Chemical reactions Study Guide

In chemistry, state symbols are universal shorthand used in chemical equations to indicate the physical state of each reactant and product under the conditions of the reaction. This information is crucial for understanding how a reaction might proceed, what apparatus would be needed for a practical, and what observations might be made. There are four primary state symbols: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solution. A solid (s) is a substance with a fixed shape and volume, such as a metal or a precipitate. A liquid (l) has a fixed volume but takes the shape of its container, like water or ethanol. A gas (g) has neither a fixed shape nor a fixed volume, and will expand to fill its container, such as oxygen or hydrogen. Finally, (aq) stands for aqueous, which means the substance is dissolved in water, forming a solution. Many ionic compounds, acids, and bases are commonly found in aqueous solutions. For example, if sodium chloride is dissolved in water, it is represented as NaCl(aq). Using these symbols accurately allows chemists to communicate reaction conditions concisely and precisely, aiding in the interpretation and prediction of chemical behaviour.

Stoichiometry is the study of the quantitative relationships between reactants and products in chemical reactions. It allows chemists to predict the amounts of substances involved in a reaction. This is based on the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction, meaning the total mass of reactants equals the total mass of products. To work out the stoichiometry from masses, you first need to convert the given masses of reactants and products into moles. This is done using the formula: moles = mass (g) / molar mass (g/mol). The molar mass (Mᵣ or Aᵣ) is the sum of the relative atomic masses of all atoms in a molecule or formula unit. Once you have the moles of each substance, you can find the simplest whole number ratio between them, which gives you the stoichiometric coefficients in the balanced chemical equation. For example, if you react 24 g of magnesium (Mg) with 16 g of oxygen (O₂) to produce 40 g of magnesium oxide (MgO), you would convert these masses to moles: Mg (24 g / 24 g/mol = 1 mol), O₂ (16 g / 32 g/mol = 0.5 mol), MgO (40 g / 40 g/mol = 1 mol). Dividing by the smallest number of moles (0.5 mol) gives a ratio of 2:1:2, leading to the balanced equation 2Mg + O₂ → 2MgO.

A limiting reactant (or limiting reagent) is the reactant that is completely used up first in a chemical reaction. Once the limiting reactant is consumed, the reaction stops, and no more product can be formed, even if other reactants are still present. The reactant that is not completely used up is called the excess reactant. The amount of product formed in a reaction is always determined by the limiting reactant. To identify the limiting reactant, you compare the mole ratio of reactants needed, as per the balanced equation, with the actual mole ratio present. The reactant present in a smaller amount, relative to its stoichiometric coefficient, is the limiting one. Understanding limiting reactants is crucial in industrial processes to maximise product yield and minimise waste by ensuring the more expensive or less available reactant is the limiting one.