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GCE A-Level (Singapore) Chemistry — practice by topic

Browse 98 questions across 2 Chemistry topics. Choose what you want to revise, then open a question to check its answer and worked solution.

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Chemistry topics

Acid-Base Equilibria (65)Fundamental Equilibrium Concepts (33)

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In a chemical equilibrium, what is true of the forward and the reverse reaction?Read the answer and worked solution →A reversible reaction run in a closed system usually gives what yield of products?Read the answer and worked solution →Liquid bromine is poured into a bottle. What condition lets a liquid to vapour equilibrium form?Read the answer and worked solution →Pure reactants alone are mixed in a flask. What is the value of Qc at that instant?Read the answer and worked solution →

Worked practice questions

In a chemical equilibrium, what is true of the forward and the reverse reaction?

  1. They run at equal rates, so amounts hold steady
  2. They both stop once the amounts hold steady
  3. They run at rates in a fixed two to one ratio
  4. They take turns, only one running at a time
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Correct answer: A. They run at equal rates, so amounts hold steady

1. At equilibrium the forward reaction and the reverse reaction proceed at the same rate. 2. Because the two rates match, the concentrations of reactants and products no longer change. 3. Neither reaction stops, so equilibrium is a dynamic balance rather than a halt. 4. Rates locked in any ratio other than one to one would leave the concentrations drifting. _Source: OpenStax Chemistry (CC BY 4.0), Ch 13 "Fundamental Equilibrium Concepts", section 13.1 Chemical Equilibria_

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A reversible reaction run in a closed system usually gives what yield of products?

  1. Exactly 100 percent, with no reactant left
  2. Less than 100 percent, with reactant left over
  3. Exactly 50 percent, whatever the conditions
  4. A yield that keeps rising without any limit
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Correct answer: B. Less than 100 percent, with reactant left over

1. In a closed system the products cannot leave, so the reverse reaction can proceed. 2. The mixture settles at a point where some reactant is still present alongside product. 3. The yield is therefore short of 100 percent, and a mixture is found at equilibrium. 4. Nothing forces the split to be 50 percent, so that value is an invented figure. _Source: OpenStax Chemistry (CC BY 4.0), Ch 13 "Fundamental Equilibrium Concepts", section 13.1 Chemical Equilibria_

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Liquid bromine is poured into a bottle. What condition lets a liquid to vapour equilibrium form?

  1. The bottle is capped so no vapour escapes
  2. The bottle is warmed above the boiling point
  3. The bottle is left open to the room air here
  4. The bottle is filled right up to the very brim
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Correct answer: A. The bottle is capped so no vapour escapes

1. Evaporation and condensation are the forward and reverse steps of a physical change. 2. A capped bottle holds the vapour in, so condensation can build up to match evaporation. 3. Once the two rates match, the amounts of liquid and vapour stop changing. 4. If the bottle were left uncapped the vapour would escape and no equilibrium would be reached. _Source: OpenStax Chemistry (CC BY 4.0), Ch 13 "Fundamental Equilibrium Concepts", section 13.1 Chemical Equilibria_

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