Monday, May 3, 2010

Chemical Reactions

2006 QUESTION FOUR: OBSERVING REACTIONS
A group of students carried out an investigation into the thermal decomposition of the following solid carbonates: sodium carbonate, calcium carbonate, and copper carbonate.

(a) Describe the appearance of each carbonate before it was heated.

(i)
Sodium carbonate - A white solid

(ii)
Calcium carbonate - A white solid

(iii)
Copper carbonate -A green solid



(b) Describe the observations the students would have made when heating each of the carbonates.

(i)
Sodium carbonate - A white solid of sodium oxide

(ii)
Calcium carbonate -A white solid of calcium oxide

(iii)
Copper carbonate - A black solid of copper oxide


(c) Write a balanced equation for the thermal decomposition of copper carbonate.

Copper carbonate --> copper oxide + carbon dioxide gas
CuCO3 --> CuO + CO2


Some grey powdered magnesium was added to a green solution of iron (II) sulfate in a beaker, and mixed well. The green colour of the solution faded and the grey powder disappeared. A new dark grey solid formed on the bottom of the beaker.

(d) (i) State what type of reaction this is.
REDOX reaction

(ii) Discuss what happened in this reaction. Your answer should link the reactants and products involved in the reaction to the observations made.
The green Fe2+ ions in the iron sulfate is displaced by the grey magnesium metal forming a colourless solution of magnesium sulfate and Iron metal. This explains why the green iron(II) sulfate fades as the magnesiun sulfate formed is colourless.
The dark grey solid froming at the bottom of the beaker is the Iron metal that is forming in the reaction

Include an appropriate balanced equation in your answer. Spectator ions may be omitted.
FeSO4 + Mg --> MgSO4 + Fe


2007 QUESTION THREE: OBSERVING CHEMICAL REACTIONS
A small amount of zinc hydroxide is heated in a test tube over a Bunsen burner. The following reaction occurs.
heat
Zn(OH)2 (s) --> ZnO(s) + H2O(g)

(a) State what type of reaction is occurring.
Thermal decomposition

(b) Fully describe the observations that would be expected if this reaction was carried out in a school laboratory. Remember to link your observations to the substances involved.
The zinc hydroxide would react to form zinc oxide, both of which are white, however the reaction produces water which would be seen as condensation on the side of the tube.


2007 QUESTION SIX: CHEMICAL REACTIONS
Part A
A colourless solution of barium nitrate is added to a pale green solution of iron(II) sulfate in a beaker. A reaction occurs.

(a) Describe the observations that would be expected for this reaction.
Upon addition of the two solutions a white precipitate of barium sulfate would form.

(b) State what type of reaction is occurring.
Precipitation reaction

(c) Discuss the chemistry of this reaction. Your discussion should refer to the observations you made in part (a). Include a balanced equation in your answer. Spectator ions may be omitted.
Barium nitrate + iron (II) sulfate --> barium sulfate + iron (ii) nitrate
Ba2+ + SO42- --> BaSO4 - ionic equatiion

The colourless solution of barium nitrate is added to the pale green solution of iron (II) sulfate. These two solutions contain ions in solution. The reaction results in the formation of a white barium sulfate precipitate where the barium ions and sulfate ions bond together to form the white solid.
The iron (II) ions and nitrate ions remain in solution.

Part B
A strip of magnesium ribbon is heated over a Bunsen burner. It burns with a bright white light and forms a white ash.

Discuss the chemistry of this reaction in terms of oxidation and reduction. Write the appropriate half equations and overall balanced equation in your answer.
(Mg --> Mg2+ + 2e-) 2 Oxidation
O2 + 4e- --> 2O2- Reduction

2Mg + O2 --> 2Mg2+ + 2O2-

The silver magnesium metal is oxidised to magnesium ions as is loses electrons. In this reaction it is acting as the reductant.
The colourless oxygen gas is reduced to oxide ions as it gains electrons. In this reaction it is acting as the oxidant.

No comments:

Post a Comment