Energetics Definitions
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Enthalpy change of reaction
energy change in a chemical rxn when molar quantities of reactants stated in chemical eqn react under standard conditions
Enthalpy change of formation (substance) Mg(s) + 0.5O2(g) –> MgO(s)
energy change when 1 mole of pure substance is formed from its constituent element in their standard state under standard conditions can be exo or endo
Enthalpy change of formation (element) O2(g) –> O2(g)
energy change when 1 mole of pure element is formed from its constituent element in their standard state under standard conditions is always zero ALWAYS ZERO
Enthalpy change of combustion
energy released when 1 mole of substance is completely burnt in excess oxygen under standard conditions always exothermic
Enthalpy change of neutralisation usually occurs b/w acid and base
energy change when acid and base react to form 1 mole of water under standard condition
Enthalpy change of atomisation (element)
energy adsorbed when 1 mole of gaseous atoms is formed from the element under standard conditons always exorthermic as bonds are broken
Enthalpy change of atomisation (compound)
energy adsorbed when 1 mole of compound is converted to gaseous atoms under standard conditions always exothermic as bonds are broken
Bond dissociation energy H2 (H-H) (g) —> 2H(g)
energy required to break 1 mole of X-Y bond in gaseous XY compound always endothermic
Bond Energy in data booklet
average energy required to break 1 mole of X-Y bond in gaseous state
1st ionisation energy Ca(g) —> Ca+(g) + e-
energy required to remove 1 mole of e- from 1 mole of gaseous atoms to form 1 mole of singly positively charged gaseous ions for 2nd IE change singly to doubly
1st electron affinity O(g) + e- —> O-(g)
energy change when 1 mole of gaseous atoms acquires 1 mole of e- to form 1 mole of singly negatively charged gaseous ions
Lattice energy Na+(g) + Cl-(aq) —> NaCl(s)
Energy released when 1 mole of solid ionic compound is formed from its constituent gaseous ions under standard conditions proportional to (cat charge + anion charge / cat + anion radius)
Enthalpy change of hydration Na+(g) + aq. —> Na+(aq)
energy released when 1 mole of gaseous ions is dissolved in water to form 1 mole of aqueous ions under standard conditions exothermic
Enthalpy change of solution ie. NaCl(s) + aq. —> Na+(aq) + Cl-(aq)
energy change when 1 mole of substance is completely dissolved in a solvent to form an infinitely dilute solution under standard conditions ΔH(soln) = −LE+ΔH(hyd) soluble if ΔH(soln) < 0 insoluble if ΔH(soln) > 0
Define entropy
a measure of disorder in the system, the more disordered the system, the larger the entropy
factors affecting entropy
entropy increases when: * temp increases: broadening of energy distribution of particles, as there are more possible energy states particles can adopt at higher temp * change in phase: (solid to liquid/gas) particles more randomly arranged, more disordered, increase in volume (for liquid to gas), entropy increases * change on no. of particles: (for gaseous system) more ways to arrange the particles, more ways to distribute energy in the system, creating more disorder, entropy increases * mixing of particles