Chapter 23 Thermodynamics What is the driving force for every process in the universe?

Review of equilibrium What you should know at this point If the Keq > 1, the equilibrium favors the products. If the Keq < 1, the equilibrium favors the reactants.

Spontaneous Processes Spontaneous process - A process that occurs on its own, without any outside intervention. Spontaneous process may occur slowly (over thousands of years) or fast (in less than a second). The amount of heat given off or absorbed is not what determines how fast a reaction takes place; however, exothermic reactions do tend to be spontaneous.

∆ H (enthalpy - heat) is a measure of this energy change.

Not all spontaneous reactions are exothermic; therefore, heat (∆H) is not the only factor that determines spontaneity. Examples: ice melts (spontaneous and endothermic) Some materials absorb heat as they dissolve in water (spontaneous and endothermic). A sports cold pack

What is it that causes a reaction to be spontaneous? What causes every reaction (and process) in the universe to take place? Lower energy is not the only driving force - there is another factor.

Entropy (S) - a measure of the disorder (randomness) of a system. The greater the disorder, the greater the S S gas > S liquid > S solid Less order more order ΔS = S products – S reactants If ΔS is positive the system goes to more disorder.

To gases from liquids and solids ΔS is positive To solutions from solids and liquids ΔS is positive If temperature is increases ΔS is positive

Second Law of Thermodynamics In any process the overall entropy of the universe always increases. ΔS universe = ΔS process + ΔSsurroundings Must be + one of these can be – if the other is + by a greater amount

Entropy change for a process and surroundings Process ΔS process ΔH process ΔS surroundings ΔSproc. + ΔSsurr. spontaneous? Exothermic yes Entropy increase Endothermic or - depends Entropy increase Exothermic or - depends Entropy decrease Endothermic no Entropy decrease

Spontaneity of a reaction depends on two factors (entropy and enthalpy). When these two factors oppose each other, the spontaneity depends on which is larger Gibbs discovered the relationship between entropy, enthalpy and spontaneity He found that the relationship is dependent on absolute temperature.

This concept is called Gibbs Free Energy (G) ∆G = ∆H - T ∆S The change in Gibbs free energy (∆G) equals the enthalpy change (∆H) minus the absolute temperature times the change in entropy. Free Energy (∆G) is the amount of energy available to do work. More importantly, it is a measure of spontaneity.

∆G = ∆H - T∆S If ∆G is negative the reaction is spontaneous. If ∆G is positive the reaction is not spontaneous and will require a high energy input to force it to occur. If ∆G is zero, the reaction is at equilibrium. The spontaneity of a system is dependent on its temperature.

∆G = ∆H - T∆S Case 2 – If both ∆H and ∆S are positive, the reaction will proceed at high temperatures and will not happen at low temperatures. (study equation) Example: melting ice How are ∆H and T∆S related at 0 o C? The two terms will be equal because the system is at equilibrium at 0 o

∆G = ∆H - T∆S Case 3 - ∆H and ∆S are both negative. If the temperature is low enough, T∆S becomes less in magnitude than the negative ∆H and the process will proceed even though there is a more ordered arrangement. Example: freezing of water See the chart at the top of page 762.

If a process is spontaneous it can be made to do work: however, the work from a reaction can never exceed ∆G. Free energy – the amount of energy “free” to do work. The remainder is unavailable because it is “lost” to the environment to meet the criterion that the entropy of the universe must increase (the 2 nd law of thermodynamics). The total energy of the universe is constant; however, the energy is continually dispersed, like the winding down of a clock.