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DESIGN : 3S5
please remember to hand in ur chem file with all the corrections done by tmr. =)
Energy Changes Worksheet
Suggested Answers
Paper 1 (MCQ)
1) A 2) C 3) C 4) D 5) C 6) A
7) C 8) C 9) C 10) A 11) D 12) C
Paper 2 - Section A
1 (a) potassium carbonate – K2CO3 temperature change – increase by 7°C
ammonium chloride – NH4Cl temperature change – decrease by 12°C
(b) endothermic change
reason: The temperature of the solution decreases, so heat energy is absorbed from
the surrounding.
2 (a) (i) C2H5OH (l) + 3O2 (g) ® 3H2O (g) + 2CO2 (g)
(ii) DH for this reaction is negative. Since the reaction is exothermic, heat energy is
given out to the surrounding. The chemicals have lost energy to the surroundings, thus the total energy content of the products is lower than the total energy content of the reactants.
(b) (i) C – O and C – H (accept also O=O)
(ii) C=O and O-H
(c) Mr of C2H5OH = 2(12) + 6(1) + 16
= 46
Mr of O2 = 2(16)
= 32
From the equation,
46 kg of C2H5OH react with 3 (32) = 96 kg of O2
thus, 23 kg of C2H5OH will react with (23/46) × 96 = 48 kg of O2
Therefore, 48 kg of liquid oxygen is needed to react with 23 kg of ethanol
3 (a) Total energy needed to break
all the bonds in 1 mole of methane, CH4 = 4×412 kJ
= 1648 kJ
(b) (i) Total energy needed to break the bonds in 1 mole of H2 and
1 mole of Br2 = 436 + 193
= 629 kJ
DHbond breaking = +629 kJ
(ii) Total energy given out in making the bonds in 2 moles of HBr = 2×-366
= -732 kJ
DHbond making = -732 kJ
(iii) The reaction between bydrogen and bromine is exothermic. This is because
the total energy absorbed to break the old bonds in the reactants (i.e., H-H and Br-Br bonds) is less than the total energy given out when new bonds are formed in the products (i.e. the 2 H-Br bonds). Thus, there is a net release of heat energy and the reaction is exothermic.
(iv) the overall enthalpy change for the reaction, DH = 629 - 732
= - 103 kJ
4. (a) aqueous sodium nitrate and water
(b) H+(aq) + OH-(aq) ® H2O(l)
(c) The reaction is exothermic. The reaction gives out heat to the surrounding, causing
the temperature of the solution to rise.
(d) Polystyrene is an insulator of heat. It minimizes heat from entering or leaving the
solution.
(e) no. of moles of NaOH = (50/1000) × 1.0
= 0.05 mol
Heat given out in this reaction = 0.05 × 57.3
= 2.865 kJ
5. (a) Mg(NO3)2 (aq) + Na2CO3 (aq) ® MgCO3 (s) + 2NaNO3 (aq)
(b) no. of moles of Mg(NO3)2 = (25/1000) × 2.0
= 0.05 mol
From the equation,
No. of moles of MgCO3 = no. of moles of Mg(NO3)2
= 0.05 mol
(c) An endothermic reaction is taking place. Heat is absorbed from the surrounding
solution, causing the temperature drop.
(d) The reaction is completed by 30 seconds. Heat from the surrounding is absorbed
by the solution, causing the temperature to increase to room temperature.
(e) Endothermic reaction because the temperature of the mixture drops. Heat energy is
absorbed and is transferred from the surroundings to the reactants.
6. (a) For the same mass, hydrogen (when burnt) produces more energy than fossil fuel.
(b) Engineers need to design a storage tank that can store liquid hydrogen at very low temperature or at very high pressure.
(c) (i) [comment: Correct labelled energy levels with Activation Energy curve
DH with correct arrows; Ea with correct arrows]
(ii) Let the amount of energy involved in the formation of carbon monoxide be x kJ
Enthalpy change = +[(4×410) + (2×460)] – [x +(3×436)] = +206
Þ x = – 206 + 1640 + 920 – 1308
Þ x = + 2560 – 1514 = 1046
1046 kJ of energy is involved/released in the formation of carbon monoxide.
(iii) Oxidation occurs as methane loses hydrogen and gains oxygen to form carbon monoxide ./ Oxidation state of carbon increases from –4 (in CH4) to +2 (in CO).
Reduction occurs as steam loses oxygen to form hydrogen./ Oxidation state of hydrogen decreases from +1 ( in H2O) to 0 ( in H2).
Since both oxidation and reduction has occurred in the reaction, this is a redox
reaction
Section B
1 (a) 6 N-H bonds
(b) The reaction is endothermic because more energy is absorbed in breaking the old
bonds in the reactants (i.e. the 6 N-H bonds) than is given out when new bonds are made in the products (i.e. the NºN and 3 H-H bonds). Thus, there is a net absorption of heat energy and the reaction is endothermic.
2 (a) (i) An exothermic reaction is one whereby heat energy is given out to the
surroundings.
(ii) More energy is given out in making new bonds in the products (i.e. the 2 H-Cl
bonds) than is absorbed in breaking old bonds in the reactants (i.e. the H-H and Cl-Cl bonds). Thus, there is a net release of heat energy and the reaction is exothermic.
(b) no. of moles of chlorine = 9600/24
= 400 mol
no. of moles of hydrogen chloride that can be made = 2 × 400
= 800 mol
thus, mass of hydrogen chloride that can be made = 800 × [1 + 35.5]
= 29 200 g
3. (a) 1.7 kJ
(b) 50 cm3
(c) 2.9 kJ
(d) no. of moles of NaOH = (50/1000) × 1.0
= 0.05 mol
(e) 0.05 mole of NaOH gives out 2.9 kJ of heat energy
thus, 1 mole of NaOH will give out 2.9/0.05 = 58 kJ of heat energy
(f) 1 mole of Ba(OH)2 ionises to produce 2 mole of OH- ions, doubles the amount of
OH- ions from 1 mole of NaOH.
Thus, twice the amount of heat is given out, i.e. 2 × 56 = 116 kJ
4. (a) 2CH3OH (l) + 3O2 (g) ® 2CO2 (g) + 4H2O (l)
(b)
(c) The magnitude of the heat of combustion increases gradually with increasing
number of carbon atom per alcohol molecule, i.e., there is a constant increase in the magnitude of the heat of combustion from one member to the next member. Successive member of the alcohol family increase by a constant amount of 14 units in relative molecular mass, caused by the CH2 unit.
(d) From the graph, the gradient of the line = -650 kJ
This is the heat released from burning one unit CH2.
Hence, the heat of combustion of the alcohol = -1180 - 650
= - 1830 kJ mol-1