Chemical reactions Study Guide

The Law of Conservation of Mass states that during any chemical reaction, the total mass of the reactants must equal the total mass of the products. This simply means that atoms are not created or destroyed, but merely rearranged to form new substances. However, in non-enclosed (open) systems, it can appear that the mass changes, either increasing or decreasing. This apparent change in mass can be accounted for by considering the physical states of the reactants and products, and whether they can escape or enter the reaction system. For example, if a gas is produced in a reaction and allowed to escape into the atmosphere, the measured mass of the remaining substances in the container will decrease. Conversely, if a gas from the surroundings (like oxygen) reacts with a solid, and the products remain within the system, the measured mass will appear to increase because the mass of the gas has been incorporated into the solid product. The particle model helps explain this: if particles of a gaseous product leave the system, their mass is no longer measured; if particles of a gaseous reactant join the system, their mass is added. It's crucial to understand that the total mass of all atoms involved, both inside and outside the measured system, remains constant. The change in measured mass only reflects particles entering or leaving the specific observation area.

In chemistry, all matter is made of fundamental building blocks called elements. Each element has a unique name and a specific chemical symbol, which is typically one or two letters. For instance, the symbol for Hydrogen is 'H', and for Oxygen it is 'O'. These symbols are universally recognised and are found on the Periodic Table, which you will be provided with in examinations. When different elements combine chemically, they form compounds. The chemical formula of a compound shows which elements are present and the ratio of atoms of each element. For example, the formula for water is H₂O, indicating two hydrogen atoms for every one oxygen atom. Subscripts after an element symbol tell you how many atoms of that element are in the compound. If there is no subscript, it means there is only one atom of that element. Chemical reactions are represented by chemical equations. A chemical equation uses symbols and formulae to show the reactants (starting materials) on the left side of an arrow, and the products (substances formed) on the right side. For example, the reaction between hydrogen gas and oxygen gas to form water can be written as H₂ + O₂ → H₂O. A balanced chemical equation ensures that the number of atoms of each element is the same on both sides of the equation, reflecting the law of conservation of mass. To balance an equation, you place large numbers (coefficients) in front of the chemical formulae. For instance, the balanced equation for water formation is 2H₂ + O₂ → 2H₂O. This means two molecules of hydrogen react with one molecule of oxygen to produce two molecules of water. Balancing equations is crucial for understanding the stoichiometry (quantitative relationships) of reactions.