Amounts of substance in equations

Some reactions appear to involve a change in mass.

A student investigated the change in mass when solids were heated.

Figure 7 shows the apparatus.

The student determined the mass of the contents of the crucible at regular intervals.

The student used a Bunsen burner instead of a water bath to heat the crucible.

Suggest one reason why.

The student planned to heat:

• 0.01 mol of magnesium

• 0.01 mol of magnesium carbonate.

The equations for the reactions are:

Figure 8 shows the student’s predicted results.

Look at Figure 8.

Explain the two trends for the first 3 minutes.

Magnesium carbonate

Figure 8 shows that both reactions are complete after 5 minutes.

Why is the final mass of the contents of the crucible the same for both reactions?

The student heated magnesium carbonate.

Figure 9 shows the results.

The student concluded:

‘The difference between the predicted results and the actual results is caused by solid escaping from the crucible.’

Give one reason why the results show the student is not correct.

Iron reacts with steam (H₂O) to produce an oxide of iron and hydrogen gas.

0.120 moles of iron reacts with 0.160 moles of steam.

Determine the balanced equation for the reaction.

You should determine:

• the simplest whole number mole ratio of Fe: H₂O

• the formula of the iron oxide produced.

Balanced equation for the reaction:

Nitric acid (HNO₃) is a strong acid.

What is meant by a ‘strong acid’?

Nitric acid is used as a dilute aqueous solution.

What is meant by ‘dilute aqueous solution’?

10 cm³ of a nitric acid solution has a pH of 1

Water is added to the nitric acid solution to change the pH of the nitric acid solution to pH 3

How does the hydrogen ion concentration change?

Tick () one box.

Write the ionic equation for the reaction between an acid and an alkali.

The equation shows the reaction between magnesium carbonate and nitric acid.

What is the ratio of the number of moles of magnesium carbonate to the number of moles of nitric acid in the reaction?

Tick () one box.

A student mixed some magnesium carbonate with excess nitric acid.

The student then added two drops of universal indicator to the solution.

What colour was the solution after the addition of universal indicator?

Tick () one box.

A student investigated the temperature change when different masses of

magnesium carbonate were reacted with excess nitric acid.

Figure 6 shows the apparatus.

This is the method used.

1. Pour 50 cm³ of nitric acid into a polystyrene cup.

2. Measure the temperature of the solution.

3. Add 0.50 g of magnesium carbonate.

4. Stir the mixture.

5. Measure the temperature.

6. Repeat steps 1 to 5 with different masses of magnesium carbonate.

Give two improvements to the method to produce more accurate results.

Do not refer to improvements to the apparatus in your answer.

Nitrogen reacts with hydrogen to produce ammonia (NH₃).

Complete the dot and cross diagram for an ammonia molecule.

The equation for the reaction between nitrogen and hydrogen to produce ammonia is:

Calculate the mass of hydrogen that is needed to produce 25 g of ammonia.

Relative atomic masses (A_r): H = 1 N = 14

Figure 7 shows the displayed formulae equation for the reaction of nitrogen

with hydrogen.

In the reaction the energy released forming new bonds is 93 kJ/mol greater than the energy needed to break existing bonds.

Table 3 shows bond energies.

Calculate the bond energy X for the H — H bond.

Use Figure 7 and Table 3.

Energy is released from the reaction to produce ammonia.

Figure 8 shows part of the reaction profile for the reaction between nitrogen and hydrogen to produce ammonia.

Complete Figure 8.

You should:

• complete the profile line

• label the energy level of the reactants and the product

• label the overall energy change.

Acids react with some metals to produce soluble salts. A student adds magnesium to hydrochloric acid until no more acid reacts and

The equation for the reaction is:

The reaction between magnesium and hydrochloric acid is a redox reaction. Explain what happens to the magnesium atoms in this reaction.

0.72 g of magnesium is added to 100 cm³ of hydrochloric acid. The hydrochloric acid is in excess. Calculate the concentration of the magnesium chloride (MgCl₂) solution produced

Metal oxides are produced when metals are heated in air. A student investigated the change in mass when 0.12 g of magnesium was heated in air. Figure 5 shows the apparatus.

The student measured the mass of magnesium oxide produced.

0.12 g of magnesium reacted to produce 0.20 g of magnesium oxide. Calculate the number of moles of oxygen gas (O₂) that reacted. Relative atomic mass (A_r): O = 16

The student repeated the experiment without a lid on the crucible. Suggest why the mass of magnesium oxide produced would be different without a lid on the crucible.

Copper reacts with oxygen to produce copper oxide. 63.5 g of copper produces 79.5 g of copper oxide. Calculate the mass of copper oxide produced when 0.50 g of copper reacts with oxygen. Give your answer to 3 significant figures.

Iron reacts with oxygen to produce an oxide of iron. 0.015 moles of iron reacts with 0.010 moles of oxygen gas (O₂). Determine:

• the formula of the iron oxide produced

• the balanced symbol equation for the reaction.

Balanced symbol equation

This question is about gold and compounds of gold.

In the alpha particle scattering experiment alpha particles are fired at gold foil. Alpha particles are positively charged. Figure 5 shows the results.

What two conclusions can be made from the results? Tick (✓) two boxes.

The gold foil is:

• 4.00 × 10^–7 metres thick

• 2400 atoms thick.

What is the diameter of one gold atom in metres? Give your answer to 3 significant figures.

Gold reacts with the elements in Group 7 of the periodic table. 0.175 g of gold reacts with chlorine. The equation for the reaction is:

Calculate the mass of chlorine needed to react with 0.175 g of gold. Give your answer in mg Relative atomic masses (A_r): Cl = 35.5 Au = 197

This question is about iron. Iron reacts with dilute hydrochloric acid to produce iron chloride solution and one other product.

Name the other product.

Suggest how any unreacted iron can be separated from the mixture.

Magnesium reacts with iron chloride solution.

0.120 g of magnesium reacts with excess iron chloride solution. Relative atomic masses (A_r): Mg = 24 Fe = 56 Calculate the mass of iron produced, in mg

Explain which species is reduced in the reaction between magnesium and iron chloride.

Your answer should include the half equation for the reduction.