A Level Chemistry OCR Practice Exam

In an addition reaction, what occurs to the reactants involved?

1. A reactant is replaced by another

2. A reactant is combined with an unsaturated molecule

3. Reactants are decomposed

4. Reactants react to eliminate a byproduct

The correct answer is: A reactant is combined with an unsaturated molecule

In an addition reaction, one of the key characteristics is that reactants combine to form a larger, more complex molecule. This typically involves an unsaturated molecule, which contains double or triple bonds. The addition reaction allows these unsaturated bonds to be 'saturated' by adding new atoms or groups of atoms to the molecule. For example, in the addition of hydrogen (H₂) to ethene (C₂H₄), the double bond in ethene is broken, and two hydrogen atoms are added, resulting in ethane (C₂H₆). This process illustrates that the reactants are effectively combined, leading to the formation of a single product without any byproducts being eliminated. The other options don't accurately describe the nature of addition reactions. Replacing a reactant would indicate a substitution reaction, while decomposition pertains to breaking molecules down into simpler substances. Additionally, the statement about reactants eliminating byproducts misleadingly suggests a different type of reaction characteristic, which is not applicable to addition reactions.