Define the terms exothermic and endothermic and state which

Question

1. The thermochemical equation for the combustion of decane (a component of petrol), is given below:

2C₁₀H₂₂ (l) + 31O₂(g) → 20CO₂(g) + 22H₂O(l)  ?H^? = -13556 kJ

a) Define the Standard Enthalpy of Combustion of a substance.

b) Define the terms Exothermic and Endothermic and state which term applies to the Standard Enthalpy of Combustion of decane.

c) Write the chemical equation for which the enthalpy change would be the standard enthalpy of combustion of decane.

d) Calculate the standard enthalpy of combustion of decane.

e) Modify the thermochemical equation above to show what happens when:

i)8.00 mol CO₂ (g) are produced;

ii)10.0 mol decane are burnt.

In both cases you will need to show the correctly balanced equation and the associated enthalpy change. (A_r(C) = 12.0, A_r(H) = 1.0)

f) Calculate the enthalpy change when 35.5 g of decane is burnt completely in excess oxygen. Show your working in full.

g) If 3g of decane is burnt and the energy liberated used to heat 3 dm³ of water, what would be the rise in temperature of the water? (Take the density of water to be 1.0 g.cm³, and the heat capacity of water to be 4.18 J.K^-1.g^-1.)

2. a) Define the Bond Dissociation Enthalpy of a bond.

b) How does the bond dissociation enthalpy differ from the mean bond enthalpy?

c) How would you expect the mean bond enthalpy to change with:

i) decreasing bond strength;

ii) decreasing bond length?

d) Use the mean bond enthalpies below to determine enthalpy of reaction for the following reactions:

i) CH₃CH₂CH=CH₂(g) + Cl₂(g) → CH₃CH₂CHClCH₂Cl(g)

ii) C₃H₈(g) + Cl₂ (g) → CH₃CH₂CH₂Cl(g) + HCl(g)

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 ^oC. M_V = 24.5 dm³ .mol^-1 at 298 K and 1.0 atm.

A_r(Na) = 23.0, A_r(N) = 14.0, A_r(C) = 12.0, A_r(H) = 1.0, A_r(Cl) = 35.5.

a) Outline the four assumptions of the kinetic molecular theory of ideal gases.

b) Helium-filled balloons are used to carry scientific instruments high into the atmosphere. Suppose that such a balloon is launched on a summer day when the temperature at ground level is 20.5 ^oC 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 ^oC? (Assume that no helium escapes.)

c)A compound has the empirical formula CHCl. A flask with volume 253 cm³ at a temperature of 373 K and pressure of 1.0 atm contains 0.8 g of the gaseous compound. Find the molecular formula of the compound.

4. a)State Hess's Law.

b) In designing a chemical plant for manufacturing polythene, you need to know 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:

2C₂H₆ (g) + 7O₂ (g) → 4CO₂ (g) + 6 H₂O(l) ?H^o = -3119.4 kJ

C₂H₄ (g) + 3O₂ (g) → 2CO₂ (g) + 2H₂O(l) ?H^o = -1410.9 kJ

2H₂ (g) + O₂ (g) → 2H₂O(l) ?H^o = -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.]

5. a) What mass of KNO₃ would have to be decomposed to produce 21.1 dm³ of oxygen, measured at SATP?

2KNO₃ (s) → 2KNO₂ (s) + O2 (g)

b) An impure sample of KNO₃ that had a mass of 55.8g was heated until all of the KNO₃ had decomposed. The liberated oxygen occupied 4.11 dm³ at SATP. What percentage of the sample was KNO₃? (Assume that no impurities decompose to produce oxygen.)