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Introduction

You are to investigate the mass changes that take place when a selection of chemical reactions take place.

Task 1: Measuring mass changes on combustion


Measuring mass changes during chemical reactions

  1. Open Yenka file Model 1.
  2. Record the mass of the evaporating dish on the balance. Take the sample of magnesium and pour it (by rotating the beaker) into the evaporating dish on the balance. Record the total mass of the magnesium and its container. Calculate the mass of the magnesium sample.
    Mass of evaporating dish + solid =
    Answer
    11 g
    Mass of empty evaporating dish =
    Answer
    10 g
    Mass of magnesium =
    Answer
    1 g
  3. Place the evaporating dish containing the magnesium on the tripod, and turn up the flame on the Bunsen burner. Select the evaporating dish and write down the word equation for the reaction occurring.
    Answer
    magnesium + oxygen → magnesium oxide
  4. After heating the sample for a minute, take it off the tripod and re-weigh it. Record the new mass, and describe what happens to the mass as the sample sits on the balance.
    Answer
    Answers in the region of 16.5g. Mass continues to rise.
  5. Keep heating and reweighing the sample until it reaches a constant mass. Fill in the table below.
    Final mass of evaporating dish + product =
    Answer
    11.66 g
    Mass of evaporating dish =
    Answer
    10 g
    Mass of product =
    Answer
    1.66 g
  6. Explain, in terms of the atoms present in the magnesium and the magnesium oxide, why the mass of the magnesium increases when it reacts with oxygen.
    Answer
    The magnesium atoms have combined with oxygen molecules/atoms from the air to form magnesium oxide. The magnesium oxide will therefore weigh more as it contains both magnesium and oxygen atoms.
  7. What is the name of the type of reaction where a substance burns in air?
    Answer
    Combustion.
  8. Add a 50 ml beaker to the scale and add 50 cm3 of 1.0 M copper(II) sulfate solution. Add 1 g of fine iron powder to the solution and observe the reading on the balance. What mass changes take place?
    Answer
    The balance reading starts at 72.98 g and increases by 1 g when the iron powder is added, and there is no further change.
  9. Now select the evaporating dish and write down a word equation for the reaction that has taken place.
    Answer
    copper sulfate + iron → iron sulfate + copper
  10. Is this chemical reaction accompanied by any mass change?
    Answer
    No.

Summary

When a solid element combines with oxygen, the mass increases as the compound formed contains atoms of both the element and oxygen.
When a chemical reaction takes place, the total mass of the products equals the total mass of the reactants. Mass is conserved in a chemical reaction.

Task 2: Decomposition of metal carbonates

  1. Open Yenka file Model 2. Record the mass of the evaporating dish in the table below. Add the copper carbonate to the evaporating dish and record the new mass. Calculate the mass of copper carbonate present.
    Mass of evaporating dish + copper carbonate=
    Answer
    12.5 g
    Mass of empty evaporating dish =
    Answer
    10 g
    Mass of copper carbonate =
    Answer
    2.5 g
  2. Now heat the copper carbonate to constant mass on the Bunsen burner. Record the final mass of the dish and remaining solid. Calculate the mass of the product formed.
    Mass of evaporating dish + product =
    Answer
    11.61 g
    Mass of empty evaporating dish =
    Answer
    10 g
    Mass of product =
    Answer
    1.61 g
  3. What has happened to the mass of the solid? Why do you think that this has happened? (Hint: use the reaction details in the mini-toolbar to see what reaction has taken place.)
    Answer
    The mass has decreased by 0.89 g as carbon dioxide gas evolved. Copper oxide remains, which contains no atoms of carbon and fewer atoms of oxygen, so it will have a lower mass.

Summary

When gases are produced during reactions, mass is lost as the gas escapes into the atmosphere.

Teacher Summary

  • This exercise simulates the measurements required in order to practically determine the mass changes during three different types of reaction.