Monitoring and controlling chemical reactions Study Guide

The rate of a chemical reaction can be studied by measuring the change in the amount of reactants or products over time. This data is often plotted on a graph, with time on the x-axis and the amount of substance (e.g., mass of product formed, volume of gas produced, decrease in concentration of reactant) on the y-axis. On such a graph, the steepness of the curve at any point indicates the reaction rate at that instant. A steeper curve means a faster reaction rate, while a less steep curve (or a flatter line) indicates a slower rate. As reactions proceed, reactants are consumed, and the concentration decreases, which typically leads to a decrease in the reaction rate. This is shown by the curve becoming less steep over time, eventually levelling off when the reaction stops because a reactant has been used up. The total amount of product formed or reactant consumed can be found from the final height of the curve on the y-axis. Comparing different reaction conditions, faster reactions will have steeper initial slopes, reach completion faster, and may produce the same or different total amounts of product depending on the limiting reactant.

Titration is a precise quantitative analytical method used to determine the concentration of an unknown solution (the titrand) by reacting it with a solution of known concentration (the titrant). This technique involves the controlled addition of the titrant from a burette into a conical flask containing the titrand, usually with an indicator. The indicator changes colour at the equivalence point, which is the point where the reaction between the titrant and titrand is complete, meaning they have reacted in exact stoichiometric proportions. The volume of titrant added is measured from the burette. By knowing the volume and concentration of the titrant, and the volume of the titrand, along with the balanced chemical equation for the reaction, the concentration of the unknown solution can be calculated. Before starting the titration, the burette and pipette are rinsed with the solutions they will contain to ensure accurate concentrations and volumes. A pipette is used to accurately measure a fixed volume of the unknown solution into the conical flask. The conical flask is placed on a white tile under the burette to make the colour change of the indicator more visible. Multiple titrations are usually performed: a rough titration to estimate the endpoint, followed by several accurate titrations until concordant results are achieved. Concordant results are those within a close range, typically ±0.1 cm³, indicating high precision. The average of these concordant results is then used in calculations.