Tuesday, May 11, 2010

Chemical reactions questions and answers.

Questions 1
For this reaction:
• Describe the observations that you would make
• Write the net ionic equation for the reaction.
• Explain what happens to each ion present in the solutions used in this reaction.

If chose silver chloride precipitate
Describe the observations that you would make.
Two colourless solutions (silver nitrate and sodium chloride) are mixed together forming a white precipitate ( silver chloride).
Write the net ionic equation for the reaction.
AgNO3 (aq) + NaCl(aq) --> AgCl (s) + NaNO3 (aq)
or
Ag+(aq) + Cl-(aq) --> AgCl (s)

Explain what happens to each ion present in the solutions used in this reaction.
This is a precipitation reaction. Silver ions will react with chloride ions to form insoluble silver chloride

The other ions in the reaction, sodium and nitrate do not react with each other, so stay in solution. They are not written in the ionic equation; they are spectator ions.

If chose aluminium carbonate precipitate
Describe the observations that you would make.
Two colourless solutions ( aluminum nitrate and sodium carbonate) are mixed together forming a white precipitate ( aluminum carbonate)
Write the net ionic equation for the reaction.
2Al(NO3)3 (aq) + 3Na2CO3 (aq) -->Al2(CO3)3 (s) + 6NaNO3(aq)
or
2Al3+(aq) + 3CO32-(aq) --> Al2(CO3)3 (s)

Explain what happens to each ion present in the solutions used in this reaction.
This is a precipitation reaction. Aluminium ions will react with carbonate ions to form insoluble aluminium carbonate.

The other ions in the reaction, sodium and nitrate, NO3–, do not react with each other, so stay in solution. They are not written in the ionic equation; they are spectator ions.


Question 2
When Copper carbonate is heated it undergoes thermal decomposition.
Discuss the chemical reaction occurring including the following:
• Describe what observations would be made and explain them using a balanced equation for the reaction taking place.
• Describe the test you could carry out to identify the gas produced in the chemical reaction. Include observations made in your answer and a balanced equation for the reaction taking place.
When solid green copper carbonate is heated in a test tube using a Bunsen burner flame it turns from green to black as it is forming black copper oxide

The equation for the reaction taking place is
Copper carbonate --> copper oxide + carbon dioxide gas
CuCO3(s) --> CuO (s) + CO2(g)

The gas produced during thermal decomposition is carbon dioxide gas
To test for this gas the gas from the reaction can be bubbled through lime water which would turn from clear to cloudy indicating carbon dioxide gas has been produced.
CO2 + Ca(OH)2 --> CaCO3 + H2O

( A splint going out is unacceptable as is not conclusive)

Question 3
When an iron nail is placed in a beaker of copper sulfate a redox reaction occurs.
Discuss the chemistry of this reaction.
In your answer:
• state what you would observe as this reaction proceeds
• explain why this is an oxidation-reduction reaction
• identify the reduction and oxidation processes
• include all appropriate equations in your discussion.

The grey nail will be covered in a reddish –brown solid which is copper and the blue colour (due to Cu2+ ions) of the solution will start to fade.

This is a redox reaction because the Fe loses electrons to form Fe2+ and the Cu2+ ions gain these electrons to form Cu. The electrons are therefore transferred from one substance to another.

The Fe is oxidised: Fe --> Fe2+ + 2e
The Cu2+ ions are reduced Cu2+ + 2e --> Cu

Overall equation: Cu2+ + Fe --> Cu + Fe2+

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