Chemical reactions Study Guide

Chemical reactions involve the breaking of existing bonds and the formation of new bonds. For this to happen, reactant particles must collide with sufficient energy. This minimum amount of kinetic energy that particles must possess for a successful collision to occur, leading to a reaction, is called the activation energy. Think of it as an energy 'hill' or 'barrier' that reactants must overcome before they can transform into products. If particles collide with less than the activation energy, they will simply bounce off each other without reacting. The higher the activation energy, the greater the energy barrier, and generally, the slower the reaction rate at a given temperature because fewer particles will have enough energy to react. Catalysts work by providing an alternative reaction pathway with a lower activation energy, which means more particles will have sufficient energy to react, thus increasing the reaction rate without being used up themselves. Endothermic reactions absorb energy from the surroundings, and exothermic reactions release energy. Both types of reactions require activation energy to initiate the process, regardless of the overall energy change. For example, a campfire (exothermic) needs a spark (activation energy) to start burning, even though it releases a lot of heat once it gets going.

The pH scale is a measure of how acidic or alkaline a solution is. It ranges from 0 to 14, where pH 7 is neutral. Values below 7 indicate acidity, with 0 being the most acidic, and values above 7 indicate alkalinity, with 14 being the most alkaline. Understanding pH is crucial in many chemical processes and daily life applications.

Two common techniques for measuring pH are using universal indicator and using a pH meter. Universal indicator is a mixture of several different indicators that changes colour gradually over a wide pH range, giving an approximate pH value. For example, it might turn red in strong acids (pH 1-2), orange in weaker acids (pH 3-6), green at neutral (pH 7), blue in weak alkalis (pH 8-11), and purple in strong alkalis (pH 12-14).

A pH meter, on the other hand, is an electronic device that measures the pH of a solution more precisely. It consists of a probe, which contains a sensor electrode, connected to a meter that displays the pH reading numerically. Before use, a pH meter must be calibrated using buffer solutions of known pH (e.g., pH 4, 7, and 10) to ensure accuracy. The probe is dipped into the solution, and the pH value is read directly from the digital display. pH meters offer higher accuracy and precision compared to universal indicator, especially when exact pH values are needed, but they are more expensive and require calibration.