Thermochemistry ENTHALPY

1st Law of Thermodynamics  When a system absorbs energy, the surroundings release it  When a system releases energy, the surroundings absorb it 1 St Law: Energy cannot be created or destroyed, but simply converted from one form to another ∆E total = ∆E system + ∆E surroundings = 0 ∆E system = -∆E surroundings

ENTHALPY  Enthalpy of a system is the energy due to a combination of kinetic energy and potential energy the molecules in that system possess.  Commonly defined as “ a measure of the heat content of a system.”  Represented by the letter H.

 It is not possible to measure the absolute value of enthalpy of a given system, H.  It is possible to measure changes in enthalpy, ∆H.  The enthalpy of a system will change when the total energy of the system changes due to a physical, chemical or nuclear change. ∆H = H final - H initial Enthalpy

If heat leaves the system then ∆H is negative (-) Process is EXOTHERMIC If heat leaves the system then ∆H is negative (-) Process is EXOTHERMIC If heat enters the system then ∆H is positive (+) Process is ENDOTHERMIC If heat enters the system then ∆H is positive (+) Process is ENDOTHERMIC ∆E system = -∆E surroundings ∆H system = ± |q surroundings | Enthalpy

Measuring Enthalpy  Calorimetry - the measurement of the heat into or out of a system for chemical and physical processes.  Based on the fact that the heat released = the heat absorbed  The device used to measure the absorption or release of heat in chemical or physical processes is called a “Calorimeter”

How we measure enthalpy: Calorimetry Constant Volume “Bomb” Calorimeter  Burn combustible sample (this is how nutritionists determine calories)  Measure heat evolved in a reaction.  Derive ∆E for reaction.

Some heat from reaction warms water q water = (sp. ht.)(water mass)(∆T) Some heat from reaction warms “bomb” q bomb = (heat capacity, J/K)(∆T) Calorimetry: How it works…

What you will use:  Foam cups are excellent heat insulators, and are commonly used as simple calorimeters under constant pressure.  For systems at constant pressure, the “heat content” is the same as a property called Enthalpy (H) of the system

A foam cup calorimeter – here, two cups are nestled together for better insulation

 The 2 stacked Styrofoam cups isolate the system from the external environment  The reaction takes place in the inner cup with a known mass of water (or dilute solution)  A thermometer is used to measure heat absorbed/released to the surroundings (water)  Limitations:  Does not work for reactions involving gases  Does not work for high temperature reactions Coffee cup calorimetry

 ∆H system = ± |q surroundings |  The enthalpy change of a system can be expressed per unit.  For example, enthalpy can be expressed per mass (e.g. Joules/gram).  Enthalpy can also be expressed per mole (e.g. Joules/mole).

 This yields us two “equations” for enthalpy change. An equation to represent specific enthalpy (Joules per gram) and molar enthalpy (Joules per mole).  Specific Enthalpy ∆ H specific = ∆ H/m  Molar enthalpy ∆ H molar = ∆ H/n

 The enthalpy change of a system can be measured from the heat energy gained/lost by the surroundings:  ∆H system = ± |q surroundings |  We calculate the heat energy gained/lost by the surroundings using the heat equation:  q = mc ∆T

Calorimetry calculation Calculate heat of combustion of octane. 2C 8 H O 2 --> 16 CO H 2 O  Burn 1.00 g of octane  Temp rises from to o C  Calorimeter contains 1200 g water  Heat capacity of bomb = 837 J/K