Physical Chemistry Definitions

enthalpy change of reaction

 enthalpy change when molar quantities of reactants as specified by the chemical equation react to form products at 1 bar and 298 K

enthalpy change of formation

enthalpy change when 1 mole of a substance is formed from its constituent elements in their standard states at 1 bar and 298K

enthalpy change of combustion

heat evolved when 1 mole of substance is completely burnt in excess oxygen at 298K and 1 bar

enthalpy change of neutralisation

heat evolved when 1 mole of water is formed in the neutralisation reaction between an acid and a base, at 298K and 1 bar

enthalpy change of atomisation of element

energy required when 1 mole of gaseous atoms is formed from an element at 298K and 1 bar

enthalpy change of atomisation (for compound)

energy required to convert 1 mole of compound into gaseous atoms at 298K and 1 bar

enthalpy change of hydration

heat evolved when 1 mole of free gaseous ions is dissolved in infinite volume of water at 298K and 1 bar → Naᐩ(g) + aq → Naᐩ(aq)

enthalpy change of solution

enthalpy change when 1 mole of solute is completely dissolved in infinite volume of solvent at 298K and 1 bar → NaCl (s) →  Naᐩ(aq) + Cl⁻ (aq)

lattice energy

1. Heat evolved when 1 mole of solid ionic compound is formed from its constituent gas ions

   1. Measure of strength of ionic bonding & stability of ionic compounds

first ionisation energy

 energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of singly charged gaseous cation

electron affinity

 enthalpy change whne 1 mole of electrons is added to 1 mole of gaseous atoms to form 1 mole of singly charged gaseous anions

entropy

Measure of randomness/disorder in a system, reflected in number of ways that the energy of a system can be distributed through the motion of its particles (J/Kmol)

Hess’ Law

Enthalpy change is determined by the initial and final states, independent of the reaction pathway

Describe the mode of action of homogenous catalysts.

Forms an intermediate between the catalyst & one of the reactants, and the catalyst is later regenerated by the end of the reaction

Describe the mode of action of heterogenous catalyst.

Act as active sites for the reactants to provide an alternative pathway of lower activation energy.

1. Reactants adsorb onto the active site of the catalyst, which brings the reactants closer together, weakening the covalent bonds within the molecules & orientates them in the correct orientation to react

2. Reaction occurs at lower activation energy

3. Products desorb from the active site to allow reactants to utilise the active sites

Define dynamic equilibrium.

Reversible reaction in a closed system, where the forward and backward reaction occur at the same rate, resulting in no overall change of concentration

Define Le Chatelier’s Principle

If the conditions of a system at equilibrium are changed, the position of equilibrium will shift to REDUCE that change

Explain why an increase in pressure does not affect the rate constant.

1. Increasing the pressure causes molecules to be forced closer together, hence the distance between molecules decreases, so the frequency of collisions between molecules increases

2. However, rate constant depends on the number of particles that possess energy greater than or equal to activation energy, as well as an increase in particles colliding in the correct orientation

3. rate equation = k [A][B]. Since the pressure is already accounted for in the concentration of the reactions, k is independent of pressure.

State the conditions of the Haber Process.

450°C, 250atm, finely divided iron catalyst

N2 + 3H2 ⇌ 2NH3

Explain why equilibrium constant is affected by temperature.

1. When temperature decreases, system will favour the exothermic reaction in order to generate additional heat (/ vice versa)

2. Temperature affects the rate constant by increasing the frequency of effective collisions, changing the rate constant ratio of the forward & backward reactions, (e.g. if forward exothermic reaction, then the rate constant of the forward reaction will increase)

3. Hence, it alters the forward & backward reaction rate constant ratio & affects the Gibbs free energy of the equation, so it affects the equilibrium constant

Define solubility.

No. of moles of a solute that can be dissolved in 1dm³ of a solvent to form a saturated solution at a given temperature

Define K_sp

Refers to the product of the molar concentrations of constituent ions in a saturated solution, raised to the appropriate powers at given temperature

Define IP

Refers to the product of molar concentration of constituent ions in a solution, raised to the appropriate powers at a given temperature