The Driving Forces of Reactions AP Chemistry

In chemistry we are concerned with whether a reaction will occur spontaneously, and under what conditions will it occur spontaneously. In chemistry we are concerned with whether a reaction will occur spontaneously, and under what conditions will it occur spontaneously. Spontaneous reactions are reactions that naturally favor the products at specific conditions. They also release free energy (the energy available to do work) Spontaneous reactions are reactions that naturally favor the products at specific conditions. They also release free energy (the energy available to do work) Non-Spontaneous reactions are ones that do not favor the products and do not release free energy, they use it! Non-Spontaneous reactions are ones that do not favor the products and do not release free energy, they use it!

Free Energy Free Energy is given the symbol “G” and we look at the change in free energy (ΔG) in a reaction to determine if it is spontaneous or not. Free Energy is given the symbol “G” and we look at the change in free energy (ΔG) in a reaction to determine if it is spontaneous or not. A positive ΔG means the reaction is not spontaneous. A positive ΔG means the reaction is not spontaneous. A negative ΔG means the reaction is spontaneous. A negative ΔG means the reaction is spontaneous. ΔG = 0 if the system is at equilibrium. (such as at melting or boiling temperature) ΔG = 0 if the system is at equilibrium. (such as at melting or boiling temperature) The change in free energy, ΔG, is determine by the Gibbs Free Energy Equation The change in free energy, ΔG, is determine by the Gibbs Free Energy Equation ΔG = ΔH - TΔS ΔG = ΔH - TΔS T = Temperature in Kelvin

ENTHALPY ΔH represents the change in enthalpy for the reaction. ΔH represents the change in enthalpy for the reaction. (remember: +ΔH is endothermic and (remember: +ΔH is endothermic and -ΔH is exothermic.) -ΔH is exothermic.) Most reactions are exothermic because there is a natural tendency in nature to move to a lower energy state, and that is what happens in an exothermic reaction (think about the potential energy diagrams!) Most reactions are exothermic because there is a natural tendency in nature to move to a lower energy state, and that is what happens in an exothermic reaction (think about the potential energy diagrams!)

ENTROPY ΔS represents the change in entropy (S) ΔS represents the change in entropy (S) Entropy is the measure of the degree of randomness in a system. Gases have higher entropy than a liquid or solid, because the particles have less order. Entropy is the measure of the degree of randomness in a system. Gases have higher entropy than a liquid or solid, because the particles have less order. There is a tendency in nature to proceed in the direction that increases entropy (to get more random). There is a tendency in nature to proceed in the direction that increases entropy (to get more random).

Entropy +ΔS means there is an increase in entropy for a reaction +ΔS means there is an increase in entropy for a reaction ( liquid  gas) gets more random. –ΔS means there is a decrease in entropy for a reaction. –ΔS means there is a decrease in entropy for a reaction. ( a liquid  solid) gets more ordered

Gibbs free energy equation We use the Gibbs free energy equation to determine that ΔG for a reaction if we know the enthalpy change, the entropy change and the temperature (in Kelvin). We use the Gibbs free energy equation to determine that ΔG for a reaction if we know the enthalpy change, the entropy change and the temperature (in Kelvin). This will tell us if a reaction will be spontaneous at that temperature.(you look at the sign (+ or -) on the answer for ΔG) This will tell us if a reaction will be spontaneous at that temperature.(you look at the sign (+ or -) on the answer for ΔG) - ΔG = spontaneous - ΔG = spontaneous + ΔG = non-spontaneous + ΔG = non-spontaneous

You can calculate ∆G o from ∆H o and ∆S o and Temperature or you can look up the standard free energies of formation ∆G of and use the products minus reactants procedure we used for ∆H and ∆S. You can calculate ∆G o from ∆H o and ∆S o and Temperature or you can look up the standard free energies of formation ∆G of and use the products minus reactants procedure we used for ∆H and ∆S. (see appendix handout for values or look at appendix in book!) (see appendix handout for values or look at appendix in book!)

ΔG = ΔH - TΔS ΔHΔSΔG -+- Always Spontaneous +-+ Never Spontaneous -- Depends on Temperature Spontaneous at Low Temps ++ Depends on Temperature Spontaneous at High Temps

Example: Determine the values of  H,  S and  G at standard conditions (25oC and 1 atm) for the following reaction using the data given. Determine the values of  H,  S and  G at standard conditions (25oC and 1 atm) for the following reaction using the data given. 2SO 2 (g) + O 2 (g)  2SO 3 (g) 2SO 2 (g) + O 2 (g)  2SO 3 (g) Substance  S (J/K mol)  H kJ/ mol SO 2 (g) O 2 (g)2050 SO 3 (g)

∆H = 2∆H SO3 ∆H SO2 + ∆H O2 ] ∆H = 2∆H SO3 - [2∆H SO2 + ∆H O2 ] = 2(-396) – [ 2(-297) + 0] = 2(-396) – [ 2(-297) + 0] = -198 kJ = -198 kJ ∆S = 2 ∆S SO3 - [2(∆S SO2 + ∆S O2 ] = 2(257) – [2(248) + 205] = 2(257) – [2(248) + 205] = -187 J/K = -187 J/K Now plug into the Gibbs Equation using T=298K (convert ∆S to kJ from J first!) ∆G = ∆H - T ∆S = (-198kJ - (298K)(-0.187kJ/K) = (-198kJ - (298K)(-0.187kJ/K) = -142 kJ = -142 kJ

H 2 O(s) = H 2 O(l) Is Enthalpy (+) or (-)? Is Enthalpy (+) or (-)?  (+) because it is an endothermic change Is Entropy Increasing or Decreasing? Is Entropy Increasing or Decreasing?  Increasing  ∆S = (+)  Is the reaction Spontaneous?  Depends on Temperature.  Spontaneous, ∆G = (-), at High Temps.  Heat is required to melt Ice.  If you are at the right temperature, it will spontaneously melt.

H 2 SO 4 = 2H + + SO Heat Is ∆H (+) or (-)? Is ∆H (+) or (-)?  (-) because it is exothermic Is ∆S (+) or (-)? Is ∆S (+) or (-)?  (+) because entropy is increasing, more particles are more random!  Is the reaction spontaneous?  Yes, it is always spontaneous.  The universe favors lower energy and higher entropy!