Thermodynamics

Question 1

This questions is about magnesium chloride.

Write the equation, including state symbols, for the process corresponding to the enthalpy of solution of magnesium chloride.

Answer 1

Enthalpy of solution of magnesium chloride:

• MgCl2( s ) - > Mg^2+( aq ) + 2Cl^-( aq )

Question 2

Use these date to calculate the standard enthalpy of solution of magnesium chloride.

Enthalpy of lattice dissociation of MgCl2 = +2493 kJ mol^-1

Enthalpy of hydration of magnesium ions = -1920 kJ mol^-1

Enthalpy of hydration of chlorude uons = -364 kJ mol^-1

( This questions is about magnesium chloride. )

( MgCl2( s ) - > Mg^2+( aq ) + 2Cl^-( aq ) )

Answer 2

Enthalpy of solution:

• MgCl2( s ) - > Mg^2+( aq ) + 2Cl^-( aq )
• ( Draw a hess’ cycle diagram )
• Arrow going from tool box to MgCl2 is -2493
• Arrow going from tool box to Mg^2+ is -1920
• Arrow going from tool box to 2Cl^- os -364
• So enthalpy of solution is ( 2493 ) + ( -1920 ) + ( 2 x -364 )
• = -155

Question 3

Solubility is the measure of how much of a substance can be dissolved in water to make a saturated solution.
A salt solution is saturated when an undissolved solid is in equilibrium with its aqueous ions.

Use your answer to part ( b ) to deduce how the soluility of MgCl2 changes as the temperature is increased.
Explain your answer.

( Enthalpy of solution = - 155 )

( MgCl2( s ) - > Mg^2+( aq ) + 2Cl^-( aq ) )

Answer 3

The solubilty of MgCL2 changes as the temperature is increased by:

• MgCl2 becomes less soluble as temperature increses
• Enthalpy of solution is exothermic
• so equilibrium shifts to the endothermic reaction ( More solid is produced )

Question 4

Define the term electron affinity for chlorine.

Answer 4

Electron affinity for chlorine is:

• The enthalpy change for the formation of one mole of chloride ions from chloride atoms
• Atoms and ions are in their gaseous state

Question 5

Complete this Born-Haber cycle for magnesium chloride by giving the missing species on the dotted lines.
Include the state sumbols where appropriate.

The energy levels are not drawn to scale

Mg^2+( g )……….

Mg^2+( g )……… Mg^2+( g )…………

Mg^+( g )…………

Mg( g )…………

Mg( s )……………

                       MgCl2( s )

Answer 5

Mg^2+( g ) + 2Cl( g ) + 2e^-

Mg^2+( g ) + Cl2( g ) + 2e^- Mg^2+( g ) + 2Cl^-( g )

Mg^+( g ) + Cl2( g ) + e^-

Mg( g ) + Cl2( g )

Mg( s ) + Cl2( g )

                       MgCl2( s )

Question 6

Table 1 contains some enthalpy data.

Table 1

1 ) Enthalpy of atomisation of magnesium

2 ) Enthalpy of atomisation of chlorine

3 ) First ionisation energy of magnesium

4 ) Second ionisation energy of magnesium

5 ) Enthalpy of formation of magnesuim chloride

6 ) Lattice enthalpy of formation of magneisum chloride

Enthalpy change / kJ mol^-1:

1 ) + 150

2 ) + 121

3 ) + 736

4 ) + 1450

5 ) - 642

6 ) - 2493

Use your Born-Haber cycle from part ( b ) and data from table 1 to calculate a value for the electron affinity of chlorine.

( Mg^2+( g ) + 2Cl( g ) + 2e^-

Mg^2+( g ) + Cl2( g ) + 2e^- Mg^2+( g ) + 2Cl^-( g )

Mg^+( g ) + Cl2( g ) + e^-

Mg( g ) + Cl2( g )

Mg( s ) + Cl2( g )

                       MgCl2( s ) )

Answer 6

Mg^2+( g ) + 2Cl( g ) + 2e^-

( 2 x 121 ) 2x

Mg^2+( g ) + Cl2( g ) + 2e^- Mg^2+( g ) + 2Cl^-( g )

( + 1450 ) ( - 2493 )

Mg^+( g ) + Cl2( g ) + e^-

( + 736 )

Mg( g ) + Cl2( g )

( + 150 )

Mg( s ) + Cl2( g )

( - 642 )
MgCl2( s )

• ( - 642 ) = ( 150 ) + ( 736 ) + ( 1450 ) + ( 2 x 121 ) + ( 2x ) + ( - 2493 )
• 2x = -727
• x = - 363.5 = - 364 kJ mol^-1

Question 7

2d