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Entropy, Enthalpy and Spontaneous Reactions!. Entropy (S), Ethalpy (H) and Gibbs Free Energy (G). What do we know about chemical reactions?. We can write chemical reactions and show how atoms and molecules come together to form products. - PowerPoint PPT Presentation
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What do we know about chemical reactions?
We can write chemical reactions and show how atoms and molecules come together to form products. We can determine the energy change in a chemical reaction ΔH.We can determine if a reaction is fast or slow (rate).We can determine how to increase the rate of the reaction.
How do we know if a reaction will occur or not?
Do the reaction in the lab!Use thermodynamic qualities to determine spontaneity.
What is spontaneous?Going downhill happens without having to put any energy into it.
Spontaneous
Going uphill only occurs if you continually add energy to the process.
Non-spontaneous
EnthalpyNature favours a decrease in potential energy. (NATURE IS LAZY!)Exothermic reactions are favoured over endothemic reactions.-ΔH means the reaction is favoured.+ΔH means the reaction is not favoured.
EntropyNature favours an increase in Entropy. (NATURE IS MESSY)This is the systems state of disorder.It is dependent on the temperature.Entropy = S, Change in Entropy = ΔS, kJ/K
Are these reactions favoured in terms of entropy?
2 K(s) + Cl2(l) 2KCl(s)
2LiBr(s) 2Li(s) + Br2(l)
N2(g) + 3H2(g) 2NH3(g)
Unfavoured
Favoured
Unfavoured
Spontaneous?NaOH(s) Na+(aq) + OH-(aq)
ΔH = -44.4 kJ
Enthalpy
Entropy
Spontaneity
-Δ H Favoured
+ΔS Favoured
This reaction will ALWAYS be spontaneous!
Spontaneous? 2Al(s) + 3O2(g) 2Al2O3(s) ΔH = -3351 J
Enthalpy
Entropy
Spontaneity?
-ΔH Strongly favoured.
-ΔS Not favoured.
Spontaneous at very low temperatures.
Spontaneous? KNO3(s) K+(aq) + NO3
-(aq) ΔH = +34 kJ
Enthalpy
Entropy
Spontaneous?
+ΔH unfavourable
+ΔS favourable
Spontaneous at very high temperatures!
Spontaneous? 6CO2(g) + 6H2O(g) C6H12O6(s) + 6O2(g) ΔH = +2868
kJ/mol
Enthalpy
Entropy
Spontaneous?
+ΔH unfavoured
-ΔS unfavoured
This reaction is NEVER spontaneous and requires a constant input of energy!
ΔG = ΔH-TΔSΔG < 0 the reaction is spontaneous.
ΔG> 0 the reaction is not spontaneous.
Gibbs Free energy is the amount of energy available to do work.