4.3 Enthalpy and Hess's law Flashcards
enthalpy, hess's law, dissolution, dissociation, bond energy, latent heat, standard states
2 things Hess’s law states
(1) Overall enthalpy change is independent of the pathway between the initial and final states
(2) the sum of the enthalpy changes around a complete circle is zero
what is enthalpy
The total heat content of system, indicating the stored chemical energy in a system at a given pressure and temperature
heat capacity
The ability to store heat (J g^-1 K^-1)
enthalpy change
ΔH = enthalpy of products (energy needed to break bonds) - enthalpy of reactants
dissociation
physical process, separation of existing ions
dissolution
physical process, a solid or a liquid breaks up to form a homogenous mixture (solution)
standard enthalpy of formation
When one mole of a compound in its standard state is formed from IRS constituent elements in their standard states.
breaking bonds:
Requires energy, therefore is endothermic
forming bonds:
Creates energy, therefore is exothermic
heat of solution equation
∆Hsoln = -mCΔT
what is latent heat
heat or energy that is absorbed or released during a phase change of a substance (without changing temperature)
latent heat of evaporation
enthalpy of vaporisation aka ∆Hvap
- at 1atm
- from 1 mol liquid to gas
latent heat of fusion
enthalpy of fusion aka ∆Hfus
- at 1atm
- from 1 mol solid to liquid
what is enthalpy
total heat content of system;
sum of the kinetic and potential energies of 1 mole of all particles
what is bond energy
amount of energy required to break 1 mol of a bond into its constituent gaseous atoms under standard conditions (25C, 1 bar)
using bond energy to calculate enthalpy
∆H = Σ(reactant bonds) - Σ(product bonds
standard state of solid and liquid
100 kPa pressure
standard state of gas
100 kPa pressure (under ideal gas law)
standard state of solutions
1M concentration
standard temperature
298.15K or 25C
standard enthalpy of formation
ΔfHθ or ∆Hot
enthalpy change when one mole of a compound in its standard state is formed from its constituent elements in their standard states
standard enthalpy of combustion
ΔcHθ
enthalpy change when one mole of a substance combusts completely with oxygen
using standard enthalpy of formation to calculate enthalpy
∆Hot = Σ∆Hot(products) - Σ∆Hot(reactants)
standard enthalpy of formation for DIATOMIC MOLECULES is…
0 kJ/mol
