Chem 1B Ch. 9 - Thermochemistry

conservation of energy

energy lost by the system = energy gained by its surroundings (ΔE system = -ΔE surroundings)

chemical energy

• when ΔE_system is negative the system loses energy and heat/work to the surroundings

• when ΔE_system is positive the system gains energy as heat/work from the surroundings

work & energy

• work performed by or on the system is energy transfer that results in macroscopic changes in the system

• w = -PΔV

• ΔE_system = q + w

quantifying heat

• (heat) q = m x c x ΔT

convention

• if energy flows into the system, it has a positive sign because the system’s energy is increasing

• if energy leaves the system, it has a negative sign because the system’s energy is decreasing

constant-volume process

• ΔV = 0

• change in internal energy is equal to the heat exchanged

constant-pressure process

• ΔH = ΔE + PΔV

• under constant pressure, the heat exchanged is defined as the enthalpy (H)

• enthalpy = most useful state function for constant pressure processes

enthalpy

• q_p = heat flow at constant pressure

• ΔE = q_p + w

• w = -PΔV

• therefore: ΔE = q_p - PΔV = ΔH - PΔV

• ΔH = ΔE + PΔV

enthalpy change of a reaction (1)

• if sign of ΔH is positive: reaction is endothermic (surroundings will cool in the reaction)

• if the sing of ΔH is negative: reaction is exothermic (the surroundings will warm up)

enthalpy change of a reaction (2)

• ΔH = ∑ (ΔH bonds broken) + ∑ (ΔH bonds formed)

• ΔH is always positive for breaking bonds and negative for forming them

• endothermic reactions = positive ΔH → strong bonds break & weak bonds form

• exothermic reactions = negative ΔH → weak bonds break & strong bonds form

conditions of constant volume and constant pressure

• under constant pressure conditions some of the energy released is to do work on the surroundings by expanding against it → less energy is manifested as heat

• under constant volume conditions all of the energy released is evolved as heat

• ex. if the same mass of a fuel is combusted under the 2 conditions, the reaction that produces a smaller heat (kJ) corresponds to constant pressure & a larger heat corresponds to constant volume

specific heat capacity

• the quantity of heat needed to raise the temperature of 1g of a substance by 1°C

• when substances are equal in mass…

  • objects with a low specific heat capacity need to absorb less heat energy to increase in temperature

  • objects with a high specific heat capacity need to absorb more heat energy to increase in temperature