Topic 5 chemi IB

When some solid ammonium nitrate was dissolved in water the temperature decreasedfrom 22 ^oC to 3 ^oC. What can be deduced from this observation?

• The dissolving is endothermic and ∆H is positive.

• The dissolving is endothermic and ∆H is negative.

• The dissolving is exothermic and ∆H is positive.

• he dissolving is exothermic and ∆H is negative.

Answer explanation

endothermic converts heat enery to chemical energy. The delta H is positive

Which is an exothermic process?

• K(s) → K(g)

• H₂O(l) → H⁺(aq) + OH^–(aq)

• Cl(g) + e^– → Cl^–(g)

• Mg(g) → Mg⁺(g) + e^–

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

As time passes the reaction rate starts to...

• Speed up

• Stay the same

• Slow down

• Not change

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)

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

Which is a correct statement concerning the reaction shown?

N₂(g) + 3H₂(g) → 2NH₃(g) ∆H = – 92 kJ

• 92 kJ of energy are absorbed for every mol of ammonia formed.

• 92 kJ of energy are released for every mol of ammonia formed.

• 46 kJ of energy are absorbed for every mol of ammonia formed.

• 46 kJ of energy are released for every mol of ammonia formed.

The heat capacity of manganese is 0.480 kJ kg^-1 K^-1. How much energy is required to raise the temperature of 3.00 kg of manganese by 20.0 K?

• 28.8 kJ

• 3.20 kJ

• 0.0720 kJ

• 288 kJ

The heat produced when 0.010 mol of propanone was combusted raised the temperatureof 250 g of water by 14 ^oC.
The specific heat capacity of water is 4.2 J g^-1 K^-1.
Which is the correct expression for value of the enthalpy of combustion of propanone (in J mol^-1)?

• 
• 
• 
• 

When one mol of magnesium and one mol of hydrogen are combusted the enthalpy changes are - 602 kJ and - 242 kJ respectively.

Mg(s) + ½ O₂(g) → MgO(s) ∆H = – 602 kJ
H₂(g) + ½ O₂(g) → H₂O(g) ∆H = – 242 kJ

What is the enthalpy change for the reduction of one mol of magnesium oxide by hydrogen?

• + 844 kJ

• + 360 kJ

• – 360 kJ

• – 844 kJ