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Hess’s Law states that the heat evolved or absorbed in a chemical change is the same whether the change is brought about in one stage or through intermediate stages. Hess's law can be represented using an
enthalpy cycle
Standard Enthalpy Changes
1. The numerical values for enthalpy changes can be affected by the following factors:
(a) temperature which experiment is carried out
(b) pressure of the gaseous reactants and products
(c) concentrations of solutions
(d) physical state of reactants and products
(e) allotropic form of elements
2. The standard conditions for measuring enthalpy change as follow:
v gases must be at pressure 1.0 atm (101.3 kPa)
v solutions must have a concentration of 1.0 mol dm-3
v the reaction must be carried out at a temperature of 298K
v the physical state of a substance which it is most stable under standard condition is taken as standard
Electron affinity
1. First affinity electron is the energy evolved when 1 mole of gaseous atom received electron to form 1 mole of uni-negative gaseous ion under the standard conditions.
Cl (g) + e- à Cl- (g) ∆Hθ = -364 kJ mol-1
O (g) + e- à O- (g) ∆Hθ = -142 kJ mol-1
2. Only the first electron affinity has the negative value, higher electron affinity all has the positive values.
3. This is because when 1 atom receives electron to form negative ion, the negative charge will reject the addition of 2nd electron (which is negatively charged). Energy is needed to force the atom to receive the 2nd electron.
S- (g) + e- à S2- (g) ∆Hθ = +532 kJ mol-1
O- (g) + e- à O2- (g) ∆Hθ = + 844 kJ mol-1
2. The standard conditions for measuring enthalpy change as follow:
v gases must be at pressure 1.0 atm (101.3 kPa)
v solutions must have a concentration of 1.0 mol dm-3
v the reaction must be carried out at a temperature of 298K
v the physical state of a substance which it is most stable under standard condition is taken as standard
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