What do metal carbonates decompose into

The experiment should take about 40—45 minutes. It is important not to inhale dust of lead carbonate or the oxide formed. Wash hands after using lead compounds. Copper carbonate, CuCO 3. Procedure Put a large spatula measure of the carbonate to be tested in a test tube. Fit a delivery tube and then clamp the test tube so that the delivery tube dips into a second test tube containing 2—3 cm 3 limewater. Heat the solid gently at first, then more strongly. Lift the delivery tube from the limewater before, or as soon as, the heating is stopped.

This is to avoid suck-back. Write down all observations. Notice what happens to the limewater and how long it takes to turn milky if at all. Notice whether any melting occurs in the heated test tube and any colour changes taking place. Write your results in tabular form. Repeat the experiment with the other metal carbonates supplied, and in each case write down your observations.

Wash your hands thoroughly at the end of these experiments, since some of the metal carbonates are toxic. Show Fullscreen. Additional information This is a resource from the Practical Chemistry project , developed by the Nuffield Foundation and the Royal Society of Chemistry. Level years years years. Use Practical experiments. Category Thermodynamics Reactions and synthesis.

Development of scientific thinking evaluate risks both in practical science and the wider societal context, including perception of risk in relation to data and consequences WJEC Combined science Working scientifically 1.

Development of scientific thinking evaluate risks both in practical science and the wider societal context, including perception of risk in relation to data and consequences. Related articles. Resource Conservation of mass on dissolving and heating TZ Help students understand how mass is conserved when a substance is dissolved or heated. The term "thermal decomposition" describes splitting up a compound by heating it. All the Group 2 carbonates and their resulting oxides exist as white solids.

If "X" represents any one of the elements, the following describes this decomposition:. The Group 2 nitrates undergo thermal decomposition to the metal oxide, nitrogen dioxide and oxygen gas. These compounds are white solids and brown nitrogen dioxide and oxygen gases are also given off when heated. Magnesium and calcium nitrates normally crystallize with water, and the solid may dissolve in its own water of crystallization to make a colorless solution before it starts to decompose.

Both carbonates and nitrates of Group 2 elements become more thermally stable down the group. The larger compounds further down require more heat than the lighter compounds in order to decompose. This page offers two different explanations for these properties: polarizability and energetics. Detailed explanations are given for the carbonates because the diagrams are easier to draw, and their equations are also easier.

Exactly the same arguments apply to the nitrates. In other words, it has a high charge density and has a marked distorting effect on any negative ions which happen to be near it. This structure two single carbon-oxygen bonds and one double bond, with two of the oxygen atoms each carrying a negative charge.

In real carbonate ions all the bonds are identical, and the charges are distributed over the whole ion, with greater density concentrated on the oxygen atoms. In other words, the charges are delocalized. The next diagram shows the delocalized electrons. The shading is intended to show that there is a greater electron density around the oxygen atoms than near the carbon. If this ion is placed next to a cation, such as a Group 2 ion, the cation attracts the delocalized electrons in the carbonate ion, drawing electron density toward itself.

The carbonate ion becomes polarized. The amount of heating required depends on the degree to which the ion is polarized. More polarization requires less heat. The smaller the positive ion is, the higher the charge density, and the greater effect it will have on the carbonate ion.

As the positive ions get larger down the group, they affect on the carbonate ions near them less. More heat must be supplied for the carbon dioxide to leave the metal oxide. The argument is exactly the same for the Group 2 nitrates.

The small cations at the top of the group polarize the nitrate ions more than the larger cations at the bottom do. This process is much more difficult to visualize due to interactions involving multiple nitrate ions. Lattice enthalpy is the heat needed to split one mole of crystal in its standard state into its separate gaseous ions. For example, for magnesium oxide, it is the heat needed to carry out 1 mole of this change:.

Note: Lattice enthalpy is more usually defined as the heat evolved when 1 mole of crystal is formed from its gaseous ions. In that case, the lattice enthalpy for magnesium oxide would be kJ mol The term we are using here should more accurately be called the "lattice dissociation enthalpy".

You can apply Hess's Law to this, and find two routes which will have an equal enthalpy change because they start and end in the same places. For reasons we will look at shortly, the lattice enthalpies of both the oxides and carbonates fall as you go down the Group. But they don't fall at the same rate. The oxide lattice enthalpy falls faster than the carbonate one. If you think carefully about what happens to the value of the overall enthalpy change of the decomposition reaction, you will see that it gradually becomes more positive as you go down the Group.

The size of the lattice enthalpy is governed by several factors, one of which is the distance between the centres of the positive and negative ions in the lattice. Forces of attraction are greatest if the distances between the ions are small. If the attractions are large, then a lot of energy will have to be used to separate the ions - the lattice enthalpy will be large. The lattice enthalpies of both carbonates and oxides fall as you go down the Group because the positive ions are getting bigger.

The inter-ionic distances are increasing and so the attractions become weaker. The lattice enthalpies fall at different rates because of the different sizes of the two negative ions - oxide and carbonate. The oxide ion is relatively small for a negative ion 0. In the oxides, when you go from magnesium oxide to calcium oxide, for example, the inter-ionic distance increases from 0. In the carbonates, the inter-ionic distance is dominated by the much larger carbonate ion.

Although the inter-ionic distance will increase by the same amount as you go from magnesium carbonate to calcium carbonate, as a percentage of the total distance the increase will be much less. I can't find a value for the radius of a carbonate ion, and so can't use real figures. For the sake of argument, suppose that the carbonate ion radius was 0.

The inter-ionic distances in the two cases we are talking about would increase from 0. The rates at which the two lattice energies fall as you go down the Group depends on the percentage change as you go from one compound to the next. On that basis, the oxide lattice enthalpies are bound to fall faster than those of the carbonates. The nitrate ion is bigger than an oxide ion, and so its radius tends to dominate the inter-ionic distance. The lattice enthalpy of the oxide will again fall faster than the nitrate.

If this is the first set of questions you have done, please read the introductory page before you start. The Facts The effect of heat on the Group 2 carbonates All the carbonates in this Group undergo thermal decomposition to give the metal oxide and carbon dioxide gas.

If "X" represents any one of the elements: As you go down the Group, the carbonates have to be heated more strongly before they will decompose. The carbonates become more stable to heat as you go down the Group. The effect of heat on the Group 2 nitrates All the nitrates in this Group undergo thermal decomposition to give the metal oxide, nitrogen dioxide and oxygen.

Again, if "X" represents any one of the elements: As you go down the Group, the nitrates also have to be heated more strongly before they will decompose. The nitrates also become more stable to heat as you go down the Group.