Changes in Energy
For Chemical and Physical Changes
Heat vs Thermal Energy
Temperature Scales and Conversions
°C = 5/9(°F-32) or °F = 9/5°C +32
Thermal Energy and Temperature
Thermochemistry-the balance between KE and PE
There are two basic forms of energy: kinetic energy (energy of motion) and potential energy (energy of position). Whenever energy is transformed (e.g., electrical from chemical (fuel)); you are turning potential into kinetic or vice versa. Potential energy is locked up in chemical bonds and can be released when those bonds are broken. When a molecule forms, potential energy drops (think ball on an incline). This is really the only reason molecules do form-the system rests at a lower energy. When chemical reactions occur, energy is put into breaking the bonds (ball has to be pushed back up the hill). Once the bonds break, the ball rolls down the hill again. There is a balance between this constant switching from potential to kinetic and that is what thermochemistry is all about.
Heats of reactions or enthalpies can also be calculated from other kinds of data (besides calorimetry). When reactants react to form products, bonds are broken and then reformed into new arrangements. It takes energy to break bonds (endothermic) and energy is released when bonds form (exothermic). This endothermic and exothermic business occurs in every chemical reaction. The balance between the two dictates whether a chemical reaction is overall exothermic (bond forming step is bigger) or endothermic (bond breaking step is bigger). If a reaction is endothermic, then enthalpy can be thought of as entering the reaction on the reactant side and if a reaction is exothermic, heat can be thought of as exiting the reaction on the product side.
C (s) + O2 (g) → CO2 (g) + 393.5 kJ
H2O (l) + 285.8 kJ → H2 (g) + 1/2 O2 (g)
Which is endothermic above? Exothermic?
Take the enthalpy out of the equations above and it could be written separately from the reaction. If it is, the sign of the enthalpy value tells you whether the reaction is endothermic or exothermic. If ∆Hrxn is negative, the reaction is exothermic. If ∆Hrxn is positive, the reaction is endothermic.