Chemical reactions Study Guide

In chemistry, stoichiometry is the calculation of quantitative relationships between reactants and products in a balanced chemical equation. A balanced chemical equation shows the ratio of moles of each reactant and product involved in a reaction. This mole ratio is crucial for calculating the masses of substances taking part in a reaction. To perform these calculations, you typically follow a four-step process. First, write down the balanced chemical equation for the reaction. This ensures you have the correct mole ratios. Second, convert the given mass of a substance (either a reactant or product) into its number of moles using the formula: moles = mass / relative formula mass (Mr). The relative formula mass is found by summing the atomic masses of all atoms in the chemical formula. Third, use the mole ratio from the balanced equation to determine the number of moles of the unknown substance. For example, if the ratio of A to B is 1:2, and you have 'x' moles of A, you will have '2x' moles of B. Finally, convert the calculated moles of the unknown substance back into its mass using the formula: mass = moles × relative formula mass (Mr). This method allows chemists to predict how much product can be formed from a given amount of reactant, or how much reactant is needed to produce a certain amount of product, which is vital for industrial processes and laboratory experiments. For instance, if you react 10 g of magnesium with oxygen to form magnesium oxide, you would first balance the equation (2Mg + O₂ → 2MgO). Then calculate moles of Mg. Use the 2:2 mole ratio to find moles of MgO, and finally calculate the mass of MgO. This method relies on the conservation of mass, meaning no atoms are created or destroyed during a chemical reaction, only rearranged.

The law of conservation of mass states that no atoms are lost or made during a chemical reaction, meaning the total mass of the products equals the total mass of the reactants. This fundamental principle holds true even when a reaction appears to involve a change in mass, such as the formation of a gas or a precipitate. For instance, if a gas is produced during a reaction in an open system, it can escape into the surroundings, leading to an observed decrease in the mass of the remaining substances. Conversely, if a gas from the surroundings reacts with a substance in an open system, such as oxygen reacting with a metal to form an oxide, the observed mass of the solid will increase. In these cases, the 'missing' or 'added' mass is simply due to the involvement of a gas that has either left or entered the reaction vessel. If the reaction were carried out in a closed system, where no substances could enter or leave, the total mass before and after the reaction would remain precisely the same, confirming the conservation of mass. Understanding this principle is crucial for balancing chemical equations and predicting reaction outcomes.