To understand why a chemical reaction goes in a particular direction, we need to study spontaneous processes. A spontaneous process is a physical or chemical change that occurs by itself. It does not require any outside force, and it continues until equilibrium is reached. e.g. Heat flows from hot to cold object. If the process was to go in the opposite direction that would be a nonspontaneous process. Heat flows from hot to cold object. Object rusts in moist air. Copyright © Cengage Learning. All rights reserved.

Copyright © Cengage Learning. All rights reserved.

Spontaneous Physical and Chemical Processes A waterfall runs downhill A lump of sugar dissolves in a cup of coffee At 1 atm, water freezes below 0 0C and ice melts above 0 0C Heat flows from a hotter object to a colder object A gas expands in an evacuated bulb Iron exposed to oxygen and water forms rust spontaneous nonspontaneous

We need a new quantity—entropy. Spontaneous reactions could be exothermic or endothermic. The first law of thermodynamics cannot help us to determine whether a reaction is spontaneous as written, hence we need another thermodynamic function to answer spontaneity of the reaction. We need a new quantity—entropy. Entropy, S, is a thermodynamic quantity that is a measure of how dispersed the energy of a system is among the different possible ways that system can contain energy. Entropy of the system plus its surroundings increases in a spontaneous process. The first law helps to tack various forms of energy. Spontaneous reactions could be exothermic or endothermic hence we need another thermodynamic function to answer spontaneity of the reaction. Entropy is the measure of the disorder of the system. When energy is concentrated in few energy states, the entropy of the system is low. However, if same energy is spread out over a great many energy states the entropy of the system is high. Entropy of the system plus its surroundings increases in a spontaneous process.

How does the entropy of a system change for each of the following processes? (a) Condensing water vapor Randomness decreases Entropy decreases (DS < 0) (b) Forming sucrose crystals from a supersaturated solution Randomness decreases Entropy decreases (DS < 0) (c) Heating hydrogen gas from 600C to 800C Randomness increases Entropy increases (DS > 0) (d) Subliming dry ice Randomness increases Entropy increases (DS > 0)

Entropy State functions are properties that are determined by the state of the system, regardless of how that condition was achieved. energy, enthalpy, pressure, volume, temperature , entropy Potential energy of hiker 1 and hiker 2 is the same even though they took different paths.

First Law of Thermodynamics Energy can be converted from one form to another but energy cannot be created or destroyed. Second Law of Thermodynamics The entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process. Spontaneous process: DSuniv = DSsys + DSsurr > 0 Equilibrium process: DSuniv = DSsys + DSsurr = 0

First Law of Thermodynamics The first law of thermodynamics is essentially the law of conservation of energy applied to a thermodynamic system. Recall that the internal energy, U, is the sum of the kinetic and potential energies of the particles making up the system: U = KE + PE The change in the internal energy ∆U is DU = Uf – Ui Internal energy can change as result of transfers of energy (energy flow) into or out of system. The first law simply says that the change in the internal energy equals to the sum of these energy transfers. Copyright © Cengage Learning. All rights reserved.

Another form of the first law for DUsystem We are interested in the energy changes associated with the system and not with the surrounding. The internal energy is changed by adding or removing heat or by doing work. DU = q + w DU is the change in internal energy of a system ‘q’ is the heat exchange between the system and the surroundings ‘w’ is the work done on (or by) the system

Putting this in an equation gives us the first law of thermodynamics. Exchanges of energy between the system and its surroundings are of two types: heat, q, and work, w. Putting this in an equation gives us the first law of thermodynamics. DU = q + w According to the first law of thermodynamics, when system undergoes a physical or chemical change, the change of internal energy = sum of heat and work done in that physical change or chemical change. Copyright © Cengage Learning. All rights reserved.

Sign Convention for q When heat is evolved by the system, q is negative. This decreases the internal energy of the system. When heat is absorbed by the system, q is positive. This increases the internal energy of the system. Heat = -q decrease in internal energy Heat = +q increase in internal energy Copyright © Cengage Learning. All rights reserved.

The heat transferred under constant pressure conditions is called as Enthalpy (Greek word Enthalpein, meaning “to warm”) Enthalpy (H) is used to quantify the heat absorbed or heat evolved in a process that occurs at constant pressure. It is an extensive property-depends on the amount of substance. At constant pressure: qP = DH & w = –P DV The first law of thermodynamics, can now be expressed as follows: DU = DH – PDV DU = q + w State functions are properties that are determined by the state of the system, regardless of how the conditions was achieved, i.e. the magnitude of change in any state function depends only on the initial and the final states of the system and not on how the change is accomplished. Most of the processes occur at constant pressure , hence change in the energy is in the form of heat lost or gain by the system. The heat transferred under constant pressure conditions is called as Enthalpy(Greek word Enthalpein, meaning “to warm”) Enthalpy (H) is used to quantify the heat absorbed or heat evolved in a process that occurs at constant pressure. It is an extensive property-depends on the amount of substance. ∆U = q + w If chemical reaction is carried at constant pressure , then, ∆U= qp+ w If we substitute w = –P∆V and qp = ∆H Then ∆U = ∆H –P∆V Copyright © Cengage Learning. All rights reserved.

Copyright © Cengage Learning. All rights reserved.

Copyright © Cengage Learning. All rights reserved.

For a constant-temperature process: Gibbs Free Energy For a constant-temperature process: Gibbs free energy (G) DG = DHsys -TDSsys DG < 0 The reaction is spontaneous in the forward direction. DG > 0 The reaction is nonspontaneous as written. The reaction is spontaneous in the reverse direction. DG = 0 The reaction is at equilibrium.

DG = DH - TDS