• Thermodynamics?literally means?heat?and?movement?and is the branch of physical chemistry that deals with heat, energy, temperature and the physical properties of matter
• Energy cycles are special representations of enthalpy changes for ionic compounds using the principles of?Hess's Law
• In order to understand how energy cycles work you need a good knowledge of some key?enthalpy change?definitions
• Enthalpy change?(ΔH) refers to the amount of heat energy?transferred?during a chemical reaction, at a constant pressure
• The definitions you need to know are:
  • enthalpy of formation
  • ionisation enthalpy
  • enthalpy of atomisation
  • bond enthalpy
  • electron affinity

   

Enthalpy of formation

• The?enthalpy of formation?(ΔHf?) is the?enthalpy change?when?1 mole of a compound?is formed from its?elements?under standard conditions
  • Standard conditions in this syllabus are a temperature of 298 K and a pressure of 100 kPa

   

• The ΔHf??can be?endothermic?or?exothermic?as the energy change is the sum of the bonds broken and formed, so the enthalpy change can have?positive?or?negative?values
• Equations can be written to show the standard enthalpy change of formation (ΔHf?) for compounds
• For example, the enthalpy of formation sodium chloride is shown as:

Na(s) + ?Cl2?(g) → NaCl?(s)? ? ? ? ? ? ΔHf??= -411 kJ mol?-1

• Notice that enthalpy of formation only refers to?compounds
  • By definition the enthalpy of formation of elements is?zero

   

Ionisation enthalpy

• The?ionisation?enthalpy?(ΔHie?) of an element is the amount of energy required to remove an electron from a gaseous atom of an element to form a gaseous ion under standard conditions
• Ionisation enthalpy?is always endothermic as energy is need to overcome the attraction between an electron and the nucleus
• The?first ionisation energy?is the energy required to remove?one mole of electrons?from one mole of gaseous atoms of an element to form one mole of gaseous 1+ ions
  • E.g. the first ionisation energy of gaseous sodium:

   

Na (g) → Na+?(g) + e–? ? ? ? ? ΔHie??= +500 kJ mol-1

Enthalpy change of atomisation

• The?standard?enthalpy change of atomisation?(ΔHat?) is the?enthalpy change?when?1 mole?of gaseous atoms?is formed from its?element?under standard conditions
• The ΔHat??is always?endothermic?as energy is always required to?break?any bonds between the atoms in the element, to break the element into its gaseous atoms
  • Since this is always an endothermic process, the enthalpy change will always have a?positive?value

   

• Equations can be written to show the standard enthalpy change of atomisation (ΔHat?) for elements
• For example, sodium in its elemental form is a?solid
• The standard enthalpy change of atomisation for sodium is the energy required to form 1 mole of?gaseous?sodium atoms:

Na (s) → Na (g)? ? ? ? ? ?ΔHat??= +108 kJ mol?-1

Answer

Answer 1:?Potassium in its elemental form is a?solid, therefore the standard enthalpy change of atomisation is the energy required to form 1 mole of K(g) from K(s)

K(s) → K(g)

Answer 2:?Mercury in its elemental form is a?liquid, so the standard enthalpy change of atomisation of mercury is the energy required to form 1 mole of Hg(g) from Hg(l)

Hg(l) → Hg(g)

Bond enthalpy

• The amount of energy required to break one mole of a?specific?covalent bond in the gas phase is called the bond dissociation energy
  • Bond dissociation energy (E) is usually just simplified to?bond energy?or?bond enthalpy

   

• In symbols, the type of bond broken is written in brackets after?E
  • Eg.?E?(H-H) is the bond energy of a mole of single bonds between two hydrogen atoms

   

• Bond enthalpy?is usually treated as a bond breaking process, so it is quoted in data tables as an?endothermic?energy change with?positive?values
  • For bond forming processes simply put a negative sign in front of the value

   

• Equations can be written to show the bond enthalpy
• For example, chlorine in its elemental form is a gas
• The bond enthalpy of chlorine is shown as

Cl2?(g) → 2Cl?(g) ???E(Cl-Cl) = +242 kJ mol?-1

• Notice this looks very similar to atomisation enthalpy for chlorine
• However, atomisation enthalpy, by definition, produces?1 mole of atoms, whereas bond enthalpy is expressed per?mole of bonds
• So the atomisation enthalpy of chlorine would be half the bond enthalpy

?Cl2?(g) → Cl?(g)? ? ΔHat??= +121 kJ mol?-1

• If the element was a liquid, instead of a gas, then atomisation enthalpy would also include vaporisation enthalpy - a change of state, before the bonds are broken

Lattice energy

• As with bond enthalpy,?lattice enthalpy?(ΔHlatt?) can be expressed as a formation or dissociation process
• As a formation process, it is the enthalpy change when?1 mole?of an?ionic compound?is formed from its?gaseous ions?(under standard conditions)
• The ΔHlatt??is therefore?exothermic,?as when ions are?combined?to form an?ionic solid lattice?there is an extremely large release of energy
  • Since this is an exothermic process, the enthalpy change will have a?negative?value
  • Because of the huge release in energy when the gaseous ions combine, the value will be a very?large negative?value

   

• The large negative value of ΔHlatt??suggests that the?ionic compound?is much more?stable?than its?gaseous ions
  • This is due to the?strong electrostatic forces of attraction?between the oppositely charged ions in the solid lattice
  • Since there are no electrostatic forces of attraction between the ions in the gas phase, the gaseous ions are?less stable?than the ions in the?ionic lattice
  • The?more exothermic?the value is, the?stronger the ionic bonds?within the lattice are

   

• The ΔHlatt??of an ionic compound?cannot?be determined?directly?by one single experiment
• Multiple experimental values and an?energy cycle?are used to find the ΔHlatt??of ionic compounds
• The lattice energy (ΔHlatt?) of an ionic compound can be written as an equation
  • For example, sodium chloride is an ionic compound formed from sodium (Na+) and chloride (Cl-) ions
  • Since the lattice energy is the enthalpy change when 1 mole of sodium chloride is formed from gaseous sodium and chloride ions, the equation for this process is:

   

Na+(g) + Cl-(g) → NaCl?(s)? ΔHlatt??= -776 kJ mol?-1

Answer

Answer 1:?Mg2+?(g) + O2-?(g) → MgO (s)

Answer 2: Li+?(g) + Cl-?(g) → LiCl?(s)

Electron Affinity

• The?electron affinity?(ΔHea?) of an element is the energy change when?one mole?of electrons is gained by?one mole?of gaseous atoms of an element to form?one mole?of gaseous ions under standard conditions
• For example, the first electron affinity of chlorine is:

Cl?(g)+ e–?→ Cl-?(g) ? ? ? ? ?ΔHea??= -364 kJ mol-1

• The?first electron affinity?is always?exothermic?as energy is released when electrons are attracted to the atoms
• However, the?second electron affinity?of an element can be?endothermic?as illustrated by oxygen:

O–?(g) + e–?→ O2-?(g) ? ? ? ? ?ΔHea??= +844 kJ mol-1

• This is because a large force of?repulsion?must be overcome between the negatively charged ion and the second electron requiring a large input of energy

转载自savemyexams