Thermochemistry

Thermochemistry is the study of the heat released (-  H) or absorbed (+  H) by chemical and physical changes. Thermochemistry

Some terminology Temperature is defined as the degree of hotness/centigrade scale, °C; kelvin, K/ Heat is a form of energy Heat capacity is the quantity of heat required to raise the temperature of a substance by 1°C C = mass x specific heat

Thermochemistry Specific heat is the heat capacity of one gram of a substance. Specific heat of water: J/(g°C) Calorimeter is a device used to measure the heat transferred in chemical reactions.

Thermodynamic Systems - Definitions Isolated System: No matter or energy cross system boundaries. No work can be done on the system. Open System: Free exchange across system boundaries. Closed System: Energy can be exchanged but matter cannot. Adiabatic System: Special case where no heat can be exchanged but work can be done on the system (e.g. PV work).

Thermodynamic Systems - Definitions Surroundings: is the portion of the universe with which a system interacts Exothermic reaction: is a chemical reaction in which heat is liberated (-  H) Endothermic reaction: is a chemical reaction in which heat is absorbed (+  H)

Thermochemical equations e.g. C 6 H 6(l) + 15,O’ (g) = 6CO 2(g) + 3H 2 O (l) -  H  H = H products – H reactants  H = heat of reaction H = enthalpy (heat content P=k)  H is given at 25°C and standard atmospheric pressure

First Law of Thermodynamics Basic concepts: 1.Work and heat are both forms of energy 2.One form of energy can be converted into another form 3.Energy cannot be created or destroyed

First law of thermodynamics is the law of conservation of energy The total energy of the univers is a constant Internal energy: E In an isolated system E = constant not known, cannot be calculated  E = E f – E i (f = final) (i= initial) can be measured

q = +heat absorbed by the system q = -heat evolved by the system w = +work done by the system w = -work done on the system w = P  V(on constant temperature)  E = q-P  V q =  E + P  V  E = q - w q = heat W = work

Definition of Enthalpy We can define a new state variable (one where the path to its current state does not affect its value) called enthalpy: H = E i + PV Enthalpy = Internal Energy + PV Heat content q =  H

Second Law of Thermodynamics  One statement defining the second law is that a spontaneous natural processes tend to even out the energy gradients in a isolated system.  Can be quantified based on the entropy of the system, S, such that S is at a maximum when energy is most uniform. Can also be viewed as a measure of disorder.  S = S final - S initial > 0 The law states that a system will always undergo a spontaneous change in such a way as to increase the entropy

 S total =  S system +  S surroundings Entropy change:  S is If the reaction is exothermic:  T  S –  H > 0 The reaction is spontaneous if  H < T  S  S = q t  S total =  S - HH T

Change in Entropy S steam > S liquid water > S ice Relative Entropy Example: Third Law Entropies: All crystals become increasingly ordered as absolute zero is approached (0 K = °C) and at 0 K all atoms are fixed in space so that entropy is zero. ISOLATED SYSTEM

Spontaneous Reaction Direction

The change in enthalpy for any chemical reaction is constant, whether the reaction occurs in one step or in several steps. Law of Hess law of constant heat summation