Monitoring and controlling chemical reactions Study Guide

In chemistry, many reactions occur in solution. To ensure a complete reaction, where reactants are fully consumed without excess, a precise relationship exists between the volumes and concentrations of the reacting solutions. This relationship is governed by the stoichiometry of the balanced chemical equation. The core principle is that the number of moles of each reactant must be in the exact ratio specified by the stoichiometric coefficients. For a substance in solution, the number of moles (n) is calculated by multiplying its concentration (c, in mol/dm³) by its volume (V, in dm³), i.e., n = c × V. Therefore, if we know the concentration and volume of one reactant, and the balanced chemical equation, we can determine the moles of that reactant. Using the molar ratio from the balanced equation, we can then calculate the moles of the other reactant required. If the volume of the second reactant is known, its concentration can be determined by rearranging the mole formula (c = n/V). Conversely, if the concentration of the second reactant is known, its required volume can be found. This principle is fundamental to quantitative analysis, such as titrations, where a solution of known concentration (a standard solution) is used to find an unknown concentration of another solution. The 'equivalence point' or 'end point' of a titration signifies when the reactants have completely reacted according to their stoichiometric ratio. Accurate measurement of volumes is crucial, typically achieved using equipment like burettes and pipettes.

The rate of a chemical reaction refers to how quickly reactants are used up or how quickly products are formed. There are several practical methods to determine reaction rates, each suitable for different types of reactions. One common method involves observing a change in mass, particularly when a gas is produced and allowed to escape. By placing the reaction mixture on a balance and recording its mass at regular time intervals, the decrease in mass over time indicates the rate of gas production and thus the reaction rate. Another method, often used for reactions producing an observable precipitate, is timing how long it takes for a cross marked beneath a reaction vessel to become obscured. This 'disappearing cross' experiment measures the time taken for a certain amount of product to form. If a gas is produced and collected, its volume can be measured over time using a gas syringe or by displacement of water. This directly shows the rate of gas formation. For reactions that cause a colour change, the time taken for a specific colour to appear or disappear can be measured. It is crucial in all these methods to control variables such as temperature, concentration of reactants, and surface area (for solids) to ensure a fair test when investigating the effect of one factor on the reaction rate.