Writing and balancing equationsIB MYP Chemistry: Subtopic test

1. 1
   In a school laboratory in Dubai, students add small pieces of zinc to dilute hydrochloric acid in a test tube. Bubbles of hydrogen gas form and the zinc slowly disappears, leaving a colourless solution of zinc chloride.

   (a)

   Which state symbol should be written after the formula of hydrogen in the symbol equation for this reaction?

   [1 mark]

   • A(s)
   • B(l)
   • C(g)
   • D(aq)

   (b)

   Which is the correct balanced symbol equation with state symbols for the reaction?

   [1 mark]

   • AZn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
   • BZn(s) + HCl(aq) → ZnCl₂(aq) + H₂(g)
   • CZn(s) + 2HCl(aq) → ZnCl(aq) + H₂(g)
   • DZn(s) + 2HCl(aq) → ZnCl₂(aq) + H(g)

   (c)

   Explain why the number 2 is written in front of HCl in the balanced equation.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student in Lagos places a conical flask containing marble chips (calcium carbonate) and dilute hydrochloric acid on a balance. Cotton wool is pushed into the neck of the flask. As the reaction takes place the balance reading falls by 0.64 g, even though no solid or liquid has left the flask.

   (a)

   What caused the balance reading to fall during the reaction?

   [1 mark]

   • ASome of the marble chips turned into energy
   • BThe calcium atoms in the marble were destroyed
   • CThe hydrochloric acid evaporated completely
   • DCarbon dioxide gas escaped through the cotton wool

   (b)

   Which statement about the atoms in this reaction is correct?

   [1 mark]

   • ASome atoms of calcium are destroyed as the marble dissolves
   • BThe atoms are rearranged and none are created or destroyed
   • CNew atoms of carbon are made when the gas forms
   • DThe number of atoms increases because a gas is made

   (c)

   A different student does the same reaction in a strong sealed container. Explain why the total mass would not change.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates whether mass changes during a reaction. She places a small test tube of sodium carbonate solution inside a conical flask containing calcium chloride solution, seals the flask with a stopper and weighs it. She tips the flask so that the two solutions mix and a white precipitate of calcium carbonate forms. She then weighs the sealed flask again. Her balance reads to 0.01 g. The mass of the sealed flask before and after mixing was: trial 1, 215.32 g then 215.32 g; trial 2, 187.45 g then 187.45 g; trial 3, 201.18 g then 201.17 g.

   (a)

   State a testable hypothesis for this investigation, give a scientific reason for it, and identify the dependent variable.

   [3 marks]

   (b)

   Evaluate the results and the method, and suggest one improvement.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A fertiliser company in India makes ammonia by reacting nitrogen gas with hydrogen gas in a sealed reactor at high pressure. The ammonia is converted into fertilisers that farmers use to grow more food. Making ammonia uses a lot of energy, and most of the hydrogen is obtained from natural gas.

   (a)

   Write the balanced symbol equation, with state symbols, for the reaction of nitrogen with hydrogen to form ammonia. Use the numbers of atoms to show that your equation is balanced, and explain why only the large numbers in front of formulae may be changed when balancing.

   [6 marks]

   (b)

   Discuss whether the benefits of making ammonia for fertilisers outweigh the drawbacks.

   [6 marks]

   Total for question 4: 12 marks

End of questions