TOPIC 5 IB Chemistry

The ore magnetite, Fe₃O₄, can be reduced by carbon monoxide

Fe₃O₄(s) + 4CO(g) → 3Fe(s) + 4CO₂(g)

The standard enthalpies of formation of magnetite (Fe₃O₄₎, carbon monoxide(CO) and carbon dioxide(CO₂₎ are −1118, −110 and −394 kJ mol⁻¹ respectively.

What is the standard enthalpy change for the reduction of magnetite by carbon monoxide?

• -3134

• -18

• +18

• +3134

Diborane, B₂H₆ has been suggested as a possible rocket fuel. The equation for its combustion is

B₂H₆(g) + 3O₂(g) → B₂O₃(s) + 3H₂O(l) ΔH_c^⦵ = − 2168 kJ mol^-1

The enthalpies of formation of boron oxide and water are −1274 and −286 kJ mol⁻¹ respectively.

What is the standard enthalpy of formation of diborane?

• +36 kj/mol

• -36kj/mol

• +608 kj/mol

• -608 kj/mol

The copper thermite reaction is used for creating electrical joints:

3Cu₂O(s) + 2Al(s) →   6Cu(s) + Al₂O₃(s)

The enthalpies of formation of Cu₂O and Al₂O₃ are −170 and −1676 kJ mol-¹ respectively.

What is the standard enthalpy change of the copper thermite reaction in kJ?

• -1506

• -1846

• -2186

• -1166

Which statements are correct about temperature?

I. A rise in temperature of 1.0 ^oC is the same as a rise in temperature of 1.0 K.

II. Both the degrees Celsius scale and the Kelvin scale are based on the temperature at which pure water melts and boils.

III. Pure water boils at 100 ^oC and 373 K at 101 kPa (1 atmosphere) pressure.

•   II and III

• I, II and III

   

   

• I and II only

• I and III only

Which are correct statements about exothermic and endothermic reactions?

I. The bonds in the products are stronger than the bonds in the reactants in endothermic reactions.

II. Both exothermic and endothermic reactions involve the transfer of heat between the system and its surroundings.

III. ΔH has a positive value for endothermic reactions.

• II and III only

   

   

• I and II only

• I and III only

•   I, II and III

Which is a correct statement about the enthalpy diagram?

• ΔH has a negative value.

   

   

   

• The absolute enthalpy value of the reactants cannot be measured.

• The products are more thermodynamically stable than the reactants.

• The diagram represents an exothermic reaction.

Which equation represents the standard enthalpy change of combustion of ethane, ΔH_c^⦵?

•   2C₂H₆(g) + 7O₂(g) → 4CO₂(g) + 6H₂O(l)

   

   

   

• C₂H₆(g) + 3.5O₂(g) → 2CO₂(g) + 3H₂O(g)

• C₂H₆(g) + 3.5O₂(g) → 2CO₂(g) + 3H₂O(l)

• 2C₂H₆(g) + 7O₂(g) → 4CO₂(g) + 6H₂O(g)

What mass of ethanol (M_r = 46.1) will need to be burned completely to raise the temperature of 1.00 kg of water by 10.0 K? Assume no heat is lost and that the measurements are made under standard conditions. (ΔH_c^⦵(ethanol) = − 1367 kJ mol⁻¹;  Specific heat capacity of water = 4.18 kJ kg⁻¹ K⁻¹.)

•   0.0306 g

   

   

• 1.41 kg

•   1.41 g

• 0.146 g

The energy cycle shows how Hess's Law can be used to find the standard enthalpy of formation of methane if the enthalpies of combustion of carbon, hydrogen and methane are known.

Which expression will give the correct value for the enthalpy of formation of methane?

•   ΔH_c^⦵(methane) − ΔH_c^⦵(carbon) + 2ΔH_c^⦵(hydrogen)

   

   

   

• ΔH_c^⦵(carbon) + 2ΔH_c^⦵(hydrogen) + ΔH_c^⦵(methane)

• ΔH_c^⦵(carbon) + 2ΔH_c^⦵(hydrogen) − ΔH_c^⦵(methane)

• ΔH_c^⦵(methane) + ΔH_c^⦵(carbon) + ΔH_c^⦵(hydrogen)

Hydrazine gas, N₂H₄(g), reacts exothermically with chlorine

N₂H₄(g) + 2Cl₂(g) → N₂(g) + 4HCl(g) ΔH^⦵ = − 463 kJ

Some relevant bond energies in kJ mol⁻¹ are: N−N 158; N≡N 945; N−H 391 and Cl−Cl 242.

What is the bond enthalpy of the H−Cl bond?

•   255 kJ mol⁻¹

   

•   371 kJ mol⁻¹

•   431 kJ mol⁻¹

• 235 kJ mol⁻¹