Monitoring and controlling chemical reactions Study Guide

In chemical reactions involving gases, the amounts of reactants and products can be related not only by mass but also by volume. This is possible because, under standard conditions of room temperature and pressure (RTP), all gases occupy the same volume per mole. The standard molar gas volume at RTP is 24 dm³ (or 24,000 cm³) per mole. This means that one mole of any gas, whether it's oxygen, hydrogen, or carbon dioxide, will take up 24 dm³ of space at RTP.

To calculate the volume of a gas involved in a reaction, you first need a balanced chemical equation. This equation provides the molar ratio between the reactants and products. Once you know the number of moles of a gas, you can use the molar gas volume to convert moles into volume. The formula for this conversion is:

Volume of gas (dm³) = Moles of gas × 24 dm³/mol

Conversely, if you know the volume of a gas, you can calculate the number of moles by rearranging the formula:

Moles of gas = Volume of gas (dm³) / 24 dm³/mol

It's crucial to ensure that all volumes are in dm³ when using 24 dm³/mol. If volumes are given in cm³, you must convert them to dm³ by dividing by 1000. This principle allows chemists to predict the volumes of gaseous products formed or the volumes of gaseous reactants needed for a reaction without directly measuring them, streamlining experimental design and industrial processes.

In chemistry, certain substances can significantly alter the rate of a chemical reaction without being used up during the reaction itself. These special substances are called catalysts. A catalyst works by providing an alternative reaction pathway that has a lower activation energy. Activation energy is the minimum amount of energy required for a chemical reaction to occur. By lowering this energy barrier, more reactant particles have sufficient energy to react upon collision, which increases the frequency of successful collisions and thus speeds up the reaction.

It's important to recognise that catalysts are not consumed in the reaction; they can be recovered unchanged at the end and used again. Although they change the rate of reaction, they do not change the overall yield of the products, nor do they affect the position of equilibrium in a reversible reaction. Catalysts are highly specific, meaning a particular catalyst will often only work for a specific reaction or type of reaction. For example, iron is used as a catalyst in the Haber process for producing ammonia, and enzymes are biological catalysts essential for life processes. PAG C1 and C8 involve investigating rates of reaction, which may include observations about the effect of catalysts.

Identifying a catalyst in a reaction involves looking for a substance that is present at the start of the reaction, appears to participate but is not a reactant, and is present in the same chemical form and quantity (by mass) at the end of the reaction. Sometimes, this can be inferred from the reaction conditions or the name of the process itself (e.g., catalytic converters in cars).