Chemistry Trial Paper – Difficulty: Moderate

Question 21)

a) Explain how a primary standard base is prepared using an example. [4 marks]

b) Justify the use of the above primary standard you used a primary standard [3 marks]


Question 22)

Outline the process used for a titration experiment with a strong acid of known concentration and a strong base of unknown concentration. [5 marks]


Question 23)

Using an example, explain the importance of buffers to living systems. [3 marks]


Question 24)

Contrast the physical properties of alcohols, carboxylic acids and aldehyde relate these to their molecular structure and their inter molecular forces. [6 marks]


Question 25)

Explain the difference between chain isomers, position isomers and functional group isomers. [4 marks]


Question 26)

With reference to Arrhenius, Bronsted-Lowry and two other scientists of your choice explain the limitations of their models. [8 marks]


Question 27)

Contrast saturated and unsaturated hydrocarbons and explain how you can differentiate between them using a chemical test. [3 marks]


Question 28)

a) Outline the process of making bio ethanol from sugar cane. [4 marks]

b) Evaluate the socioeconomic and environmental implications of producing ethanol from sugar can as opposed to deriving it from fossil fuels through fractional distillation and catalytic cracking. [6 marks]

Question 29)


Answers

Question 21)

a) Explain how a primary standard base is prepared using an example. [4 marks]

A primary standard is prepared by measuring out a known mass of a substance such as anhydrous Na2CO3 using electronic scales and also a known volume of water which is placed in a volumetric flask . Gravimetric analysis will determine the molar mass of the substance where the number of moles can be calculated. After that, the molarity of the primary standard can be determined by dividing the number of moles of Na2CO3 by the volume of water in that solution (M = n/v). 

CriteriaMark
Define primary standard1
Provides an example1
Explains process of making a primary standard2

b) Justify the use of the above primary standard you used as a primary standard [3 marks]

  • Easily obtainable substance with a large and known molecular weight
  • Soluble in water meaning that no precipitates will form
  • Is anhydrous (does not absorb water from its surroundings)
  • Does not react with the atmosphere
CriteriaMark
Large molecular mass allowing for more  accurate measuring1
Mentions it is anhydrous1
Not reactive with the atmosphere1
Or any other suitable points if above are missing

Question 22)

Outline the process used for a titration experiment with a strong acid of known concentration and a strong base of unknown concentration. [5 marks]

Letting the strong base be NaOH + the strong acid be HCl 

  1. Rinse the burette with a small amount of HCl solution. Rinse the conical flask with distilled water and rinse the bulb pipette with NaOH. 
  2. Using a funnel, pour 50mL of HCl into a glass burette clamped to a retort stand. 
  3. Using the rinsed bulb pipette measure 100mL of NaOH in a beaker and pour into a conical flask 25mL of NaOH and place into the rinsed conical flask. 
  4. Place 2-3 drops of bromothymol blue indicator into the conical flask containing the NaOH solution.
  5. Slowly release the tap on the burette and allow for the HCl to drip into the conical flask. Swirl the conical flask until the equivalence point (green) is reached.  
  6. Record the amount of HCl required to reach the equivalence point from the burette reading. 
  7. Discard the neutralised solution into a waste bucket and rinse the conical flask with distilled water.
  8. Repeat the experiment 3 more times and record all data. 
Marking CriteriaMarks
Identifies the correct method4
Identifies appropriate indicator1

Question 23)

Using an example, explain the importance of buffers to living systems. [3 marks]

  • A buffer solution is one that resists a change in pH. 
  • Carbonic acid – hydrogen carbonate buffer (equation)
  • (include states!!)
  • Maintains pH of the blood at 7.35 to 7.45
  • This prevents alkalosis and acidosis and allows for normal function of humans
CriteriaMark
Define what a buffer is1
Provide an example of a buffer with an equation1
Link to the importance for living systems1

Question 24)

Contrast the physical properties of alcohols, carboxylic acids and aldehyde relate these to their molecular structure and intermolecular bonding. [6 marks]

Low boiling point = high volatility

Alcohols:

  • High boiling point – the dispersion forces are weaker and can be disrupted easier 
  • Soluble in water – the hydroxyl group causes hydrogen bonding between the highly electronegative oxygen and hydrogen atom. This makes the molecule polar, thus being able to fully dissolve in water. 

Carboxylic acids

  • Higher boiling point –  
  • Soluble in water – carboxylic acids are polar substances due to the presence of the hydroxyl (O-H) group  in the carboxyl group. As a result, they are able to form hydrogen bonds with other molecules and can therefore dissolve in water.

Aldehydes 

  • Higher boiling point due to the carbon to oxygen double bond present in the molecule. 
  • Soluble in water
CriteriaMarks
Explains the physical properties of each molecule1 per molecule (total of 3)
Links the physical properties to the molecular functional group of each molecule1 per molecule (total of 3)

Question 25)

Explain the difference between chain isomers, position isomers and functional group isomers. [4 marks]

An isomer is where two molecules have the same molecular formula however contain a different molecular structure. 

Chain isomers: differ due to chain branching 

E.g Hexane and 3 – methylpentane

Position Isomers – differ due to different positions of the functional group

E.g 1- fluro pentane vs 2 fluro pentant


Functional Group Isomers – differ due to different function groups

Ketones and aldehydes e.g pentanal and pentanone

Definition of isomer1 mark
Definition of each type with example1 mark each (total of 3)

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