Chap 5A - Enthalpy

Up to election affinity

Draw basic energy level and profile diagrams of endo and eco reactions

Compare endo and eco reactions with examples

Endothermic	Exothermic
The products have higher energy content than the reactants	The products have lower energy content than the reactants
Absorbs heat from the surroundings The temperature of the surroundings decreases	Releases heat to the surroundings The temperature of the surroundings increases
Total enthalpy of products > total enthalpy of reactants Since enthalpy of the system is increased, energetically, the system is destabilised	Total enthalpy of products < total enthalpy of reactants Heat is released from the system to the surroundings -> enthalpy of the system decreased -> Energetically, the system is stabilised
Examples : Chemical - thermal decomposition + Electrolysis + photosynthesis Dissolving of ammonium chloride in water  Physical - boiling + melting + sublimation + dissolution of ionic compounds in water	Examples :  Chemical - neutralization + combustion + respiration + metal displacement reaction  Physical : freezing + condensation + dissolution of acids and alkalis in water (process)

State the standard conditions and states

What is standard state

Standard state : a substance in its normal , most stable physical state at 298K and 1 Bar 

• Elements in their standard state under standard conditions are assigned 0 enthalpy 

• To compare energy changes, need to specify conditions under which the reaction was performed

What is the physical states of substances under standard conditions

1. Solids (majority)

2. Liquids 

• Mercury, Br, Gallium, Cesium, Francium 

• Diatomic gases - O2, H2, CI2 

• Monoatomic gasses - noble gases

Define enthalpy

• Enthalpy (Def.): Total heat content of a substance but it cannot be measured directly

State the characteristics of enthalpy

1. H of a particular reaction (Hreaction or Hr) is associated with a balanced chemical equation

• It refers to the enthalpy change for the stoichiometric molar amounts of reactants and products as indicated in the balanced chemical equation

2. H is directly proportional to the molar amounts of reactants

3. H of a reverse reaction is exact in magnitude but opposite in sign to the H of the forward reaction

4. H changes with the physical states (solid, liquid and gaseous) of the reactants and products

• Chemical equations must be accompanied by state symbols

Define and describe SEC of reaction

• The enthalpy change when molar quantities of reactants as specified by the chemical equation react to form products at 1 bar and 298 K (standard conditions)

• This is the heat absorbed or evolved, usually associated with breaking and forming of chemical bonds.

• H = (Total enthalpy of products) – (Total enthalpy of reactants)

Define SEC of formation

• (Def.): The enthalpy change when one mole of a pure substance is formed from its constituent elements in their standard states, under standard conditions of 298K and 1 bar

• Coefficient of HI MUST be 1

State some important considerations of SEC of formation

1. If an element has allotropes , the more stable allotrope is chosen (Eg. Graphite (not diamond) is chosen as the standard state for carbon) 

2. From the definition, the enthalpy change of formation of elements in their standard state is 0 kJ mol–1

3. Hf⦵ of a compound represents the heat transferred to or from the surroundings when chemical bonds in the elements are broken and new bonds are formed in the compound

• Measures the stability of the compound relative to its constituent elements

• If positive (endo) : compound is energetically less stable than its constituent elements ->  greater likelihood for compound to decompose into its constituent elements

• More negative (more exo) -> more stable 

4. Hf⦵ can be used for calculation of enthalpy change of various reactions

5. Enthalpy changes of formation are often theoretical

• Some reactions may not take place in practice

Define SEC of combustion, what it is useful for and its considerations

• (Def.): The enthalpy change when 1 mole of a substance is completely burnt in excess oxygen under standard conditions of 298K and 1 bar

• Useful for: 

  • Identification of substances with large Hc values as sources of fuel

  • Determination of calorific value of foodstuffs

  • Calculation of enthalpy change of various reactions

Note: 

1. Always exothermic 

2. Applies to complete combustion of 1 mole of substance

• For carbon, the combustion product is CO2 and NOT CO

Define SEC of neutralisation

• (Def.): The enthalpy change when 1 mole of water is formed when an acid neutralises a base and is carried out in an infinitely dilute aqueous solution under standard conditions of 298K and 1 bar

• Hneut⦵ is always negative as heat is always evolved due to formation of bonds between H+ and OH- to form H2O during neutralisation

Compare SEC of neutralisation for strong acids and bases VS weak acids and bases

1. Almost the same for all strong acids and bases (--57.0 mol-1)

• Strong acids and strong bases ionise completely in dilute aqueous solution

• Reaction between them is effectively the reaction between aqueous H+ ions and OH– ions

2. Slightly less exothermic than --57.0 mol-1 for weak acid and bases 

• The neutralisation of a weak acid and a strong base will involve an additional endothermic process – ionisation of the weak acid

• Part of the energy released during the neutralisation process is absorbed to break up the undissociated acid molecules, resulting in a less exothermic Hneut⦵

Define SEC of atomisation and Hatom of a compound

• (Def.):  The enthalpy change when 1 mole of isolated gaseous atoms are produced from the element in its standard state under standard conditions of 298K and 1 bar

• (Def.): Hatom m for a compound is the energy required to convert 1 mole of the compound into gaseous atoms at 298 K and 1 bar

• H Ꝋ atom values are always positive since heat is always absorbed as the bonds in the elements must first be broken before atoms can be released (endo)

Define SEC of hydration

(Def.): The enthalpy change when one mole of gaseous ions is dissolved in a large amount of water under standard conditions of 298 K and 1 bar

State some considerations of SEC of hydration

• Always negative (exo): 

  • For both cations and anions because the gaseous ions are always attracted to the oppositely charged ends of the polar water molecules, leading to the formation of ion-dipole interactions

  • Heat is evolved during the formation of ion-dipole interactions in the hydration process

• The ions are dissolved in such a large amount of water that further addition of water produces no further heat changes

• The higher the charge-radius ratio (charge density), the stronger is the ion-dipole interactions formed and the hydration energy released is more exothermic

Define SEC of solution

• (Def.): The enthalpy change when one mole of a solute is completely dissolved in a solvent to form an infinitely dilute solution under standard conditions of 298K and 1 bar

• The solute is usually an ionic compound while the solvent is usually water

• Can be either positive or negative

  • Highly positive : salt is likely insoluble 

  • Negative : salt is likely soluble

Define lattice energy

• Definition : The energy released when one mole of an ionic solid is formed from its isolated gaseous ions from an infinite distance apart

• Always negative as heat is evolved in forming electrostatic forces of attraction between oppositely charged ions

• X+ (g) + Y+ (g) -> XY (s) 

• Measure of the strength of ionic bonding and the stability of the ionic compounds

  • The more exothermic the lattice energy, the stronger the ionic bonding and the more stable the ionic compound

Describe the factors of lattice energy

1. Charge on the ions 

• The greater the ionic charges on ions, the stronger the electrostatic forces of attraction between the ions and hence lattice energy is more exothermic

2. Sizes of ions or inter-ionic distance 

• The smaller the ions, the closer they can approach each other in the solid crystal -> shorter inter-ionic distance -> electrostatic forces of attractions between ions are stronger and lattice energy is more exothermic

3. Crystal structure 

• The arrangement of ions in the crystal lattice will affect the attraction and repulsion between ions in the compound

Compare difference in theoretical and experimental lattice energies of BaI2 and BaF2

• Comparing between BaF2 and BaI2, I− has a larger ionic radius and hence a larger and more polarisable electron cloud

• This leads to a larger extent of distortion of the electron cloud of in I−, which results in a more covalent character in BaI2

• Hence, there is a bigger difference between the theoretical and experimental lattice energies of BaI2 compared to BaF2

• Experimental lattice energy : refers to the value found from experimental results using the Born-Haber cycle

• Theoretical lattice energy : refers to the value calculated based on a model which assumes that the compound is completely ionic

Define bond energy

• (Def.): The energy absorbed when 1 mole of covalent bonds between 2 atoms in a gaseous molecule are broken

• Always positive because energy is needed to pull the two atoms apart (endothermic process) to break the covalent bond

Why is bond energy defined with respect to molecules in the gaseous state?

If we were to define bond energy based on solids e.g. I2(s), the energy change for

I2(s) → 2I(g) would also include the energy needed to sublime the solid iodine. Hence, it

does not actually reflect the actual amount of energy that is needed to break the I−I

covalent bond

State the limitations of bond energies

• Due to the reasons above, the enthalpy change of reaction calculated using bond energies might differ from the actual enthalpy change of the reaction

1. Bond energies are average values, and not specific to particular bonds in any molecule

2. Bond energies are only applicable to molecules in the gaseous state

Define electron affinity

• (Def.): The enthalpy change when one mole of a gaseous atoms or negatively charged ions gain one mole of electrons

Compare the 1st EA and 2nd EA

1. 1st EA 

• Usually negative (eco) as the effective nuclear charge of the atom leads to an attraction of the incoming electron

• When the attraction is stronger, the energy given off is greater and hence E.A. is more negative

2. 2nd and above EA

• Always positive (endo) because energy is absorbed to overcome the electrostatic repulsion between the incoming electron and the anion