Monitoring and controlling chemical reactions Study Guide

Enzymes are a special type of protein that act as biological catalysts. In chemistry, a catalyst is a substance that speeds up the rate of a chemical reaction without being used up itself. Enzymes perform this crucial role within living organisms, making vital reactions happen quickly enough to sustain life. For instance, processes like digestion, respiration, and synthesis of new molecules all rely on enzymes. Without enzymes, these reactions would occur too slowly at normal body temperatures to be efficient or even sustainable. Each enzyme is highly specific, meaning it usually only catalyses one particular reaction or a very small group of structurally similar reactions. This specificity arises from the unique three-dimensional shape of an enzyme's 'active site', which is where reactant molecules (called substrates) bind. When a substrate binds to the active site, an 'enzyme-substrate complex' is formed, which reduces the activation energy needed for the reaction to proceed, thus speeding it up. After the reaction, the products are released, and the enzyme is ready to catalyse the same reaction again with new substrate molecules. Factors like temperature and pH can affect enzyme activity by changing the shape of the active site, a process called denaturation. Maintaining optimum conditions is vital for enzyme function.

When a chemical reaction occurs, reactants are consumed and products are formed. The rate of reaction describes how quickly this change takes place. We can monitor the rate of a reaction by measuring a change in a quantifiable property over time. This property could be the mass of reactants decreasing, the volume of a gas produced increasing, or the concentration of a reactant decreasing.

Graphs are an excellent way to visualise and interpret reaction rates. Typically, time is plotted on the x-axis, and the changing quantity (e.g., volume of gas, mass loss) is plotted on the y-axis.

Key features to look for on such a graph include:

1. Steepness of the curve (gradient): The steeper the curve, the faster the reaction. A steep slope indicates a large change in quantity over a short period. As the reaction proceeds, reactants are used up, and the reaction slows down, causing the curve to become less steep.
2. Initial rate: The steepest part of the curve, usually at the very beginning (time = 0), represents the fastest rate of reaction. This is because reactant concentrations are at their highest.
3. Completion of reaction: The reaction is complete when the curve becomes horizontal (flat). At this point, there is no further change in the measured quantity, indicating that one or more reactants have been entirely consumed.
4. Total change: The total change in the measured quantity (e.g., total volume of gas produced) can be read from the final horizontal level of the curve.

By comparing different curves on the same graph, we can compare the rates of reaction under various conditions (e.g., different temperatures, concentrations, or surface areas). A reaction that produces a given amount of product faster will have a steeper initial gradient. A reaction that produces more product will have a higher final plateau if it's an 'amount produced' graph, or a lower final plateau if it's 'amount remaining' graph.