Chapter 16, Spontaneity of Reaction

Relationship between Delta G, Delta H and Delta S

A negative value of Delta H, Exothermic reaction, tends to be spontaneous. A positive value of delta S the term -TS will make a negative contribution to delta G, hence there will be a tendency for a reaction to be spontaneous if the products are less orderly than the reactants both these factors make G negative

Pressure, Concentration

Delta G = Delta Gdegrees + RTlnQ (check bookmark for better understanding) it is the standard Free Energy change Sometimes if delta G is close to 0, raising the pressure from 1ATM can change direction of spontaneity

Spontaneous Processes

Delta H, change in Enthalpy, if negative causes a spontaneous reaction Delta S, Change of entropy , a positive value causes a spontaneous reaction Delta G, the change in free energy, if at a constant temperature and pressure and NEGATIVE will always cause a spontaneous reaction

Delta S for reactions

Delta S = Sum Sproducts - Sum Sreactants

Standard Molar Entropies

Elements have nonzero standard entropies Standard molar entropies of elements and compounds are always positive Aqueous Ions may have negative entropy values

Free Energy

Enthalpy and entropy both affect the spontaneity of a reaction for a reaction taking place at a constant pressure and temperature, Delta G represents the total energy change that is free to do work G=H-TS If Delta G is negative, the reaction is spontaneous If Delta G is positive, the reaction will not occur spontaneously, instead the reverse reaction will occur If Delta G is 0 no reaction will occur since it is at equilibrium If products have a lower free energy than the reactants, the forward reaction will occur

The Free energy Change and the Equilibrium Constant

For a reaction at standard conditions, Delta G naught must be negative. If K is greater than 1, the reaction is spontaneous Delta G = Delt G naught +RTlnQ once equilibrium is established, Delta G =0 and Q=K Therefore 0=Delt G naught +RTlnK Delt G naught = -RTlnK If Delt G naught is negative, K will be large implying that the forward reaction will go almost to completion If Positive, the reverse reaction will go nearly to completion

The second law of thermodynamics

In a spontaneous process, there is a net increase in entropy, taking into account both system and surroundings, Delta Suniverse = (delta Ssystem + Delta Ssurr) and if it is greater than 0 it is a spontaneous process

The Energy Factor

Most spontaneous processes occur when there is a decrease of energy, it is proposed that spontaneous processes are exothermic, however this is not always the case There are a number of endothermic spontaneous processes, which occur at very high temperatures

The Randomness Factor

Nature tends to move spontaneously from a state of lower probability to one of higher probability Think of how it is more likely to roll a 7 with two dies than a 12, so that is the higher probability that we are moving to N2 and O2 in a container, opened and they will evenly distribute, moving towards its own most probable distribution independent of the other gas Nature tends to move from orderly states (N2 and O2 separated) to random states (O2 and N2 diffusing evenly on both sides

Temperature

Temperature affects spontaneity. When increased, the temperature may or may not change the sign of delta G If delta S and Delta H have opposite signs, it is impossible to reverse direction of spontaneity by a change in temperature alone When Delta S and Delta H have the same signs (more common) enthalpy and entropy factors oppose each other. Delta G will change sign and the spontaneity reverses At low T delta H dominates and the exothermic reaction occurs Once T increases to the point where -TS overtakes H in magnitude, an increase in entropy occurs

Entropy

The randomness factor, the more probable a state or the more random the distribution of molecules, the greater the entropy. Entropy depends on the state of the system Some simple Entropy Rules: A liquid has a higher entropy than the solid from which it is formed (solid is much more ordered, liquid more space for particles to move and be in random position) a gas has a higher entropy than the liquid from which it is formed Increasing the temperature of a substance increases the entropy (raising T increases the KE which also increases their freedom of motion Pure crystalline has an entropy of 0 at 0 kelvin