A Level Chemistry OCR Practice Exam

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What is the role of a limiting reagent in a chemical reaction?

  1. The substance that is present in excess

  2. The reactant that determines the amount of product formed

  3. The catalyst that speeds up the reaction

  4. The product that is formed last in the reaction

The correct answer is: The reactant that determines the amount of product formed

The role of a limiting reagent in a chemical reaction is to determine the amount of product that can be formed during the reaction. This is because the limiting reagent is the reactant that will be completely consumed first, which restricts the reaction from continuing further. Once this reagent is used up, no additional product can be generated, regardless of the amounts of other reactants present. For example, if you have a reaction where one reactant is in short supply compared to the others, that reactant limits how much product can ultimately be produced. The presence of excess amounts of other reactants does not influence the total yield of the product formed, as the reaction can only occur until the limiting reagent is depleted. Understanding the concept of limiting reagents is crucial for stoichiometry and predicting the outcomes of chemical reactions.

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