Thermochemistry & thermodynamics Flashcards
Standard enthalpy change of formation
The enthalpy change when 1 mole of a substance is formed from its constituent elements in their standard states at 298K and 1 bar
Standard enthalpy change of combustion
The heat evolved when 1 mole of a substance is completely burnt in excess oxygen at 298 K and 1 bar
Standard enthalpy change of reaction
The enthaply change when molar quantities of reactants as specified by their chemical equation react to form products at 298K and 1 bar
Hess’s Law
The enthalpy change of a reaction is determined only by the initial and final states and is independent of the reaction pathway taken
General formula for enthalpy change of reaction
enthalpy change of reaction = enthalpy change of combustion/formation of (reactants)- (products)
following stoichiometric coeff of eqn
Standard enthalpy change of neutralisation
The heat evolved when 1 mole of water is formed in the neutralisation reaction between an acid and a base at 298K and 1 bar
Nature of neutralisation reaction (endo or exothermic)
Neutralisation is an exothermic reaction since it involves the attraction of H+ and OH- ions to form an O-H bond
Why neutralisation of stronger acids and bases is more exothermic
Strong acids and strong bases ionise completely in dilute aqueous solution and reaction between them is effectively the reaction b/w H+ and OH- ions.
Weak acids/bases do not ionise completely in aqueous solution. During neutralisation, more energy is absorbed to ionise the un-ionised weak acid/base so less energy is released and the resulting enthalpy change of neutralisation is less exothermic.
eqn for heat change of solution (q)
q=mc delta T
eqn for enthalpy change of reaction
delta H = q/n, -q/n if reaction is exothermic
assumptions in calculation of delta H
- no heat loss or gain from surroundings
- heat capacity of calorimeter is omitted
Bond dissociation energy
Bond dissociation energy is the energy required to break 1 mole of a particular covalent bond in a specific molecule in the gaseous state
What affects the magnitude of bond dissociation energy?
Bond dissociation energy is a measure of the strength of covalent bonds. The more endothermic the bond dissociation energy, the stronger the covalent bond
Bond energy
Bond energy is the average energy required to break 1 mole of a covalent bond in the gaseous state
Standard enthalpy change of atomisation (element)
The energy required when 1 mole of gaseous atoms is formed from the element at 298K and 1 bar
Standard enthapy change of atomisation (compound)
The energy required to convert 1 mole of the compound into gaseous atoms at 298K and 1 bar
Lattice energy
The heat evolved when 1 mole of solid ionic compound is formed from its constituent gaseous ions
Factors affecting magnitude of lattice energy
- the charges on the ions
- the sizes of ions or inter-ionic distance
Ionisation energy
The first ionisation energy is the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of singly charged gaseous cations
Electron affinity
The first electron affinity is the enthalpy change when 1 mole of electrons is added to 1 mole of singly charged gaseous anions
Why 1st EA is -ve
1st EA is usually negative as the effective nuclear charge of the atom leads to an attraction of the incoming electron
Why 2nd EA is +ve
2nd and subsequent EA are always positive because energy is required to overcome the electrostatic repulsion between the incoming electron and anion
Experimental versus theoretical lattice energy
Theoretical lattice energy refers to the value calculated based on the model that assumes that the compound is completely ionic
A large differences shows that there is covalent character in the ionic compound. This is most apparent when a cation with a high charge density distorts an anion with a large electron cloud
Standard enthalpy change of hydration
The heat evolved when 1 mole of free gaseous ions is dissolved in an infinite volume of water at 298K and 1 bar
Sign of enthalpy change of hydration
enthalpy change of hydration is always negtaive as heat evolved in forming ion-dipole interactions between ion and water molecules
What affects the magnitude of enthalpy change of hydration?
The magnitude of enthalpy change of hydration depends on charge density.
The higher the charge density of the ion, the stronger the ion-dipole interaction and enthalpy change of hydration will be more exothermic
Standard enthalpy change of solution
The enthalpy change when 1 mole of solute is completely dissolved in an infinite volume of solvent at 298K and 1 bar
\+ve = likely insoluble -ve = likely soluble
spontaneous changes
spontaneous changes are changes that have a natural tendency to occur
entropy (S)
entropy is a measure of the randomness/disorder in a system, reflected in the number of ways that the energy can be distributed through the motion of its particles
Effect of change in temperature on S
As temp increases, the average kinetic energy of the particles and the range of energies increase. There are more ways to disperse the energy among the particles. Hence S increases
Effect of change in phase on S
During (stated change in phase), the particles move more freely in the liquid/gaseous state and become more disordered. Hence there is an increase in entropy as there are more ways to distribute the particles and their energy in the liquid/gaseous state
Effect of change in number of particles on S (for gaseous systems)
Increase in number of gas particles causes a large increase in entropy. This is because the particles are the most disordered so the number of ways that the particles and the energy can be distributed increase greatly.
Effect of mixing of particles on S
As the volume available for each gas is increased, there are more ways to distribute the particles and hence their energy. Thus, entropy increases
Effect of dissolution of an ionic solid on S
Entropy both increases and decreases.
- Entropy increases because the ions in the solid are free to move in solution
- Entropy decreases because the water molecules that were originally free to move become restricted in motion as they arrange themselves around the ions
