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Question 1.
Bill and Bob have been testing a new aluminised PET (Polyethylene Terephthalate) film and they now use the material to make helium filled balloons. Bill’s balloon is less inflated than Bob’s, and the latter jokes about it. However, Bill insists he has to be careful because if he were to take the balloon on hills and mountains, the atmospheric pressure would decrease and the balloon would expand and burst. Bill then makes a bet that he can walk, drive or climb somewhere high enough for his balloon to burst…
The balloons are partially filled with helium and they unfold as helium expands into them. The balloons burst once they reach a volume of 22.0 dm3; the pressure and temperature of the helium inside the balloons are always equal to the surrounding atmospheric pressure and temperature. Bill and Bob added respectively 8600 cm3 and 18000 cm3 of helium in their balloons, by the sea side where the altitude is 0 meters, the temperature is 25.0oC and the atmospheric pressure is 760mm Hg.
The conditions on Mount Everest during climbing season are typically: P = 0.330atm and
T = -20.0oC.
(a) Calculate the volume that would be occupied by the helium in Bill’s balloon on Mount Everest (3 sig. fig.). Can Bill ever hope to win his bet? [3 marks]
(b) The dependence of the atmospheric pressure with altitude can be approximated by the following formula:
Patm = -7.57×10-2 h + 1
where h is the altitude in km, Patm is the atmospheric pressure in atm.
Calculate the altitude at which Bob’s balloon would burst, assuming that the temperature remains constant and equal to 250C (3 sig. fig). [3 marks]
Question 2.
A famous antacid and pain reliever is sold as effervescent tablets that fizz, producing carbon dioxide when added to water.
(a) Write a balanced chemical equation with state symbols for the following reaction occurring in water: hydrochloric acid reacts with sodium carbonate to form water, carbon dioxide and sodium chloride. [2 marks]
The commercial tablets use citric acid (a tricarboxylic acid) and sodium bicarbonate for their effervescent properties, and the balanced equation for the reaction is:
H3C6H5O7(aq) + 3 NaHCO3(aq) ? 3H2O(l) + 3CO2(g) + Na3C6H5O7(aq)
citric acid sodium bicarbonate sodium citrate
Each tablet weighs 1.0 g and contains several components, including an excess of citric acid and 38% by mass of sodium bicarbonate.
(b) i Calculate the molar mass of sodium bicarbonate and hence find the amount,
(3 sig. fig.), in moles, of NaHCO3 in one tablet.
ii Given that the molar volume of an ideal gas at 20.0oC and P = 1.00 atm is 24.0 dm3 mol-1, calculate (3 sig. fig.) the volume in cm3 of CO2(g) evolved under the same conditions by a single tablet when all the sodium bicarbonate is reacted.
[3 marks]
(c) How many tablets would be required to fill a 1500 cm3 flask with CO2 at 20.0oC and P = 1.00 atm? [1 mark]
(d) We have no tablets available, but would still like to fill the 1500 cm3 flask with CO2. We have at our disposal 5.32g of NaHCO3 and some lemon juice. If 1.0 dm3 of lemon juice contains 45.0g of citric acid, what is the minimum volume of lemon juice that would be required to react with all the sodium bicarbonate (3 sig. fig.)? [4 marks]
Question 3.
(a) i. Write a balanced chemical equation with state symbols for the combustion of ethane. [1 mark]
ii. ?Hoc[C2H6] = -1560kJ.mol-1, write the corresponding thermochemical equation. [1 mark]
iii. Calculate the heat generated by the complete combustion of 15.0g of ethane. [2 marks]
(b) Molar volume of an ideal gas at S.A.T.P. (25oC and 1.00 bar is 24.8 dm3 mol-1.
Assume that gases are at 225oC and P=1.00 atmosphere before and after the combustion and behave as ideal gases.
The chemical equation for the combustion of butane is:
2C4H10(g) + 13O2(g) ? 8CO2(g) + 10H2O(l)
At 225oC, we ignite a mixture of 5.00 dm3 of butane and 75.0 dm3 of O2
i. Is there enough oxygen for the complete combustion of the butane present? Explain your answer. [2 marks]
ii. What gases are present in the resulting mixture after combustion? [2 marks]
iii. Calculate the final volume of the mixture after combustion (3 sig. fig). [4 marks]
iv. Calculate the amounts (number of moles) of O2 and butane consumed using the
molar volume of an ideal gas at S.A.T.P (3 sig. fig). [4 marks]
v. The final mixture is cooled down from 225oC to 25oC. Calculate the volume of
the resulting gaseous mixture (3 sig. fig.). [3 marks]
Question 4.
(a) When a covalent bond is formed between 1 mole of atoms A and 1 mole of atoms B, the overall energy of these atoms decreases and heat is released into the surroundings.
i. Is the process of a covalent bond formation exothermic or endothermic?
ii Is the change of enthalpy for the formation at constant pressure of a mole of covalent bonds between A and B positive or negative? Why? [3 marks]
(b) i. In terms of energy, what must be done to break a covalent bond?
ii. Is the process of breaking a covalent bond exothermic or endothermic?
[2 marks] (c) For each of the following reactions:
decomposition of water: 2H2O(g) ? 2H2(g) + O2(g)
chlorination of methane: CH4(g) + 4Cl2(g) ? CCl4(g) + 4HCl(g)
i. List the broken bonds (and how many there are), and using table of standard mean bond enthalpies below, calculate the corresponding enthalpy change (energy absorbed from the surroundings);
ii. List the new bonds formed and calculate the corresponding enthalpy change (energy released into the surroundings);
iii. Calculate ?Horeaction and state if the reaction is exothermic or endothermic:
Bond Mean Bond Enthalpy
/kJmol-1
H-H 436
O=O 497
O-H 463
C-H 412
Cl-Cl 242
C-Cl 338
H-Cl 431
[8 marks]
(d) Here are four standard enthalpies of formation:
?Hof[CH4(g)] = -75 kJ.mol-1
?Hof[Cl2(g)] = 0 kJ.mol-1
?Hof[HCl(g)] = -92 kJ.mol-1
?Hof[CCl4(l)] = -135 kJ.mol-1
i. Write the four thermochemical equations relevant to the above enthalpies of
formation. [4 marks]
ii. Referring to the following reaction equation for the synthesis of CCl4 above
CH4(g) + 4Cl2(g) ? CCl4(l) + 4HCl(g)
Draw an energy cycle and use Hess’s Law to calculate ?Horeaction using the standard enthalpies of formation given above. [4 marks]
iii. Compare the value found in 4.d.ii and the value of ?Horeaction found in question 4.c.ii. These two values should be similar; give two reasons for the difference between the two calculated values. [2 marks]
Question 5.
(a) Balance the following neutralisation reaction between phosphoric acid and sodium hydroxide:
H3PO4(aq) + NaOH(aq) ? Na3PO4(aq) + H2O(l) [1 mark]
(b) When 25.0ml of 0.500 mol.dm-3 H2SO4(aq) solution is added to 0.025dm3 of 1.00 mol.dm-3 KOH solution, the temperature of the reaction mixture rises from 23.50oC to 30.17oC. The specific heat capacity and density of the reaction mixture are the same as the specific heat capacity and density of water: 4.20 J.g-1.oC-1 and 1.00 g.cm-3. The equation of the reaction is:
H2SO4(aq) + 2 KOH(aq) ? K2SO4(aq) + 2 H2O(l)
i. Is the reaction endothermic or exothermic? Why? [2 marks]
ii. Calculate the amounts of reactants available (number of moles) before reaction. Are these amounts in a stoichiometric ratio (3 sig. fig)? [3 marks]
iii. Calculate the volume of water produced by the reaction and state whether it is significant or not if the overall volume of the reaction mixture is calculated with the correct number of decimal places. What is the total volume of the reaction mixture after the reaction is completed. (3 sig. fig.)? [5 marks]
iv. Calculate the amount of heat that was transferred to the water (3 sig. fig.).
[2 marks]
v. Calculate ?Horeaction, the enthalpy of reaction for this neutralisation (3 sig. fig).
[2 marks]
vi. What increase of temperature would be observed if 0.5 mol of sulphuric acid reacted with 1 mol of aqueous potassium hydroxide and the final volume of the reaction mixture was 1.0 dm3? [3 marks]
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