Analysis and Synthesis

Question 1

By which two methods can positive ions be identified?

Answer 1

Using a flame test

Using Sodium hydroxide solution

Question 2

The hydroxides of most metals that have ions with 2+ and 3+ charges are what in water?
When sodium hydroxide solution is added to solutions of these ions what forms?

Answer 2

Insoluble

A precipitate of metal hydroxide forms

Question 3

What will form sodium hydroxide is added to solutions of aluminium, calcium and magnesium ions?

Answer 3

A white precipitate

Question 4

What happens when excess sodium hydroxide is added to the precipitate of aluminium hydroxide?

Answer 4

It dissolves

Question 5

What will form sodium hydroxide is added to solutions of:
Copper (II) ions?
Iron (II) ions?
Iron (III) ions?

Answer 5

A blue precipitate of copper (II) hydroxide
A green precipitate of Iron (II) hydroxide
A brown precipitate of Iron (III) hydroxide

Question 6

What is the test for carbonate ions?

Answer 6

Add dilute hydrochloric acid to the substance to see if it fizzes, if it does and the gas produced turns limewater milky, the substance contains carbonate ions.

Question 7

What is the test for halide ions?
What colour precipitate will chloride ions produce?
What colour precipitate will bromide ions produce?
What colour precipitate will iodide ions produce?

Answer 7

Add dilute nitric acid and then silver nitrate solution.
Chloride ions give a white precipitate.
Bromide ions give a cream precipitate
Iodide ions give a yellow precipitate.

Question 8

What is the test for sulfate ions? What colour will the precipitate be if sulfate ions are present?

Answer 8

Add dilute hydrochloric acid and then barium chloride solution, if a white precipitate forms, sulfate ions are present.

Question 9

What is a neutarlisation reaction?

Answer 9

A neutralisation reaction occurs when solutions of an acid and an alkali react to form a salt and water.

Question 10

What can be found using a titration?

Answer 10

The volumes of solutions that react exactly in a neutralisation solution.

Question 11

What method is used to carry out a titration?

Answer 11

1) A pipette is used to measure accurately the volume of alkali that is put into a conical flask.
2) An indicator is added to the alkali
3) The conical flask is placed on top of a white tile and under a burette.
4) The burette is filled with acid, the volume of acid in the burette should be recorded, the acid is then added gradually to the alkali in the conical flask.
5) When the indicator changes colour the end points has been reached and you should stop adding acid immediately by closing off the tap of the burette.
6) The final volume of acid should be recorded.
7) The volume of acid used can be calculated from the initial and final burette readings.

Question 12

Why should titrations be carried out several times?

Answer 12

To improve the repeatability of the results.

Question 13

What two units are used to measure the concentrations of solutions?

Answer 13

Grams per decimetre cubed

Moles per decimetre cubed

Question 14

If the mass or the number of moles of a substance dissolved in a given volume (cm cubed) of a solution is known, what method should be used to calculate the concentration of the solution?

Answer 14

1 dm cubed of solution contains Mass/Number of moles ÷ volume = Concentration (x1000g)

Question 15

If the volume of a solution and its concentration are known, what can be calculated?
What equation can be used to to calculate this?

Answer 15

The mass or the number of moles of the substance in any volume of solution.
Volume x Concentration = Number of moles

Question 16

1 decimetre cubed is equal to how many centimetres cubed?

Answer 16

1000 centimetres cubed

Question 17

If the concentration of one solution that reacts exactly in a titration is known, and the volumes that react together are know, what method would you use to calculate the unknown concentration?
If concentration is known in mol/dm cubed how would concentration be calculated in g/dm cubed?

Answer 17

Acid: Conc x Volume = Alkali: Conc x volume
Substitute in the values and calculate answer

By multiplying mol/dm cubed by mr

Question 18

What are qualitative methods of analysis used for?

Examples of qualitative methods?

Answer 18

To find out if a substance is in a sample

Tests for ions

Question 19

What are quantitate methods of analysis used for?

Examples of quantitive methods?

Answer 19

To find out how much of a substance is in a sample

Titrations, mass spectrometry and gas chromatography

Question 20

Large databases of the results of analysis have been built up with the aid of computers, what are the databases used for?

Answer 20

To identify substances in samples
To identify individuals
To monitor changes in amounts of substances over time

Question 21

What is a reversible reaction?

Answer 21

A reaction where the products can react to make the reactants again.

Question 22

In a closed system when is the equilibrium of a reversible reaction reached?
What happens at equilibrium?

Answer 22

When the rate of the forward reaction is equal to the rate of the reverse reaction.
At equilibrium both reactants continue to happen, but the amounts of reactants and products remain constant.

Question 23

For a reversible reaction in a closed system, what effect will changing the pressure have on the reaction and why?

Answer 23

Changing the pressure will change the amount of product formed as the system tries to achieve equilibrium.

Question 24

For a reversible reaction in a closed system, what effect will removing a product have on the reaction and why?

Answer 24

More reactants will react to try and achieve equilibrium and so more product is formed.

Question 25

If the forward reaction produces more molecules of gas…
What will happen if the pressure is increased?
What will happen if the pressure is decreased?

Answer 25

An increase in pressure decreases the amount of product formed.
A decrease in pressure increases the amount of product formed.

Question 26

If the forward reaction produces fewer molecules of gas…
What will happen if the pressure is increased?
What will happen if the pressure is decreased?

Answer 26

An increase in pressure will increase the amount of products formed.
A decrease in pressure will decrease the amount of products formed.

Question 27

If the forward reaction is exothermic…
What will happen if the temperature is increased?
What will happen if the temperature is decreased?

Answer 27

An increase in temperature will decrease the amount of products formed.
A decrease in temperature will increase the amount of products formed.

Question 28

If the forward reaction is endothermic…
What will happen if the temperature is increased?
What will happen if the temperature is decreased?

Answer 28

An increase in temperature will increase the amount of products formed.
A decrease in temperature will decrease the amount of products formed.

Question 29

What is the Haber process used to manufacture, and what are the uses of this substance?

Answer 29

Ammonia

Used to make fertilisers and other chemicals

Question 30

In the Haber process, where is the nitrogen and hydrogen usually sourced from?

Answer 30

Nitrogen - Air

Hydrogen - Natural gas

Question 31

Detail the main steps involved in the Haber process and include the % yield of ammonia.

Answer 31

1) Nitrogen from air and hydrogen obtained from natural gas are purified and mixed in the correct proportions.
2) The gases are passed over an iron catalyst at a temperature of around 450 degrees celsius and a pressure of 200 atmospheres.
3) Some of the ammonia produced breaks down into hydrogen and nitrogen, the yield of ammonia is around 15%.
4) The gases that come out of the reactor are cooled so the ammonia condenses, the liquid ammonia is the separated from the unreacted gases.
5) The unreacted gases are recycled so they are not wasted.

Question 32

Explain why the reaction is conducted at a pressure of 200 atmospheres.

Answer 32

The products have fewer molecules than the reactants, so the higher the pressure, the greater the yield of ammonia.
However the higher the pressure the more energy is needed to compress the gas and the stronger the reaction vessels and pipes need to be, which increases costs.

Therefore this pressure is seen a s a compromise between the costs and the yield.

Question 33

Explain why the reaction is conducted at a temperature of 450 degrees celsius?

Answer 33

The forward reaction is exothermic so the lower the temperature the greater the yield of ammonia.
However the rate of reaction decreases as the temperature is lowered and the iron catalyst becomes ineffective, so it would take longer to produce any ammonia.

Therefore this temperature is seen as a compromise to give a reasonable yield in a short time.