MC test: Energy cycles

Multiple choice test on 15.1 Energy cycles

Use the following 'quiz' to test your knowledge and understanding of this sub-topic. You will need access to a periodic table (Section 6 of the IB data booklet).

If you get an answer wrong, read through the explanation carefully to learn from your mistakes.

Which process accounts for the fact that the formation of magnesium oxide from its elements is exothermic? 

The formation of the ionic lattice from its gaseous ions is always highly exothermic and is more than the sum of all the endothermic factors in the Born-Haber cycle.

 

Which processes are always endothermic during the process of forming an ionic compound from its elements in their standard states?

I. atomization enthalpy

II. ionization energy

III. electron affinity

For the formation of ions with a single negative charge the electron affinity is always exothermic. The overall process of electron affinity is only endothermic when more than one electron is being removed as, for example, in the formation of the O2−(g) ion.

 

The diagram shows a Born-Haber cycle for the reaction between an alkali metal, M and a halogen, X.

Which row gives the correct enthalpy terms for a, b and c?

Row a b c
1 atomization electron affinity formation
2 atomization atomization formation
3 ionization atomization lattice
4 dissociation dissociation lattice

a is the atomization of M, b is atomization of X (which is equal to half the dissociation enthalpy) and c is the enthalpy of formation of MX..

 

The diagram shows the Born-Haber cycle for the formation of calcium oxide.

Which enthalpy terms have negative values?

All the processes except the first electron affinity of oxygen, the lattice enthalpy of calcium oxide (in the direction shown by the arrow) and the enthalpy of formation of calcium oxide are endothermic.

 

For which substance is there the greatest difference between the value for the lattice energy arrived at by using the Born-Haber cycle and the theoretical value obtained on the basis of a purely ionic model?

The greater the percentage of covalent bonding in the compound the more the theoretical value based on an ionic model deviates from the experimentally determined value using the Born-Haber cycle. Covalent character tends to increase as the difference in electronegativities between the metal and non-metal decreases.

 

Which salt will have the greatest lattice enthalpy?

The smaller and more highly charged the ions the greater the lattice enthalpy

 

Which is the correct value for the enthalpy of formation of potassium bromide?

Given:
Atomization energy of potassium = 90.0 kJ mol−1
Atomization energy of bromine = 112 kJ mol−1
1st. Ionization energy of potassium = 419 kJ mol−1
Electron affinity of bromine = −325 kJ mol−1
Lattice enthalpy of potassium bromide = 691 kJ mol−1

From the Born-Haber cycle ΔHf⦵(KBr) = ΔHat⦵(K) + ΔHIE⦵(K) + ΔHat⦵(Br) + ΔHEA⦵(Br) + ΔHlatt⦵(KBr) where the lattice energy is for the formation of the salt i.e. has an exothermic value. ΔHf⦵(KBr) = 90.0 + 419 + 112 + (−325) + (−691) = −395 kJ mol−1

 

What can be deduced from the fact that when sodium chloride dissolves in water the temperature of the water falls by a small amount?

Sodium chloride is soluble because the hydration enthalpy is slightly less than the lattice enthalpy and the small difference in energy required to break the lattice into its free ions is provided by the water.

 

The enthalpy of solution of rubidium bromide is +21.9 kJ mol−1 and the specific heat capacity of water is 4.2 kJ kg−1 oC−1.

What will happen to the temperature of the water when 1.00 mol of rubidium bromide is dissolved in 1.00 kg of water (at an initial temperature of 25 oC) in an insulated container?

1.00 mol of RbBr requires 21.9 kJ to dissolve in 1 kg of water. The temperature of 1.0 kg of water will decrease by 21.9 ÷ 4.2 = 5.2 oC. This means the resulting temperature will be 25.0 - 5.2 = 19.8 oC.

 

Which describes the enthalpy of hydration of the caesium ion?

Hydration always refers to the gaseous ion becoming hydrated.

 

Total Score:
All materials on this website are for the exclusive use of teachers and students at subscribing schools for the period of their subscription. Any unauthorised copying or posting of materials on other websites is an infringement of our copyright and could result in your account being blocked and legal action being taken against you.