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Assignment 3
1. Answer
ALL questions.
2. Write your answers on only one side of the
paper.
3. Ensure your name is at the top of each sheet
of paper.
4. The application of the rules for significant
figures in quantities calculated byaddition
or subtraction, and multiplication or division must be demonstrated.
5. Include appropriate units, where necessary, in
all numerical results.
6. Show
ALL your working.
8. Use the following relative atomic weights:
| Element | Ar | Element | Ar |
| C | 12:0 | Ca | 40:1 |
| O | 16:0 | N | 14:0 |
| H | 1:0 | Na | 23:0 |
| K | 39:1 | Cl | 35:5 |
9. At SATP (298 K,1 bar) the volume of one mole of an ideal gas is24:8 dm3.
1. A thermochemical equation is a chemical
reaction equation and its associatedstandard
enthalpy of reaction. The thermochemical equation for the combustion of decane
(a component of petrol), is given below:
2 C10H22 (l) + 31 O2 (g) 20 CO2 (g) + 22 H2O (l) H = 13 556 kJ
a) Define the
Standard Enthalpy of Combustion of a substance.
(2 marks)
b) Define the terms Exothermic and Endothermic and
state which termapplies to the
Standard Enthalpy of Combustion of decane.
(3 marks)
c) Write the chemical reaction equation for which
the enthalpy change wouldbe the
standard enthalpy of combustion of decane.
(2 marks)
d) Calculate the standard enthalpy of combustion
of decane.
(2 marks)
e) Modify the thermochemical equation above to
show what happens when:
i)8:00
molCO2 (g) are produced under standard conditions;
ii)10:0
moldecane are burnt under standard
conditions.
In both cases you will need
to show the correctly balanced equation and the associated enthalpy change. (Ar(C) = 12:0, Ar(H) = 1:0)
(4 marks)
f) Calculate the enthalpy change when71:0 gof decane is burnt completely inexcess oxygen. Show your working in full.
(3 marks)
g) If5
gof decane is burnt and the energy
liberated used to heat3 dm3 ofwater, what would be the rise in
temperature of the water? (Take the density of water to be 1:0 g cm 3, and the heat capacity of
water to be 4:18 J K 1
g 1.)
(3 marks)
2. a) Define the
Bond Dissociation Enthalpy of a bond.
(1 mark)
b) How does the bond dissociation enthalpy differ
from the mean bond enthalpy?
(1 mark)
c) How would you expect the mean bond enthalpy to
change with:
i) decreasing bond strength;
ii) decreasing bond length?
(2 marks)
d) Use the mean bond enthalpies below to
determine the enthalpy of reactionfor
the following reactions:
i) CH3CH2CHCH2 (g) + Cl2 (g) CH3CH2CHClCH2Cl (g)
ii)
C3H8 (g)
+ Cl2 (g)
CH3CH2CH2Cl (g) + HCl (g)
(8 marks)
| Bond Broken | Mean Bond Enthalpy / | ||||||
| kJ mol 1 | |||||||
| C | H | 413 | |||||
| Cl | Cl | 242 | |||||
| H | Cl | 431 | |||||
| C | Cl | 338 | |||||
| C | C | 612 | |||||
| C | C | 348 | |||||
3. Data:0 K= 273:15 C. MV = 24:5 dm3 mol 1 at 298 K and 1:0
atm.
a) Outline thefourassumptions
concerning the gas particles in the kineticmolecular theory of ideal gases.
(4 marks)
b) Helium-filled balloons are used to carry
scientific instruments high into theatmosphere.
Suppose that such a balloon is launched on a summer day when the temperature at
ground level is 22:5 C and the barometer reading is 756 mmHg. If the balloon’s
volume is 1:00 106
dm3 at launch, what will its
volume be at a height of 37 km, where the pressure is 74:0 mmHg and the
temperature is 35 C? (Assume that no helium escapes and that helium is an ideal
gas.)
(4 marks)
c) A compound has the empirical formula CHCl. A
flask with volume253 cm3 at
a temperature of 373 K and pressure of 1:0 atm contains 0:8 g of the gaseous
compound. Assuming the gaseous compound is an ideal gas, find its molecular
formula.
(4 marks)
4. a) State
Hess’s Law.
(2 marks)
b) In designing a chemical plant for
manufacturing polythene, you need toknow
the enthalpy change for the removal of H2 from C2H6 (ethane) to give
C2H4 (ethene).
C2H6 (g) C2H4 (g) + H2 (g) (1)
From experiments you know these thermochemical
expressions:
| 2 C2H6 |
(g) + 7 O2 |
(g) | 4 CO2 (g) + 6 H2O (l) | H = | 3119:4 kJ | |
| C2H4 | (g) + 3 O2 | (g) | 2 CO2 (g) + 2 H2O (l) | H = | 1410:9 kJ | |
| 2 H2 (g) + O2 | (g) | 2 H2O (l) | H = | 571:66 kJ | ||
Use this information to
determine the enthalpy change for the formation of ethene from ethane (Reaction
(1)). [Hint: a purely ’algebraic’ approach or thermal chemical cycle can be
used.]
(8 marks)
5. a) What mass of KNO3 would
have to be decomposed to produce22:3
dm3 ofoxygen, measured at SATP? [At SATP the
volume of one mole of an ideal gas is 24:8 dm3.]
2 KNO3 (s) 2 KNO2 (s) + O2 (g)
(3 marks)
b) An impure sample of KNO3 that
had a mass of55:8 gwas heated until all ofthe KNO3 had decomposed. The
liberated oxygen occupied 4:11 dm3 at SATP. What percentage by mass of the sample
was KNO3? (Assume that no impurities
decompose to produce oxygen.)
(4 marks)
TOTAL = 60 marks
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