C3- Analysis and Synthesis

Question 1

Describe a flame test to test for positive ions.

Answer 1

1) Clean a piece of nichrome wire with sandpaper, then water.
2) Dip wire into test substance.
3) Hold wire at the edge of a blue Bunsen flame.

Question 2

What colour flame do each of these metal ions produce in a flame test:

• Lithium?
• Sodium?
• Potassium?
• Calcium?
• Barium?

Answer 2

• Lithium➡️Crimson.
• Sodium➡️Yellow.
• Potassium➡️Lilac.
• Calcium➡️Red.
• Barium➡️Green.

Question 3

Describe a sodium hydroxide test for positive ions.

Answer 3

1) Dissolve substance in water.
2) Add a sodium hydroxide solution until in excess.

Question 4

In a sodium hydroxide test for positive ions, what ions produce no precipitates?

Answer 4

Sodium (Na⁺) or Potassium (K⁺)➡️Because these hydroxides are soluble.

Question 5

In a sodium hydroxide test for positive ions, what ions produce white precipitates which do not redissolve?

Answer 5

Calcium (Ca²⁺) or Magnesium (Mg²⁺)➡️Because these hydroxides are both white and insoluble.

Question 6

In a sodium hydroxide test for positive ions, what ions produce white precipitates which dissolve when excess sodium hydroxide is added?

Answer 6

Aluminium (Al³⁺)➡️Because the hydroxide is insoluble in water but soluble in sodium hydroxide solution.

Question 7

In a sodium hydroxide test for positive ions, what ions produce a coloured precipitate and what colours are produced?

Answer 7

• Copper (Cu²⁺)➡️Blue.
• Iron (II)(Fe²⁺)➡️Dirty green.
• Iron (III)(Fe³⁺)➡️Brown.

Question 8

Describe a test for carbonates for negative ions.

Answer 8

1) Add dilute hydrochloric acid.
2) If it fizzes, test for CO₂ with lime water (which goes cloudy).

This indicates a carbonate (CO3^2-).

Question 9

Describe a test for sulphates for negative ions.

Answer 9

1) Add a little dilute hydrochloric acid followed by barium chloride solution.
2) A sulphate (SO₄^2-) gives a white precipitate.

Question 10

Describe a test for halides for negative ions.

Answer 10

1) Add a little nitric acid followed by a few drops of silver nitrate solution.

Question 11

What coloured precipitates do non-metal ions produce in a test for halides?

Answer 11

• Chloride (Cl^-)➡️White.
• Bromide (Br^-)➡️Cream coloured.
• Iodide (I^-)➡️Yellow.

Question 12

What are the advantages of using instrumental methods in chemical analysis?

Answer 12

✅Quicker.

✅You can use very small samples.

✅More sensitive.

✅Accurate.

Question 13

Balance this ionic equation:

Fe³⁺+3OH^-➡️…

Answer 13

Fe³⁺+OH^-➡️Fe(OH)₃

Question 14

Describe the method for titration.

Answer 14

1) Use a graduated pipette and safety filler to put 25cm³ of 0.1M sodium hydroxide solution into a 250cm³ conical flask.
2) Add 3-5 drops of indicator.
3) Place a white tile under the burette, then fill to the 0 mark with hydrochloric acid.

*Remember the bottom of meniscus must be in line with the 0 line*

4) Add the acid from the burette 1cm³ at a time until you approach the end point (given your indicator colour change). Swirl the conical flask constantly. Now add the acid one drop at a time until you have reached the end point, as seen by the colour change.
5) Repeat the experiment until 3 concordant results are obtained.

Question 15

What is a titration used for?

Answer 15

A titration is used to measure accurately how much acid and alkali react together completely.

Question 16

What is the difference between a pipette and burette?

Answer 16

• A pipette measures a fixed volume, has a single graduation and has no tap.
• A burette measures different volumes, has graduations and has a tap.

Question 17

What are concentrations of solutions measured in?

Answer 17

g/dm³ or mol/dm³.

Question 18

How can the number of moles of a solution be calculated?

Answer 18

n=vc/1000

n= no. of moles (mol).

v= volume (cm³).

c= concentration (moldm^-3 or M).

Question 19

We found that 25.2cm³ of 0.1moldm^-3 hydrochloric acid was needed to neutralise a 25cm³ sample of sodium hydroxide.

Calculate the concentration of NaOH.

Answer 19

1) n=(25.2 x 0.1)/1000= 0.00252 moles of HCl.
2) HCl + NaOH➡️NaCl + H₂O= 1 mole of HCl always reacts with exactly 1 mole of NaOH.
3) c= 1000n/v= (1000 x 0.00252)/25= 0.1008M.

Question 20

How can the mass of a solute be calculated from the number of moles in it?

Answer 20

n= m/M_r

n= number of moles (mol).

m= mass (g).

M_r= relative formula mass.

Question 21

What is the mass of sodium hydroxide in 100cm³ of a solution with a concentration of 0.2mol/dm³?

1 mol NaOH= 40g.

Answer 21

1) (100 x 0.2)/1000= 0.02mol.
2) 0.02 x 40= 8g.

Question 22

How are qualitative methods for chemical analysis used?

Answer 22

Qualitative methods are used to find out if a substance is in a sample.

Question 23

How are quantative methods for chemical analysis used?

Answer 23

Quantative methods can tell us how much of a substance is in a sample.

Question 24

What is meant by equilibrium?

Answer 24

When the rates of the forward and reverse reactions of a reversible reaction are equal or when the amounts of reactants and products in a reversible reaction are constant.

Question 25

How does changing the concentration of a reactant change the product?

Answer 25

Increasing the concentration of a reactant will cause more products to be formed as the system tries to achieve equilibrium. If a product is removed, more reactants will react to try to achieve equilibrium and so more product is formed.

Question 26

To make SO₃, the reaction 2SO₂ +O₂⇌2SO₃ is done in a reactor over a heated catalyst. Why is the SO₃ removed from the reactor as soon as it is made?

Answer 26

So that more of reactants react or so more SO₃ is produced.

Question 27

How does changing the pressure affect reversible reactions involving gases at equilibrium?

Answer 27

- Increasing the pressure favours the reaction with fewer molecules. - Decreasing the pressure favours the reaction with more molecules.

Question 28

How does changing the temperature in a reversible reaction affect the amount of products formed at equilibrium?

Answer 28

- Increasing the temperature favours the endothermic reaction. - Decreasing the temperature favours the exothermic reaction.

Question 29

What is the Haber process?

Answer 29

The Haber process is used to manufacture ammonia, which can be used to make fertilisers and other chemicals.

Question 30

What is ammonia made from?

Answer 30

Nitrogen and hydrogen.

Question 31

What conditions are used in the Haber process and why?

Answer 31

- Gases passed over iron catalyst. - Temperature of about 450^oC➡️Although lower temperatures would increase the yield of ammonia, the rate of reaction would be too slow. - Pressure of about 200 atmospheres➡️To increase amount of ammonia produced. Although higher pressures would produce more ammonia, they would make the chemical plant too expensive to build and run.

Question 32

Why do higher pressures increase the costs of an industrial process?

Answer 32

More energy is needed and stronger reaction vessels and pipes are needed which both cost more.

Question 33

How is ammonia made in the Haber process separated from unreacted nitrogen and hydrogen?

Answer 33

The gases are cooled and ammonia is liquified.