Thermochemistry The study of the energy changes that accompany physical, chemical or nuclear changes in matter
Definitions: recall physics Energy (E): The ability to do work; measured in Joules (J) Work: Amount of energy applied or transferred over a distance Potential Energy (Ep): Energy of object due to its position or composition; stored energy Kinetic Energy (Ek): Energy of object due to its motion; movement energy
Definitions: recall physics Thermal Energy (H): Total quantity of kinetic & potential energy within a substance, Ep + Ek = H Heat (q): The transfer of thermal energy from a warm body to cooler body Temperature: A measure of the average kinetic energy of particles in a substance/object
Definitions: new ones Chemical System: Surroundings: Open System The group of reactants & products being studied Surroundings: The environment in which the chemical reaction takes place; substance that is not part of the reaction Open System Matter & Energy able to leave system Closed System Energy able to leave system Isolated System Neither matter nor energy able to leave system
-qsurroundings = +qsystem Calorimetry Measuring energy changes in a reaction q = mcΔT We measure the ΔT of the surroundings and calculate the thermal energy, q, lost or gained by the surroundings -qsurroundings = +qsystem
q = mcΔT q = thermal energy (or heat) (Joules, J) m = mass of surroundings (grams, g) ΔT = Tfinal – Tinitial = change in temperature (oC or K) c = specific heat capacity; the energy (in Joules) needed to raise 1 gram of surroundings by 1 degree; (J/g oC ) or kJ/kg oC
Specific Heat Capacity,(c);
Definitions: new ones Enthalpy (H): another term for thermal energy Enthalpy Change (ΔH): the change in thermal energy from reactants to products; this equals q under constant pressure: ΔHsystem = qsystem ΔHsystem = Hproducts – Hreactants Where qsurroundings is the measurable quantity Endothermic: intake of energy, -qsurroundings & +qsystem & + ΔH Exothermic: release of energy, +qsurroundings & -qsystem & - ΔH
Molar Enthalpy Change ΔHn: Molar Enthalpy Change (ΔHn): enthalpy change associated with any change (physical, chemical or nuclear) in one mole of a substance; J/mol or kJ/mol ΔHrxn = nΔHn
Common Molar Enthalpies
Example: Ethanol is used to disinfect skin before receiving a flu shot. How does this feel on your skin? The enthalpy of vaporization (ΔHvap)of ethanol is 38.6 kJ/mol. What is the enthalpy change (ΔHrxn) of this reaction if 1.0g of ethanol is rubbed on your skin?
Expressing Molar Enthalpy Changes: As part of chemical equation: Associated ΔH value: Potential Energy Diagram:
3) Potential Energy Diagram: