Chemical reactions Study Guide

The Law of Conservation of Mass is a fundamental principle in chemistry. It states that during any chemical reaction or physical change, the total mass of the reactants before the change is equal to the total mass of the products after the change. In simpler terms, mass cannot be created or destroyed. This means that atoms are rearranged to form new substances, but no atoms are lost or gained. For example, if you burn a piece of wood, it seems like mass is lost because the ash weighs less than the original wood. However, if you were to capture all the gases released during combustion (like carbon dioxide and water vapour) and measure their mass along with the ash, you would find that the total mass is exactly the same as the mass of the original wood plus the oxygen that reacted with it. This law is crucial for balancing chemical equations, as it ensures that the same number of each type of atom is present on both sides of the equation. It is also important for understanding reactions in closed and open systems. In a closed system, all reactants and products are contained, making it easy to observe the conservation of mass. In an open system, some products (like gases) might escape, making it appear as though mass has been lost, even though it has just dispersed.

Electrolysis is a chemical process that uses electrical energy to drive non-spontaneous chemical reactions. It's essentially using electricity to break down compounds. This process occurs in an electrolytic cell, which consists of an electrolyte (an ionic compound molten or dissolved in water, allowing ions to move) and two electrodes connected to a power supply. The positive electrode is called the anode, and the negative electrode is called the cathode.

The key to electrolysis lies in the movement of ions. Positive ions (cations) are attracted to the negatively charged cathode, where they gain electrons (reduction). Negative ions (anions) are attracted to the positively charged anode, where they lose electrons (oxidation). The products formed at each electrode depend on the identity of the ions present and the reactivity series, especially when water is also present.

A crucial aspect of electrolysis is the nature of the electrodes themselves. Electrodes can be either 'inert' or 'non-inert' (also known as 'active').

1. <b>Inert electrodes</b>: These electrodes do not participate in the chemical reaction themselves; they merely provide a surface for electron transfer. Common inert electrodes include carbon (graphite) and platinum. When inert electrodes are used, the products of electrolysis are solely determined by the ions in the electrolyte and, if applicable, the solvent (e.g., water). For example, electrolysing molten lead(II) bromide with inert graphite electrodes produces lead at the cathode and bromine gas at the anode.

1. <b>Non-inert (active) electrodes</b>: These electrodes actively participate in the chemical reaction, meaning they can be oxidized or reduced themselves. A common example is the electrolysis of copper(II) sulfate solution using copper electrodes. At the anode, instead of the sulfate ions or water being oxidised, the copper anode itself loses electrons and dissolves, forming copper(II) ions. At the cathode, copper(II) ions from the solution gain electrons and deposit as copper metal. This technique is used in electrorefining to purify metals or in electroplating to coat one metal with another. The choice of electrode type significantly influences the products formed and the overall purpose of the electrolytic process.