Conservation Of Mass…

When a chemical reaction occurs, mass/matter is conserved. This means that the reactants and the products have the same mass. No new atoms are created during the reaction and no atoms are destroyed; they are just rearranged. The following short video attempts to illustrate this principle and although it contains a rather big mistake, it is still worth watching.

The video shows a few different examples of chemical reactions. The first example is barium chloride reacting with sodium sulphate. Both compounds are soluble, so when they are mixed the ions can rearrange themselves. This is a reaction known as a double displacement because the sodium ions grab the chloride ions and the barium ions grab the sulphate ions. Barium sulphate is insoluble so it settles out at the bottom as a white precipitate and the sodium chloride remains in solution. The total mass of the flask’s contents does not change because no matter has been created or destroyed.

The second reaction is also a double displacement, this time with sodium chloride and silver nitrate. Silver is a very unreactive metal so most of its salts are insoluble. You may remember me telling you that all nitrates are soluble so that is why the silver nitrate is in solution here. When they mix, the sodium ions displace the silver ions to form sodium nitrate. This leaves the silver ions to link up with the chloride ions – silver chloride is insoluble so this settles out as a white precipitate. Again, the total mass of the flask and contents does not change. Did you spot the mistake in the video?

Finally he shows three symbol equations to demonstrate that no atoms are added or removed during a reaction. The first one is calcium oxide (quicklime) being slaked (reacting with water) to form calcium hydroxide (slaked lime). The second is potassium chlorate (an excellent oxidising agent) being thermally decomposed to leave potassium chloride and release oxygen. If this was done in an open system, the oxygen would escape and the reaction would appear to lose mass. If it was done such that the gas could be collected in a gas syringe then there would be no loss of mass. Lastly there is a familiar displacement reaction with zinc and copper sulphate.

There are five kinds of reaction that you are likely to encounter in chemistry at this level. They are (thermal) decomposition, combination, neutralisation, displacement and RedOx reactions. Each one obeys the Law of Conservation of Matter (of course – that’s why it is a Law!)

Thermal decomposition

Some molecules fall apart when they are heated. The most common group of compounds to do this are the carbonates.

You can see here that we started with one copper atom, one carbon atom and three oxygen atoms. After the reaction, we had the same number and type of atoms but in a different arrangement.

Combination reaction

When two elements combine, they form a compound. Compounds always form in fixed proportions. Energy is either released or absorbed when a compound forms. You need another chemical reaction to separate the elements from a compound again.

One copper atom reacts with one sulphur atom to produce a single molecule of copper sulphide.

Neutralisation

Whenever an acid reacts, a neutralisation reaction occurs. Acids react with bases (alkalis), carbonates & some metals.

In each case, if you add up the number of atoms on the left hand side, it is identical to the number of atoms on the right hand side. It has to be – that’s the Law!

Displacement

Sometimes a more reactive element can displace a less reactive element in a solution.

This reaction doesn’t require any heating to make it happen. It happens at room temperature as soon as the piece of zinc is placed into the copper sulphate solution. The zinc is more reactive than the copper so it can knock it out of the solution.

RedOx

This is short for “reduction/oxidation” reaction. One element gains oxygen (oxidised) whilst another loses oxygen (reduced). The two substances reacting will be solids at the start and will need strong heating to get them to react.

The magnesium rips the oxygen away from the copper because magnesium is more reactive than copper. This is similar to the smelting process used to extract iron from iron ore in a blast furnace.

Questions…

1. What colour change would you observe when copper carbonate thermally decomposes?
2. What gas would be released if magnesium were dropped into hydrochloric acid?
3. What gas is released when sodium bicarbonate reacts with vinegar?
4. What compound is produced when zinc and sulphur react?
5. What would you see if you placed a piece of copper into silver nitrate solution?
6. What two products would you get if you mixed copper sulphate solution and sodium carbonate solution?
7. If you reacted sodium chloride solution and copper nitrate solution, would you expect a solid precipitate to form? Explain your answer.
8. Did you spot the mistake in the video? What was it?