refers to the average speed at which reactants are converted into products, typically calculated over a timescale where the concentrations of substances change perceptibly.

  • Instantaneous rate of reaction is the speed of a reaction at a particular instant in time, often determined by drawing a tangent to the curve at that point and calculating its gradient.

  • The rate of a reaction can be influenced by factors such as temperature, concentration of reactants, surface area of solid reactants, and the presence of a catalyst.

  • When graphing the concentration of reactants against time, a steeper line indicates a faster rate of reaction.

  • Reversible reactions can also be analysed in the same way, but bear in mind that in a closed system, they eventually reach a state of dynamic equilibrium where the concentrations of all reactants and products remain stable. However, the rates of the forward and backward reactions continue to be equal.

  • A rate equation expresses the rate of a reaction in terms of the concentrations of the reactants. The order of reaction with respect to a certain reactant is determined experimentally.

  • Zero order reactions have rates that are independent of the concentration of the reactants, while first order reactions have rates that directly depend on the concentration of one reactant. Another type, second order reactions, have rates that are proportional to the square of the concentration of one reactant, or to the product of two reactant concentrations.

  • Be able to construct and interpret rate-concentration graphs for zero order, first order and second order reactions.

  • Remember that classroom calculations often simplify reality, and real-life scenarios might involve more complex calculations.