Energy AS Revision: Energy terms Enthalpy change of reaction.ΔH r Enthalpy change of formation ΔH f Enthalpy change of combustion ΔH c Standard conditions Hess’ Law

Enthalpy Change of Reaction ΔH r Enthalpy, H, is the stored energy in a compound. We cannot measure this. Enthalpy changes, ΔH are transfers of energy in or out of the system at constant pressure. ΔH r Enthalpy change for a reaction under standard conditions when the molar quantities from the balanced equation are in their standard states. E.g.2H 2(g) + O 2(g) → 2H 2 O (l) ΔH r =-571.6kJ

Enthalpy Change of Formation ΔH f The energy change when 1 mole of a compound is made from its constituent elements in their standard states, under standard conditions. E.g.H 2(g) + ½O 2(g) → H 2 O (l) ΔH f =-285.8kJmol -1 The same reaction but half the energy given out. This is per mole of water made, NOT for the balanced equation which is the case in ΔH r.

Enthalpy Change of Combustion ΔH c The energy change when 1 mole of a substance burns completely in oxygen under standard conditions. E.g.H 2(g) + ½O 2(g) → H 2 O (l) ΔH c =-285.8kJmol -1 Same equation, same conditions, same energy change, different name. Standard Conditions: Temp: 298K Pressure:1 atmosphere (101kPa) Solutions: 1 mol dm -3

Energy Cycles Since some energy changes cannot be measured directly, at AS Hess’ Law was used. “If a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route.” This is usually used when combustion or formation data is available. E.g. C 2 H 4(g) + HBr (g) → C 2 H 5 Br (l)

Lattice Enthalpy At A2 Hess’ Law is applied to IONIC Compounds. This is called a Born-Haber Cycle. It is usually used to calculate lattice enthalpy which is a measure of ionic bond strength. Lattice Enthalpy is the enthalpy change that accompanies the formation of 1 mole of an ionic compound from its GASEOUS ions under standard conditions. E.g. K + (g) + Cl - (g) → KCl (s) ΔH LE = -711 kJmol -1

Week 22 © Pearson Education Ltd 2009 This document may have been altered from the original Ionic lattice of sodium chloride

Lattice Enthalpy This term is ALWAYS negative as energy is ALWAYS given out as oppositely charged ions come together. The higher the magnitude, the greater the electrostatic force between the oppositely charged ions. Lattice enthalpies cannot be measured directly because it is impossible to form one mole of an ionic lattice from gaseous ions.

Other Key Terms To complete Born-Haber Cycles, other key terms are needed as listed on p.167. Copy these into the spaces provided on the sheet. Complete the questions on page 167. Born-Haber Cycles can also be done as scale diagrams using graph paper or even lined paper. Then do q.1,2 on p Q. p. 167., Q. p.169 Q. p.171, q. p.173

Week 22 © Pearson Education Ltd 2009 This document may have been altered from the original Completed Born–Haber cycle for sodium chloride

Week 22 © Pearson Education Ltd 2009 This document may have been altered from the original Construction of the Born–Haber cycle for caesium chloride

Week 22 © Pearson Education Ltd 2009 This document may have been altered from the original Born–Haber cycle for sodium oxide

Week 22 © Pearson Education Ltd 2009 This document may have been altered from the original Born–Haber cycle for calcium chloride

Week 22 © Pearson Education Ltd 2009 This document may have been altered from the original Born–Haber cycle for copper(II) oxide

Trends in Lattice Enthalpy Lattice enthalpy is the result of electrostatic attractions between ions of opposite charge. The value of lattice enthalpy is affected by: i) size of ions ii) charge on the ions Why? See handout for group 1 halides. The lattice enthalpy becomes less exothermic as the size of the ion increases. True for both anions and cations.

Ionic radius As the radius of the ion increases and the charge density on the surface of the ions decreases, the attraction between the ions decreases. Small ions pack more closely together in a lattice than larger ions. The lattice energy is less exothermic if the attraction is lower because less energy is given out as the lattice forms. This leads to weaker ionic bonding.

Week 23 © Pearson Education Ltd 2009 This document may have been altered from the original Trend in lattice enthalpies of the sodium halides

Charge on the ions Lattice enthalpy becomes more exothermic as the charge on the ion increases. It is most exothermic for small, highly charged ions. See this by comparing LiF with MgO. LiF ΔH latt = kJmol -1 MgO ΔH latt = kJ mol -1 Shows that doubly charged ions attract each other more strongly than singly charged ions. Charge Density: Assumes that charge is spread over the outer surface of the ion and is charge per unit surface area: Charge density = charge on ion ionic radius Surface area α radius 2

Week 23 © Pearson Education Ltd 2009 This document may have been altered from the original Lattice enthalpy becomes more exothermic as the ionic charge increases

Magnesium oxide as a refractory The exceptionally high lattice enthalpy of magnesium oxide means that that the lattice is so strong that it takes a great deal of heat to decompose it. (mp 2853 o C) It can be used to line furnaces – as a refractory. It is also used in ceramics, wire coatings and flame retardant particle boards

Exercise For each pair of compounds, suggest which will have the most exothermic lattice enthalpy: A) CaO and CaCl 2 B) KCl and K 2 O C) BaI 2 and SrI 2 Place the following compounds in order of increasingly exothermic lattice enthalpy. Li 2 O LiF MgO Explain why you have put them in this order.

Exercise For each pair of compounds, suggest which will have the most exothermic lattice enthalpy: A) CaO and CaCl 2 B) KCl and K 2 O C) BaI 2 and SrI 2 Place the following compounds in order of increasingly exothermic lattice enthalpy. LiF Li 2 O MgO Because LiF is singly charged ions, Li 2 O has one doubly charged on and MgO has two doubly charged ions.

Testing The Ionic Model Lattice Enthalpies/kJmol -1 CompoundActual Value (from B-H cycle) Theoretical Value NaF NaCl NaBr NaI AgF AgCl AgBr AgI

Theoretical Values compared with Actual Values By applying electrostatic calculations and knowing the arrangement of ions in the lattice scientists can work out a theoretical value for standard lattice enthalpies. The model is based on perfect ionic bonding, with perfectly spherical ions with even charge density. Alkali metal halides show close agreement so they are close to perfectly ionic. Silver halides are stronger, having covalency in their bonding so there is less agreement between actual and theoretical lattice enthalpies. Bonding is not purely ionic.

Enthalpy Change of Solution ΔH s The standard enthalpy change of solution is the enthalpy change that takes place when one mole of a compound is completely dissolved in water under standard conditions. This involves breaking the lattice so that the individual ions are free to move through the solution and then hydrating the ions. This happens because the δ+ hydrogen atoms in water molecules are attracted to anions and the δ- oxygen atoms are weakly attracted to the cations. As a result both anions and cations are surrounded by water molecules. =57 =57

Week 23 © Pearson Education Ltd 2009 This document may have been altered from the original Ionic lattice dissolves in polar solvents – each ion is surrounded by water molecules to become ‘hydrated’

Enthalpy of Hydration ΔH hyd The standard enthalpy change of hydration is the enthalpy change that takes place when one mole of isolated gaseous ions is dissolved in water forming one mole of hydrated ions under standard conditions. Each ion in the lattice has its own value for enthalpy of hydration. The overall value for the enthalpy change of solution depends on the relative values of the reverse lattice enthalpy (endothermic) and the exothermic hydration enthalpies. Do q. p.175

Week 23 © Pearson Education Ltd 2009 This document may have been altered from the original Born–Haber cycle for calculating the enthalpy change of solution of KCl(s)

Effect of Ion Charge and Size on Hydration Enthalpy Ionic size Small ions have a large charge density and attract water molecules to them strongly, increasing the exothermic value of hydration enthalpy. Ionic Charge More highly charged ions attract water molecules to them more strongly, increasing the exothermic value of hydration enthalpy. Do q.p.177