Thermochemistry

Thermochemistry

• Definition: The study of energy changes associated with chemical reactions or physical transformations

  • Ex: phase changes, dissolving

• focuses on a system’s exchange of energy within its surroundings (releasing energy into surroundings> surroundings absorb reaction’s energy)

• Endothermic Process - a system that absorbs energy from its surroundings (heat is on the reaction side, 🔼H is positive) (low to high)

• Exothermic Process - a system that releases energy to its surroundings (heat is on the product side, 🔼H is negative) (high to low)

• Heat - energy that’s transferred due to a difference in temperature

  • always occurs from warmer (high kinetic energy) to cooler (low kinetic energy)

    • results in both objects reaching the same temperature, known as thermal equilibrium

• Thermochemical equations - any equation with heat in it

• Enthalpy (H)

  • a form of chemical energy (potential energy)

  • referred to as “heat contact.”

  • can’t be measured directly —> but can measure how it changes during a reaction

  • 🔼H = Hf - Hi (final - initial)

Heat Capacity & Substance

• Definition: a measure of how well the substance stores/transfers energy

• Heat Capacity (uppercase C) - amount of heat absorbed or released / change in temperature

• Specific heat capacity (“specific heat”) (lowercase c) - amount of heat required to raise the temperature of 1 gram of a pure substance by 1 °C (or 1 Kelvin)

  • specific heat of water: 1 cal/g•°C or 4.184 J/g•°C

  • q = mc🔼T (c is always unique to the substance)

• 1 mL = 1 gram

Bomb Calorimeter

• Definition: often used to measure 🔼H of combustion reactions (🔼Hc)

• The reaction chamber is surrounded by an H2O “jacket” and then an insulation layer with steel boxes separating each layer, a thermometer, and stir sitting in the H2O jacket

• -q(reaction) = +q(calorimeter) —> exothermic, calorimeter absorbs the heat released by the reaction —> temperature goes up in calorimeter

• +q(reaction) = -q(calorimeter) —> endothermic, calorimeter releases heat absorbed by the reaction —> temperature goes down in calorimeter

• The calorimeter is constructed with different substances, each with a different specific heat capacity

  • q(whole calorimeter) = mc🔼T (steel) + mc🔼T(H2O) + mc🔼T(insulation) + …

    • q(cal) = 🔼T(mc(steel) + mc(H2O) + mc(insulation))

      • q(cal) = C🔼T (C —> uppercase, heat capacity of whole calorimeter, units: J/°C or kJ/°C)

      • multiple trials are run with the same calorimeter to determine “C” (heat capacity)

Phase Change

endothermic (adding heat, KE) —————————————>

      melting (fusion)      boiling (vaporization)

solid ————-————→ liquid ——-———————-→ gas

solid ←——————--—-- liquid <——————————-gas

        freezing (solidification)         condensation

                                    sublimation

solid ————————————————————-——>gas

solid <—————--——————————————-——-gas

                                    deposition

←————————————-exothermic (taking out heat, KE)

• H(f) = heat of fusion

  • amount of energy required to melt 1 gram of a solid at its melting point

• H(v) = heat of vaporization

  • amount of energy required to vaporize 1 gram of liquid at its boiling point

• KE is when temperature is being raised, heat is added, energy increases

• PE is when there is no change in temperature'

• On the heating graph, you need melting point (solid —> liquid) and a boiling point (liquid —> gas)

• Equations:

  • q = mc🔼T (whenever temp changes, c depends on whether it is solid/gas or liquid)

  • q = mH(f) (no temp change)

  • q = mc🔼T

  • q = mH(v) (no temp change)

  • q = mc🔼T