5 ENERGETICS 1CHEMISTRY FOR THE IB DIPLOMA © CAMBRIDGE UNIVERSITY PRESS 2011

5 Energetics

Revision checklistI am able to:

state whether the temperature goes up or down and whether heat is given out or taken in an exothermic reaction

state the sign of ∆H for an exothermic reaction

state whether the products are more or less stable than the reactants in an exothermic reaction

draw enthalpy level diagrams for exothermic and endothermic reactions

state whether combustion is an exothermic or endothermic process

state whether the neutralisation reaction when an acid reacts with an alkali is an exothermic or endothermic process

calculate the enthalpy change when 1 mole of hexane is burnt, given that when 1.20 g of hexane (C6H14) is burnt the temperature of 250.0 g of water (speci� c heat capacity 4.18 J g−1 °C) is raised by 56.0 °C

give two reasons why the enthalpy change in the reaction described above is less exothermic than the literature value

describe an experimental procedure that could be used to measure the enthalpy change of a reaction occurring in aqueous solution

calculate the enthalpy change of neutralisation given that, when 200.0 cm3 of 0.150 mol dm−3 hydrochloric acid is mixed with 100.0 cm3 of 0.300 mol dm−3 sodium hydroxide solution the temperature rose by 1.36 °C

de� ne average bond enthalpy

write an equation to represent the bond enthalpy (energy) of hydrogen � uoride

state whether bond breaking is an exothermic or endothermic process

use the bond energies given below to calculate the enthalpy change for the reaction:

CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g)

Bond Bond energy / kJ mol−1

H–Cl 430

C–Cl 340

C–H 410

Cl–Cl 240

explain in terms of bond enthalpies why the reaction above is exothermic

use the data below to calculate the enthalpy change for the process:

C2H5OH(l) → C2H5OH(g)

C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l) ∆H = −1371 kJ mol−1

C2H5OH(g) + 3O2(g) → 2CO2(g) + 3H2O(l) ∆H = −1415 kJ mol−1

de� ne the standard enthalpy change of formation for a substance

write an equation that represents the standard enthalpy change of formation of CH3COOH(l)

explain what is meant by the term ‘standard state’

state the standard conditions for enthalpy changes

write an equation that represents the standard enthalpy change of combustion of methanol (CH3OH(l))

HL

2 CHEMISTRY FOR THE IB DIPLOMA © CAMBRIDGE UNIVERSITY PRESS 20115 ENERGETICS

work out the standard enthalpy change of formation of benzene given the enthalpy change of combustion data below:

Substance Standard enthalpy change of combustion / kJ mol−1

hydrogen (g) −290

carbon (s) −400

benzene (l) −3300

calculate the standard enthalpy change of combustion of methanol using the standard enthalpies of formation, ∆Hf , below:

Compound CH3OH(l) CO2(g) H2O(g)

∆Hf / kJ mol−1 −240 −400 −240

de� ne the term ‘lattice enthalpy’

write an equation to represent the lattice enthalpy of sodium chloride

de� ne the term ‘electron a� nity’

write equations for the � rst and second electron a� nities of O

draw a Born–Haber cycle for the formation of sodium chloride and use it and the following data to calculate the standard enthalpy change of formation of sodium chloride:

∆Hat (Na) 100 kJ mol−1

∆Hat (Cl2) 120 kJ mol−1

IE (Na) 500 kJ mol−1

EA (Cl) −350 kJ mol−1

∆Hlatt (NaCl) 800 kJ mol−1

draw Born–Haber cycles for the formation of Na2O, CaO and MgCl2

place the following compounds in order of increasing lattice enthalpy:

KF MgCl2 MgF2 CaO CaS KI

explain why there is a signi� cant di� erence between the theoretical and experimental lattice enthalpy values for magnesium iodide

state and explain whether each of the reactions below involves an increase or decrease in entropy

NH3(g) + HCl(g) → NH4Cl(s)

C2H5OH(g) + 3O2(g) → 2CO2(g) + 3H2O(l)

2NO(g) + O2(g) → 2NO2(g)

state the sign of ∆S for each of the reactions above

use the absolute entropies given below to calculate the standard entropy change for the following reaction:

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)

Substance ∆S / J K−1 mol−1

CH4(g) 186

O2(g) 103

CO2(g) 214

H2O(l) 70

state whether ∆G is positive or negative for a spontaneous reaction

state the equation that connects ∆G to the enthalpy and entropy change of a reaction

HL

5 ENERGETICS 3CHEMISTRY FOR THE IB DIPLOMA © CAMBRIDGE UNIVERSITY PRESS 2011

calculate ∆G for the following reaction and state whether it is spontaneous or not at 298 K, given that ∆H = −2220 kJ mol−1 and ∆S = −370 J K−1 mol−1

C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l)

calculate ∆G for the following reaction given the ∆Gf values in the table:

C5H12(l) + 8O2 → 5CO2(g) + 6H2O(l)

Substance ∆Gf

C5H12(l) −10

H2O(l) −240

CO2(g) −400

explain whether a reaction for which ∆S is positive becomes more or less spontaneous as the temperature increases

work out the temperature at which the reaction below becomes spontaneous:

2Pb(NO3)2(s) → 2PbO(s) + 4NO2(g) + O2(g)

∆H = + 600 kJ mol−1 ∆S = 900 J K−1 mol−1

HL