17-2 Driving Force of Reactions

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Entropy Section 16-2.

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Presentation transcript:

17-2 Driving Force of Reactions

p. 526 Are most reactions endothermic or exothermic?

p. 526 Are most reactions endothermic or exothermic? Something other than enthalpy (ΔH) determines whether endothermic occurs spontaneously A system that can go from one state to another WITHOUT an enthalpy change does so by becoming more ___________________

p. 526 Are most reactions endothermic or exothermic? Something other than enthalpy determines whether endothermic occurs spontaneously A system that can go from one state to another WITHOUT and enthalpy change does so by becoming more DISORDERED

2NH4NO3(s)  2N2 (g) + 4H2O(l) + O2 (g) Right side is more random arrangement Tendency in nature is an increase in disorder What is a disordered system? (p. 527)

2NH4NO3(s)  2N2 (g) + 4H2O(l) + O2 (g) Right side is more random arrangement Tendency in nature is an increase in disorder What is a disordered system? (p. 527) Lacks arrangement of its parts Entropy (s) -

2NH4NO3(s)  2N2 (g) + 4H2O(l) + O2 (g) Right side is more random arrangement Tendency in nature is an increase in disorder What is a disordered system? (p. 527) Lacks arrangement of its parts Entropy (S) - measure of randomness of particles Solids Liquids gases

2NH4NO3(s)  2N2 (g) + 4H2O(l) + O2 (g) Right side is more random arrangement Tendency in nature is an increase in disorder What is a disordered system? (p. 527) Lacks arrangement of its parts Entropy (S) - measure of randomness of particles Solids – low entropy Melt – Liquids, increase entropy Evaporate – Gas, increase entropy

Entropy Change ΔS Difference of products and reactants Increase in entropy = + ΔS Decrease in entropy = - ΔS Making solutions increases ΔS Mixing gases Liquid into another liquid Solids dissolved in liquid Fig. 17-7 (p. 528)

p. 528 Processes are driven in 2 directions: Toward lowest enthalpy (lowest ΔH) OR Highest entropy (highest ΔS) Free Energy (G) – combined H and S function Only change can be measured Free energy change – ΔG Equation:

p. 528 Processes are driven in 2 directions: Toward lowest enthalpy (lowest ΔH) OR Highest entropy (highest ΔS) Free Energy (G) – combined H and S function Only change can be measured Free energy change – ΔG Equation: ΔG = ΔH - T ΔS (T in Kelvin)

Table p. 529 ΔG = ΔH - T ΔS Each can be positive or negative When change in different directions one will usually predominate

Example p. 530

More Order or More disorder? ΔS = + or -? 2 Fe2O3(s) --> 4 Fe(s) + 3 O2(g) W(s) + 6 CO(g) --> W(CO)6(s) C6H12O6(s) --> C6H12O6(aq) H2O(g) --> H2O(ℓ)