Chemical Energetics

Hydrogen gas burns in air to form water.

2H₂ (g) + O₂ (g) → 2H₂O(I) ΔH= - 572 kJ

How much heat energy (in kJ) is given off if a rocket carrying 200.0 kg of hydrogen gas is burnt in excess oxygen?

• 1.43 x 10⁶

• 1.43 x 10⁷

• 2.86 x 10⁷

• 4.92 x 10⁷

The gaseous oxides of nitrogen such as NO, N₂O, NO₂ and N₂O₄ have positive enthalpies of formation because

• oxygen has high electron affinity.

• the atomisation enthalpy of nitrogen is very high.

• the nitrogen atom has a high electronegativity.

• nitrogen has an extra stability due to its half-filled p orbitals.

The standard enthalpy of combustion of 2-methylbut-1-ene, C₅H₁₀ is -3114 kJ mol^-1. Which of the following compounds is expected to have the same enthalpy of combustion?

• Cyclopentane, C₅H₁₀

• Cyclopentene, C₅H₈

• Pent-2-ene, C₅H₁₀

• Methylcyclobutane, C₅H₁₀

When an unknown mass of ethanol was burnt, the temperature of 1 dm³ water contained in a copper beaker increased by 7.2 °C. If the enthalpy of combustion

of ethanol is -1350 kJ mol-1, determine the mass of ethanol burnt. [Specific heat capacity of solution = 4.20 J g^-1 °C^-1; density of solution = 1.0 g cm^-3; M_r of ethanol = 46]

• 1.03 g

• 1.55 g

• 1.88 g

• 2.25 g

The following table shows the change in the enthalpy of combustion (kJ mol^-1) of a hydrocarbons.

C₂H₆ = -1560 ; C₄H₁₀ = -2880 ; C₇H₁₆ = -4848

The complete combustion of 0.05 mol of a hydrocarbon W releases 176.5 kJ. What could be the molecular formula of W if W is also an alkane?

• C₃H₈

• C₅H₁₂

• C₆H₁₄

• C₈H₁₈

In Thermite Reaction, a reactive metal such as aluminium is used to reduce iron(lll) oxide to iron. What is the enthalpy change when 2.0 moles iron(lll) oxide is reduced by 3.0 moles aluminium? [ΔH_f (Al₂O₃) = -1676 kJ mol^-1 ; ΔH_f (Fe₂O₃) = -825 kJ mol^-1]

• -855.0 kJ

• -1276.5 kJ

• -1702.2 kJ

• -3404.8 kJ

The standard enthalpies of combustion of graphite and hydrogen are -394 kJ mol^-1 and -286 kJ mol^-1 respectively. If the standard enthalpy of formation of ethane is -20.0 kJ mol^-1, what is the standard enthalpy of combustion of ethane in kJ mol^-1?

• -1 220

• -1 305

• -1 537

• -1 626

The standard heats of combustion per mole of butane, C₄H₁₀(g), hydrogen, H₂(g), and 1,3-butadiene, C₄H₆(g) are -2878 kJ, -286 kJ and -2540 kJ respectively. What is the heat of hydrogenation of 1,3-butadiene?

C₄H₆(g) + 2H₂(g) → C₄H₁₀(g)

• -234 kJ mol^-1

• -325 kJ mol^-1

• -370 kJ mol^-1

• -408 kJ mol^-1

When heated strongly, potassium chlorate(V) decomposes to form potassium chloride and oxygen.

2KCIO₃(s) →- 2KCI(s) + 3O₂(g)

If the heat of formation of KCIO₃ and KCI are -391.2 kJ mol^-1 and -436.7 kJ mol^-1 respectively, what is the enthalpy change when 20 g KCIO₃ is heated?

[Relative atomic mass: O = 16; Cl = 35.5; K = 39]

• 6.88 kJ

• 7.42 kJ

• 8.15 kJ

• 8.80 kJ

When 28.0 g of nitrogen reacts with oxygen to form nitric oxide, NO, and nitrogen dioxide, NO₂, 103.0 kJ of heat was required. If no nitrogen remained, calculate the mass of nitric oxide, NO formed.

[ΔH_f (NO) = 90.3 kJ mol^-1, ΔH_f (NO₂) = 33.2 kJ mo^-1]

• 4.8 g

• 9.6 g

• 19.2 g

• 24.3 g