Chemical Reactions and Equations — Questions & Answers

Sample questions from Chemical Reactions and Equations

1. Q1. Reaction 12: The reaction 2AgBr → 2Ag + Br₂ is best classified as:

   • A.decomposition✓
   • B.only a physical change
   • C.only an evaporation process
   • D.no chemical reaction
   Solution

   One compound breaks into simpler substances.

2. Q2. When the black coating of copper oxide is heated in a stream of hydrogen, the shining brown colour returns. This shows that:

   • A.Hydrogen is an oxidising agent
   • B.Copper oxide is a reducing agent
   • C.Hydrogen is a reducing agent and removes the oxygen✓
   • D.The copper melts and recrystallises
   Solution

   C is correct: the hydrogen takes the oxygen from the copper oxide, so copper oxide is reduced back to copper and hydrogen is itself oxidised to water, which makes hydrogen the reducing agent here. A is the opposite of what hydrogen is doing. B names the wrong substance, since copper oxide supplies the oxygen and so is the oxidising agent. D is not a chemical change at all and would not restore the metal.

3. Q3. In the reaction CuO + H₂ → Cu + H₂O, the oxidising agent is:

   • A.H₂
   • B.CuO✓
   • C.Cu
   • D.H₂O
   Solution

   CuO gives up its oxygen to hydrogen; the substance that supplies oxygen (and is itself reduced) is the oxidising agent.

4. Q4. A reaction in which two or more reactants combine to form a single product is called a:

   • A.Decomposition reaction
   • B.Displacement reaction
   • C.Double displacement reaction
   • D.Combination reaction✓
   Solution

   By definition, a combination (synthesis) reaction has two or more reactants forming one single product, e.g. CaO + H₂O → Ca(OH)₂.

5. Q5. Observation 5: Copper powder becomes black on heating because it forms:

   • A.copper(II) oxide✓
   • B.copper chloride
   • C.copper sulphide only
   • D.copper nitrate
   Solution

   Copper combines with oxygen to form black CuO.

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