Thermochemistry

Thermochemistry is the branch of thermodynamics that deals with  the heat energy involved in chemical reactions.

Terminology/symbols:

Potential energy (PE)

Energy due to position or composition.

Kinetic energy (KE)

Energy due to motion associated with mechanical work. It is calculated from a moving object's mass (m) and velocity (v) by the equation KE=1/2 mv².

Internal energy (E)

The sum of the potential and kinetic energy for all particles in a system (the part of the universe whose change we are going to observe). When the internal energy of a system decreases, the change in energy (D E) is lost to the surroundings; therefore, D E of the system (E _final - E _initial) is negative. When the system's internal energy increases, D E is gained from the surroundings and is positive.
If: E final < E initial, then D E < 0 (negative).
If: E final > E initial, then D E > 0 (positive).

Heat  (q)

The energy transferred between a system and its surroundings as a result of a difference in their temperatures only. All other forms of energy transfer (mechanical, electrical, etc.) involve some type of work. A positive value means the system gains heat (energy in); a negative value means the system loses heat (energy out).

Work (w)

The energy transferred when an object is moved by a force. A positive value means work is done on the system; a negative value means work is done by the system. PV work is the work done by an expanding gas. (w = - P D V)  P is the external pressure and D V = V _final - V _initial.

 

 

Change in energy (DE) 

D E = q + w 

q	+	w	=	D E
+		+		+
+		-		depends on the sizes of q and w
-		+		depends on the sizes of q and w
-		-		-

Joule (J)

The energy of a 2 kg mass moving at a velocity of 1 m/s. The joule is the correct SI unit for energy.
1 J = kg·m²/s²

calorie (cal)

The calorie (cal) is an older unit that is defined as the amount of energy needed to raise the temperature of one gram of water by 1°C (from 14.5°C to 15.5°C). 
1 cal = 4.184 J
The nutritional Calorie (Cal) is actually a kilocalorie.

Calorimetry

The science of measuring heat flow.

Specific heat capacity (C) or (C_p)

The amount of heat needed to raise the temperature of 1 gram of a substance by 1°C (read, one calorie per gram per degree Celsius change), specifically the degree between 14.5 and 15.5°C. The equation for specific heat is: C= heat energy / (mass x D T). The unit is J/g°C.

Heat capacity

An object's ability to absorb or release heat which depends on the product of its specific heat and the mass (C x m), or heat divided by the change in temperature (q/D T). The unit is usually J/°C.

Enthalpy (H)

A property of a system, at constant pressure, which reflects its capacity to exchange heat with its surroundings. The enthalpy of a system is defined as the internal energy + the product of the pressure and volume: H = E + PV. Likewise, the change in enthalpy (D H) equals the heat gained or lost at constant pressure. Combining equations:
w = - P D V
DE = q + w 
DE = q + w = q + (- P D V) = q - P D V
DE + P D V= q_p (the heat at constant pressure)  
q_p =  D E + P D V = D H 
So, to find D H, measure q_{p .}

For many reactions, D H = or » D E.

A change that releases heat is exothermic (D H < 0); one that absorbs heat is 
endothermic   (D H > 0).

 

D H = H _final - H _initial = H _products - H _reactants

Types of Enthalpy Change:
heat of combustion (D H°_comb): the amount of heat energy evolved in a combustion reaction

heat of formation (D H°_f): symbol used only to denote the enthalpy change for the formation of one mole of a compound from its elements, all in their standard states. The heat of formation of an element is zero. 

heat of fusion (D H°_fus): the amount of heat evolved when a liquid freezes to form a crystalline solid .

heat of vaporization (D H°_vap): the amount of heat required for a molecule to overcome intermolecular forces and move from the liquid to the gas phase

heat of reaction (D H°_rxn): the amount of heat gained or lost by a reacting system at constant pressure

Hess's Law

The value of D H° for any reaction that can be written in steps equals the sum of the D H° of each of the individual steps. Hess's Law is mainly used to calculate the enthalpy change for a reaction for which such data cannot be determined experimentally or are otherwise unavailable. 

Standard states

The superscript ° on a thermodynamic function such as D H, D E, etc. means that the process has been carried out under standard conditions (not the same as STP, standard temperature and pressure for gases). The standard conditions are 1 atm and 25°C. For a pure substance in a condensed state (liquid or solid), the standard state is the pure liquid or solid. For a substance present in a solution, the standard state is a concentration of exactly 1 M. 

State function

A process which is independent of the pathway is referred to as a state function. Examples are energy, enthalpy, Gibbs free energy (G), entropy (S), pressure, volume, and temperature.

To be continued ...