Chemical reactions Study Guide

Electrolysis is a crucial industrial process that uses electrical energy to drive non-spontaneous chemical reactions. It's essentially the decomposition of an electrolyte using a direct electric current. An electrolyte is a substance, either molten or in solution, which contains free-moving ions and can therefore conduct electricity. When an electric current is passed through an electrolyte, the ions migrate to oppositely charged electrodes. There are two main types of electrodes: the anode and the cathode. The anode is the positive electrode, attracting negatively charged ions called anions. At the anode, anions lose electrons through oxidation, meaning they are oxidised. For example, chloride ions (Cl⁻) might lose electrons to form chlorine gas (2Cl⁻ → Cl₂ + 2e⁻). The cathode is the negative electrode, attracting positively charged ions called cations. At the cathode, cations gain electrons through reduction, meaning they are reduced. For instance, copper ions (Cu²⁺) might gain electrons to form solid copper metal (Cu²⁺ + 2e⁻ → Cu). The products of electrolysis depend on the specific electrolyte being used and whether it's molten or in an aqueous solution. For aqueous solutions, water itself can be oxidised or reduced, leading to the formation of oxygen, hydrogen or hydroxide ions at the electrodes, which can complicate the process, requiring careful consideration of the reactivity series.

Chemical reactions involve the breaking of existing bonds and the formation of new bonds. Energy is required to break bonds, making this an endothermic process. Conversely, energy is released when new bonds are formed, which is an exothermic process. The overall energy change in a chemical reaction, known as the enthalpy change (ΔH), can be calculated by comparing the total energy absorbed to break bonds with the total energy released during bond formation. To do this, you first sum all the bond energies for the bonds broken in the reactants. Then, you sum all the bond energies for the bonds formed in the products. The overall energy change is calculated as: Energy absorbed (bond breaking) - Energy released (bond making). If the calculated value is negative, the reaction is exothermic, meaning more energy is released than absorbed, and heat is given out to the surroundings. If the value is positive, the reaction is endothermic, meaning more energy is absorbed than released, and heat is taken in from the surroundings. This method provides an estimation of the energy change, as average bond energies are used, which are values averaged over many different compounds containing that specific bond.