Suggest why calcium oxide is used to neutralise acidic soils

07 May Suggest why calcium oxide is used to neutralise acidic soils

  1. Cement is made by heating clay with crushed calcium carbonate. During this process, the calcium carbonate is first converted to calcium oxide.

 

(a)        Suggest why calcium oxide is used to neutralise acidic soils.

………………………………………………………………………………………………………

 

 

(b)        Concrete is made from cement, sand and water. Concrete is slightly porous. When rain water soaks through concrete, some of the uncombined calcium oxide dissolves to form calcium hydroxide.

 

(i)         Write a balanced chemical equation, with state symbols, for this reaction.

…………………………………………………………………………………………………………

 

(ii)        The aqueous calcium hydroxide in wet concrete reacts with carbon dioxide in the air.

Ca(OH)2 + CO2 → CaCO3 + H2O

 

The diagram shows the pH at various points inside a cracked concrete beam.

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Describe and explain the change in pH from the surface to the centre of the beam.

……………………………………………………………………………………………………………

……………………………………………………………………………………………………………

………………………………………………………………………………………………………..

(iii)       25.0 cm3 of an aqueous solution of calcium hydroxide is exactly neutralised by 18.0 cm3 of 0.040 mol/dm3 hydrochloric acid.

 

Ca(OH)2 + 2HCl → CaCl 2 + 2H2O

 

Calculate the concentration, in mol/dm3, of the aqueous calcium hydroxide.

concentration = ……………………………….mol/dm3

 

(c)        Bromine is a liquid with a low boiling point and a strong smell.

 

A technician spilt some bromine in the corner of a room which is free of draughts. After thirty seconds the bromine could be smelt on the other side of the room.

 

Use the kinetic particle theory to explain why the bromine could be smelt on the other side of the room.

…………………………………………………………………………………………………………….

…………………………………………………………………………………………………………..

 

(d)        Chloroethene can be polymerised to form poly(chloroethene).

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Draw a section of a poly(chloroethene) chain to show at least two repeating units.

 

  1. Electrolysis can be used to remove unwanted hair. The customer holds a

metal bar which acts as a positive electrode. A needle, which acts as the negative electrode, is held by the operator.

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(a)        What do you understand by the term electrolysis?

(b)        The solution around the tip of the needle is mainly a dilute aqueous solution of sodium chloride.32

(i)         Name all the ions present in the solution during this electrolysis.

 

(ii)        During electrolysis a small amount of chlorine is formed at the surface of the skin. Write an ionic equation for this reaction.

 

(iii)       During electrolysis, a gas forms at the tip of the needle and the solution changes from pH 7 to pH 10. Explain both these changes.

 

(c)        Explain why aqueous sodium chloride solution conducts electricity but solid sodium chloride does not.

 

(d)        The sweat glands in the skin produce small amounts of lactic acid.

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Lactic acid reacts with ethanol to form an ester.

(i)         State the conditions needed to form an ester.

 

(ii)        Draw the structure of the ester produced by the reaction of lactic acid with ethanol.

 

 

 

Miracle Learning Centre (O Level Chemistry Tuition/ IP Chemistry Tuition)

Did you answer the Secondary Chemistry Tuition Question correctly?

 

9.a. it is (a) basic (oxide)/it is a base/it is (an) alkaline oxide

ALLOW: it is alkaline/an alkali (in solution)/has a high pH (when it reacts with

water)/forms hydroxide ions (when reacts with water)

NOT: it contains hydroxide ions

NOT: answers about effect on plant growth

 

b.i. CaO (s) + H2O (l) à Ca(OH)2 (l)

 

  1. any two of:
  • pH increases inside beam/
  • carbon dioxide (in solution) is slightly acidic/
  • on the surface CO2 reacts with neutralises Ca(OH)2 OR implication that pH neutral on the surface/
  • reaction of carbon dioxide with calcium hydroxide reduces alkalinity (or lowers pH)/
  • further inside (beam), less (or no) CO2/little or no reaction (of carbon dioxide) with calcium hydroxide inside (beam)/
  • crack allows carbon dioxide to enter the inside of the beam/
  • near crack alkalinity less/pH lower

 

iii. 0.0144 mol/dm3

 c. Particles move into spaces in the air, moves from a region of higher conc to a region of lower conc.

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10.a.  breaking down/splitting up/decomposition (of electrolyte / compound / substance) by electricity/electric current

ALLOW: causing a chemical reaction to occur by an electric current

ALLOW: producing elements (from compounds) by using an electric current

 

b.i. sodium, chloride, hydrogen, hydroxide (ALLOW: hydroxyl) (all 4 needed)

ALLOW: Na+, Cl , H+ and OH

ALLOW: mixture of symbols and words

NOT: chlorine ions

ii. 2Cl → Cl2 + 2e

IGNORE: state symbols

ALLOW 2e instead of 2e

ALLOW: 2Cl – 2e → Cl2

 

iii. hydrogen ions form hydrogen (gas)/hydrogen ions removed

hydroxide/OH ions (remaining in solution) are alkaline OR hydroxide/OH ions give high pH/alkalinity caused by OH ions

NOT: hydroxide ions remain in solution (must be a link to pH

c. in solution ions can move

NOT: ions are free

ALLOW: ions carry the charge

REJECT: if reference to electrons moving

ions cannot move in solid/ions held together (by strong forces)

IGNORE: electrons can’t move for this mark

NOT: ions not present

d. reflux ALLOW: heat/high temperature/boil/warm

ALLOW: temperature range of 30–200 °C

NOT: distil

(sulphuric) acid catalyst/sulphuric acid

ALLOW: other named mineral acids/hydrogen ion catalyst

NOT: acid without qualification (otherwise confusion with the lactic acid)

NOT: catalyst (unqualified)

e. structure of lactic acid correct i.e. CH3CHOHCO2C2H5

ALLOW: RCO2C2H5

REJECT: if OH group altered

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