II.3 Predicting Whether a Rx is

Spontaneous or Not p. 43 - 49

Spontaneous

a change that occurs by itself, without any help

some rxs are spontaneous, others are not

Enthalpy

∆H (heat)

systems drive towards the lowest possible energy state

endothermic rx = +∆H

exothermic rx = -∆H

Entropy

∆S

possibility for disorder, the drive towards randomness

increasing entropy = +∆S

decreasing entropy = -∆S

There are 2 drives in any rx:

the tendency for a rx to go to the side with MAXIMUM RANDOMNESS (MAX. ENTROPY)

the tendency for a rx to go to the side with MINIMUM ENERGY (MIN. ENTHALPY)

Ex. showing an Increase ∆S:

gas formed from a solid

gas formed from a solution

# mol of gaseous product is greater than # mol of gaseous reactant

when a solid dissolves in water

Ex. of Minimum Enthalpy:

PE

Time

Exofavoursproducts

Ex. of Minimum Enthalpy:

PE

Time

Exofavoursproducts

PE

Time

Endofavours

reactants

The Probability of a Rx Occuring:∆H ∆S Spontaneity(-)

exothermic favours _______

(+)favours _______

(-) exothermic favours _______

(-)favours _______

(+) endothermic

favours _______

(+)favours _______

(+) endothermic

favours _______

(-)favours _______

The Probability of a Rx Occuring:∆H ∆S Spontaneity(-)

exothermic Favours products

(+)favours products

(-) exothermic favours _______

(-)favours _______

(+) endothermic

favours _______

(+)favours _______

(+) endothermic

favours _______

(-)favours _______

The Probability of a Rx Occuring:∆H ∆S Spontaneity(-)

exothermic Favours products

(+)favours products

Spontaneous Rx goes 100%

(-) exothermic favours _______

(-)favours _______

(+) endothermic

favours _______

(+)favours _______

(+) endothermic

favours _______

(-)favours _______

The Probability of a Rx Occuring:∆H ∆S Spontaneity(-)

exothermic Favours products

(+)favours products

Spontaneous Rx goes 100%

(-) exothermic favours products

(-)favours reactacts

(+) endothermic

favours _______

(+)favours _______

(+) endothermic

favours _______

(-)favours _______

The Probability of a Rx Occuring:∆H ∆S Spontaneity(-)

exothermic Favours products

(+)favours products

Spontaneous Rx goes 100%

(-) exothermic favours products

(-)favours reactacts

SpontaneousRx goes to Equil.

(+) endothermic

favours _______

(+)favours _______

(+) endothermic

favours _______

(-)favours _______

The Probability of a Rx Occuring:∆H ∆S Spontaneity(-)

exothermic Favours products

(+)favours products

Spontaneous Rx goes 100%

(-) exothermic favours products

(-)favours reactacts

SpontaneousRx goes to Equil.

(+) endothermic

favours reactants

(+)favours products

(+) endothermic

favours _______

(-)favours _______

The Probability of a Rx Occuring:∆H ∆S Spontaneity(-)

exothermic Favours products

(+)favours products

Spontaneous Rx goes 100%

(-) exothermic favours products

(-)favours reactacts

SpontaneousRx goes to Equil.

(+) endothermic

favours reactants

(+)favours products

SpontaneousRx goes to Equil.

(+) endothermic

favours _______

(-)favours _______

The Probability of a Rx Occuring:∆H ∆S Spontaneity(-)

exothermic Favours products

(+)favours products

Spontaneous Rx goes 100%

(-) exothermic favours products

(-)favours reactacts

SpontaneousRx goes to Equil.

(+) endothermic

favours reactants

(+)favours products

SpontaneousRx goes to Equil.

(+) endothermic

favours reactants

(-)favours reactants

The Probability of a Rx Occuring:∆H ∆S Spontaneity(-)

exothermic Favours products

(+)favours products

Spontaneous Rx goes 100%

(-) exothermic favours products

(-)favours reactacts

SpontaneousRx goes to Equil.

(+) endothermic

favours reactants

(+)favours products

SpontaneousRx goes to Equil.

(+) endothermic

favours reactants

(-)favours reactants

Rx will NOT occur

Ex: 2NH3(g)+ 92 kJ➔N2(g) +

3H2(g)

∆H favours___________

∆S favours___________

therefore the rx ________________

Ex: 2NH3(g)+ 92 kJ➔N2(g) +

3H2(g)

∆H favours reactants

∆S favours products

therefore the rx ________________

Ex: 2NH3(g)+ 92 kJ➔N2(g) +

3H2(g)

∆H favours reactants

∆S favours products

therefore the rx goes to equilibrium

BaCO3(s) ➔BaO(s) + CO2(g) ∆H=600kJ

∆H favours___________

∆S favours___________

therefore the rx ________________

BaCO3(s) ➔BaO(s) + CO2(g) ∆H=600kJ

∆H favours reactants

∆S favours products

therefore the rx ________________

BaCO3(s) ➔BaO(s) + CO2(g) ∆H=600kJ

∆H favours reactants

∆S favours products

therefore the rx goes to equilibrium

3H2(g)+ CO(g)➔CN4(g) + H2O(g)

∆H = -51 kJ

∆H favours___________

∆S favours___________

therefore the rx ________________

3H2(g)+ CO(g)➔CN4(g) + H2O(g)

∆H = -51 kJ

∆H favours products

∆S favours reactants

therefore the rx ________________

3H2(g)+ CO(g)➔CN4(g) + H2O(g)

∆H = -51 kJ

∆H favours products

∆S favours reactants

therefore the rx goes to equilibrium

H2O(l)+ heat➔ H2O(g)

∆H favours___________

∆S favours___________

therefore the rx ________________

H2O(l)+ heat➔ H2O(g)

∆H favours reactants

∆S favours products

therefore the rx ________________

H2O(l)+ heat➔ H2O(g)

∆H favours reactants

∆S favours products

therefore the rx goes to equilibrium

Do: 14 - 16