O level Chemistry Questions and Answers(3)

Question 1

Ethanol, CH3CH2OH, is a liquid fuel. Ethanol can be manufactured either from glucose,
C6H12O6, or from ethene.

(a) Briefly describe the manufacture of ethanol from glucose.
Include the balanced equation in your answer.
(b) (i) Draw the displayed formula for ethene.
(ii) Name the substance that reacts with ethene to make ethanol.
(iii) Give the conditions needed for this reaction.

Solution 1

(a) Ethanol is manufactured by the anaerobic fermentation of glucose using an enzyme in yeast called zymase. Yeast is added to a solution of glucose and zymase breaks down the glucose into ethanol and carbon dioxide.

C6H12O6(aq) → 2C2H5OH(aq) + 2CO2(g)

When the concentration of ethanol produced reaches about 15%, the zymase is denatured (killed) by the ethanol. The ethanol solution is then refined by fractional distillation.

(b)(i) Ethene

(ii) Water (Steam)

(iii) Temperature 300 degrees Celsius, pressure 70 atm, and phosphoric acid as a catalyst.

Question 2

(a) Potassium reacts with water as shown in the equation.
2K(s) + 2H2O(l) → 2K+(aq) + 2OH(aq) + H2(g)
Describe what you would see when potassium reacts with water.
(b) A sample of 0.195 g of potassium was added to 500 cm3 of cold water. When the reaction was finished, 100 cm3 of 0.100 mol/dm3 hydrochloric acid was added to form solution X.
(i) Calculate the number of moles of hydroxide ions formed when the potassium was added to water.
(ii) Calculate the number of moles of hydrogen ions in 100 cm3 of 0.100 mol/dm3 hydrochloric acid.
(iii) Give an ionic equation to represent the neutralisation reaction.
(iv) Suggest a pH value for solution X and explain your answer.

Solution 2

(a) Potassium reacts violently with water producing a colourless gas and a colourless solution.

(b)(i) According to the equation, 2 moles of potassium produces 2 moles of hydroxide ions, so to find the moles of the hydroxide ions, we find the moles of potassium.

Number of moles (potassium) = mass ÷ Mr
= 0.195/39
= 0.005 moles.

Therefore the moles of hydroxide ions produced is also equal to 0.005 moles.

(ii) First we convert the volume to dm3.
V = 100/1000 = 0.1 dm3.

Number of moles = concentration × volume
= 0.1 × 0.1
= 0.01 moles of HCl
Since HCl is a monobasic acid, 0.01 moles of HCl contains 0.01 moles of Hydrogen ions.

(iii) First you write down the balanced chemical equation.
HCl(aq) + KOH(aq) → KCl(aq) + H2O(l)

Then split it into ions.
H+(aq) + Cl(aq) + K+(aq) + OH(aq) → K+(aq) + Cl(aq) + H2O(l)

Remove spectator ions (ions that have the same charge both sides of the equation).
H+(aq) + OH(aq) → H2O(l)

(iv) Solution X has a pH of 7 because it is a product of neutralisation.

Question 3

Iron is one of the most important metals. It is a transition element. Most iron is used in the alloy steel.
(a) Explain, in terms of metallic bonding, why iron is a good electrical conductor.
(b) Describe how different proportions of carbon can modify the physical properties of steel.
(c) When underwater, iron pipes will rust relatively rapidly.
(i) State the essential conditions needed for the rusting of iron.
(ii) Pieces of magnesium are often attached to underwater iron pipes. Explain how the magnesium protects the iron pipes against rusting.
(d) Write two typical properties that are generally common only to transition elements.
(e) A sample of a compound of iron is analysed. The sample contains 0.547 g of potassium, 0.195 g of iron, 0.252 g of carbon and 0.294 g of nitrogen.
Calculate the empirical formula of this compound.

Solution 3

(a) Metallic bonding is formed when metal atoms lose their electrons to form positive ions bound by a sea of mobile electrons. These delocalised electrons are free to move and carry charge from one point to another, thereby conducting electricity.

(b) Increasing carbon content in steel increases hardness and strength. But however carbon also increases brittleness.

(c)(i) Oxygen and Water.

(ii) Magnesium is more reactive than iron so the oxygen will react with magnesium instead of the iron, leaving the iron protected.

(d) High melting point, good electrical and thermal conductivity.


Molar ratio4661


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