A2 Chemical Energetic

CHEMICAL ENERGETIC A2BORN HABER CYCLE

LEARNING OUTCOMEexplain and use the term lattice energy.Explain the effects of ionic charge and ionic radius on the numerical magnitude of lattice energy.apply Hess law to construct Born Haber cycle and carry out calculations involving such cycles and relevant energy terms ( including ionization energy, electron affinity and enthalpy change of formation of simple ionic solid )construct simple energy cycles for dissolving of ionic solids in water.apply Hess law and carry out calculations involving such cycles.

LATTICE ENERGYLattice enthalpy is a measure of the strength of the forces between the ions in an ionic solid. The greater the lattice enthalpy, the stronger the forces.Those forces are only completely broken when the ions are present as gaseous ions, scattered so far apart that there is negligible attraction between them.Because of high lattice energies, ionic solids tend to be hard and have high melting points.

Lattice Formation EnthalpyThe enthalpy change when ONE MOLE of an ionic lattice is formed from its isolated gaseous ions.highly exothermicstrong electrostatic attraction between oppositely charged ionsa lot of energy is released as the bond is formedrelative values are governed by the charge density of the ions.always negative.LATTICE ENERGY

Lattice Formation Enthalpy

Lattice Dissociation EnthalpyThe enthalpy change when ONE MOLE of an ionic lattice dissociates into isolated gaseous ions.highly endothermicstrong electrostatic attraction between oppositely charged ionsa lot of energy must be put in to overcome the attraction.relative values are governed by the charge density of the ions.always positive.

LATTICE ENERGY

Lattice Dissociation Enthalpy

The strength of the bond between the ions of opposite charge in an ionic compound therefore depends on the charges on the ions and the distance between the centers of the ions when they pack to form a crystal.

FACTORS AFFECT LATTICE ENERGY

- The greater the charge on the ion, the greater is the force. The lattice energies become more exothermic as the charge on the ion increases. - Smaller ions will have a greater attraction for each other because of their higher charge density. -They will have larger lattice enthalpies and larger melting points because of the extra energy which must be put in to separate the oppositely charged ions.

FACTORS AFFECT LATTICE ENERGY

The sodium ion has the same charge as a potassium ion but is smaller. It has a higher charge density so will have a more effective attraction for the chloride ion. More energy will be released when they come together. Greater charge densities of ions causes greater attraction and larger lattice enthalpy

FACTORS AFFECT LATTICE ENERGY

CANNOT MEASURE LATTICE ENTHALPY DIRECTLY

it is CALCULATED USING A BORN-HABER CYCLE

EffectsMelting pointThe higher the lattice enthalpy, the higher the melting point of an ionic compound

SolubilitySolubility of ionic compounds is affected by the relative values of lattice and hydration enthalpiesCALCULATING LATTICE ENERGY

The Born-Haber cycle for lithium fluoride

Step 1: Convert elements to atoms in the gas statefor Li, Li (s) Li (g) H1 = Hatomizationfor F, 1/2 F2 (g) F (g) H2 = 1/2 (Bond Energy)Step 2: Electron transfer to form (isolated) ionsLi (g) Li+ (g) + eH3 = IE1F (g) + e F (g)H4 = EA1Step 3: Ions come together to form solidLi+ (g) + F (g) LiF (s) H5 = Lattice EnergyLi (s) + 1/2 F2 (g) LiF (s) H = Hf = (H15)Lattice Energy = Hf (H1 + H2 + H3 + H4)CALCULATING LATTICE ENERGY

Born-Haber Cycle For Sodium Chloride

Born-Haber Cycle - NaCl165432Na(s) + Cl2(g) NaCl(s)Na(g) + Cl2(g) Na(g) + Cl(g) Na+(g) + Cl(g) Na+(g) + Cl(g) 123456Lattice Enthalpy is exothermic. Oppositely charged ions are attracted to each other.

Construct Born-Haber Cycle for the formation of MgCl2

165432Mg(s) + Cl2(g) MgCl2(s)Mg(g) + Cl2(g) Mg(g) + 2Cl(g) Mg2+(g) + 2Cl(g) 7Mg+(g) + 2Cl(g) Mg2+(g) + 2Cl(g) Enthalpy of formation of MgCl2Mg(s) + Cl2(g) > MgCl2(s)

Enthalpy of sublimation of magnesiumMg(s) > Mg(g)

Enthalpy of atomisation of chlorineCl2(g) > Cl(g)x2

Ist Ionisation Energy of magnesiumMg(g) > Mg+(g) + e

2nd Ionisation Energy of magnesiumMg+(g) > Mg2+(g) + e

Electron Affinity of chlorineCl(g) + e > Cl(g) x2

Lattice Enthalpy of MgCl2Mg2+(g) + 2Cl(g) > MgCl2(s)1234576Born-Haber Cycle - MgCl2

Q: 1(b) Calculate the lattice energy of magnesium nitride using the following data, in addition to relevant data from the Data Booklet. 3[A2 June 2011 Paper 41 Q1]

Solution

Q: 1(c) (i) Use the following data and data from the Data Booklet to construct a Born-Haber cycle and calculate the lattice energy of BaS.

(ii) Explain whether the magnitude of the lattice energy of BaS is likely to be greater or less than that of BaO. 4[A2 Nov 2009 Paper 42 Q1]

Solution

Q: 2 Calcium chloride, CaCl2, is an important industrial chemical used in refrigeration plants, for de-icing roads and for giving greater strength to concrete.(a) Show by means of an equation what is meant by the lattice energy of calcium chloride. 1(b) Suggest, with an explanation, how the lattice energies of the following salts might compare in magnitude with that of calcium chloride. (i) calcium fluoride, CaF2 (ii) calcium sulfide, CaS 3

(c) Use the following data, together with additional data from the Data Booklet, to calculate the lattice energy of CaCl2. 3

[A2 Nov 2009 Paper 41 Q2]

Q: 2(c) (i) Explain what is meant by the term lattice energy. (ii) Predict, with a reason, how the lattice energy of BaSO4 might compare with that of MgSO4. 3[A2 June 2006 Paper IV Q2]

Q: 1(c) (i) Write a chemical equation representing the lattice energy of AgBr. (ii) Use the following data to calculate a value for the lattice energy of AgBr (s).First ionization energy of silver = +731kJ mol-1Electron affinity of bromine = -325kJ mol-1Enthalpy change of atomization of silver = +285kJ mol-1Enthalpy change of atomization of bromine = +112kJ mol-1Enthalpy change of formation of AgBr = -100kJ mol-1 (iii) How might the lattice energy of AgCl compare to that of AgBr? Explain your answer. 4[A2 Nov 2005 Paper IV Q1]

Solution

Q: 1(d) (i) Explain the term lattice energy. (ii) Write a balanced equation including state symbols to represent the lattice energy of magnesium chloride. 4(e) Suggest, with an explanation in each case, how the lattice energy of magnesium chloride might compare with that of (i) Sodium chloride, NaCl (ii) Calcium chloride, CaCl2 4(f) Use the following data to calculate a value for the lattice energy of sodium chloride. 3

[A2 June 2004 Paper IV Q1]

Solution

Q: 2 (a) Write an equation to represent the lattice energy of sodium oxide, Na2O. 1(b) The Born-Haber cycle may be used to calculate the lattice energy of sodium oxide. (i) In the space below, identify the species A and B in the cycle, including the appropriate state symbols.Species A..Species B (ii) Identify the enthalpy changes labeled by the numbers 1 to 4 in the cycle. 31.2.3.4.[A2 Nov 2002 Paper IV Q2]

Q: 2 (c) Use your cycle, the following data and further data from the Data Booklet to calculate a value for the lattice energy of sodium oxide. 3Enthalpy change of atomization for Na (s) = +107kJ mol-1First electron affinity of oxygen = -141kJ mol-1Second electron affinity of oxygen = +798kJ mol-1Enthalpy change of formation of Na2O = -414kJ mol-1Enthalpy change of atomization of oxygen = half bond energy for O2(d) (i) How would you expect the magnitude of lattice energy of magnesium oxide to compare with that of sodium oxide. Explain your reasoning. 4[A2 Nov 2002 Paper IV Q2]

Draw a fully-labelled BornHaber cycle for the formation of solid barium chloride, BaCl2, from its elements. Include state symbols for all species involved.Use your BornHaber cycle and the standard enthalpy data given below to calculate a value for the electron affinity of chlorine.

Enthalpy of atomisation of barium +180 kJ mol1 Enthalpy of atomisation of chlorine +122 kJ mol1 Enthalpy of formation of barium chloride 859 kJ mol1 First ionisation enthalpy of barium +503 kJ mol1 Second ionisation enthalpy of barium +965 kJ mol1 Lattice formation enthalpy of barium chloride 2056 kJ mol1

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