Chemical reactions Study Guide

Chemical compounds are formed when atoms combine. When atoms gain or lose electrons, they form charged particles called ions. Metals tend to lose electrons and form positively charged ions (cations), while non-metals tend to gain electrons and form negatively charged ions (anions). In an ionic compound, the overall charge must be neutral. This means that the total positive charge from the cations must exactly balance the total negative charge from the anions. To deduce the formula of an ionic compound, you need to know the charge of each ion involved.

For example, consider sodium chloride. Sodium (Na) is in Group 1, so it loses one electron to form a Na+ ion. Chlorine (Cl) is in Group 7, so it gains one electron to form a Cl- ion. Since the charge of +1 from Na+ perfectly balances the charge of -1 from Cl-, only one of each ion is needed. The formula is thus NaCl.

Now consider magnesium chloride. Magnesium (Mg) is in Group 2, forming a Mg2+ ion. Chlorine (Cl) forms a Cl- ion. To balance the +2 charge of one Mg2+ ion, you need two Cl- ions (2 x -1 = -2). Therefore, the formula is MgCl2.

For compounds involving polyatomic ions (ions made of multiple atoms, like sulfate SO4^2- or nitrate NO3^-), the same principle applies. For instance, magnesium sulfate: Mg2+ and SO4^2-. The charges are already balanced (+2 and -2), so the formula is MgSO4. For sodium sulfate: Na+ and SO4^2-. You need two Na+ ions to balance the -2 charge of one SO4^2- ion, resulting in the formula Na2SO4. The key is to find the lowest whole number ratio of ions that results in a net charge of zero.

When carbonates react with acids, a specific type of chemical reaction occurs where the carbonate compound is converted into a salt, water, and carbon dioxide gas. This reaction is characterised by effervescence (fizzing) as the carbon dioxide gas is produced. For example, calcium carbonate (found in limestone and chalk) reacting with hydrochloric acid produces calcium chloride (a salt), water, and carbon dioxide. The general word equation for this reaction is: Carbonate + Acid → Salt + Water + Carbon Dioxide. To write balanced chemical equations, it's crucial to identify the correct chemical formulas for the reactants and products and then adjust the coefficients to ensure the number of atoms of each element is the same on both sides of the equation.

Some metals also react with acids, but the products differ from those of carbonate reactions. When a reactive metal (above hydrogen in the reactivity series) reacts with an acid, it displaces the hydrogen from the acid to form a salt and hydrogen gas. This is a redox reaction where the metal is oxidised and the hydrogen ions in the acid are reduced. For instance, magnesium reacting with hydrochloric acid produces magnesium chloride and hydrogen gas. The general word equation for this reaction is: Metal + Acid → Salt + Hydrogen. Less reactive metals, such as copper, will not react with dilute acids. Predicting the products requires knowledge of the metal's reactivity and the composition of the acid. Balancing these equations also involves ensuring that the number of atoms for each element is equal on both sides of the reaction arrow. These reactions are important in various industrial processes and in understanding corrosion.