CHAPTER 13 - THERMOCHEMISTRY
Systems:
- Isolated: no exchange energy/matter from system with surrounding
- Closed: fixed amount of matter, exchange energy possible
- Open: both matter and energy can be exchanged with surrounding
Heat capacity(C) is the heat needed to raise the temperature of a substance by 1K.
Specific heat capacity Cs is for 1 gram substance.
q
Cs=
m∗dT
Molar heat capacity is of 1 mol:
q
C m=
n∗dT
Value of molar heat capacity depends for gases on if the experiment is carried out under
constant pressure or volume Cp/Cv
Heat capacities are intensive properties of substances: don’t depend on quantity
substances. (extensive properties do depend).
Enthalpy change: dH is heat transferred at constant pressure by chemical
reaction/process. It’s a state function: only depends on current state system, not how it
got there. dX=Xfinal-Xinitial.
Standard enthalpy change of fusion: dfusH: energy to melt 1 mol substance at
melting point at 1 bar. dvapH at boiling point. dfusH<dvapH.
Sublimation: solid goes directly to gas: dsubH=dfusH+dvapH
Hess’s law: the total enthalpy change for a chemical
reaction is independent of the path by which the reaction
occurs, provided the starting and finishing states are the
same for each reaction path.
Standard enthalpy change of formation: 1 mol compound
formed under standard conditions from elements in standard
states. Enthalpy of formation for element in standard
state=0. Standard state= most stable form
elements under standard conditions.
, Vi is stoichiometric coefficient: number of moles of reactant of product involved in
balanced thermochemical equation.
Standard enthalpy change of combustion dcH: heat released when substance burns at
constant pressure a.k.a. the heat of combustion. Variation of dcH: energy desity or
enthalpy change per unit mass of a compound in units kJ(or MJ)/g = enthalpy change on
burning 1 (k)g of compound.
Standard enthalpy change of a solution dsolH: when 1 mol substance dissolves in
large excess pure solvent at 1 bar, a.k.a. integral enthalpy change of solution.
Bond dissociation enthalpy D(A-B): enthalpy change per mol when particular
chemical bond, A-B, is broken under standard conditions in the gas phase.
Different quantities of energy needed to break
each bond because of the different electron
densities in the particular C-H bonds. The
mean bond enthalpy is the average of the
bond dissociation enthalpies.
These mean bond enthalpies are
approximated values calculated from a large
number of compounds.
Enthalpies of substances increase when the temperature increases. Enthalpy change =
heat transferred at constant pressure. If qp is energy used to heat substance in open
container from T1 to T2, enthalpy change is:
dH T 1→ T 2=H T 2−H T 1 =q p
P stands for constant pressure. dH is an enthalpy change of a substance when
temperature is raised form initial to final temperature, not an enthalpy change of a
reaction (like before).
qp
Cm , p= , so qp=Cp∗n∗dT =dH T 1 →T 2=H T 2 −H T 1, so for 1 mol:
n∗dT
qp=Cp∗dT =dH T 1 → T 2=H T 2−H T 1, so H T 2 =H T 1 +Cp∗dT
Systems:
- Isolated: no exchange energy/matter from system with surrounding
- Closed: fixed amount of matter, exchange energy possible
- Open: both matter and energy can be exchanged with surrounding
Heat capacity(C) is the heat needed to raise the temperature of a substance by 1K.
Specific heat capacity Cs is for 1 gram substance.
q
Cs=
m∗dT
Molar heat capacity is of 1 mol:
q
C m=
n∗dT
Value of molar heat capacity depends for gases on if the experiment is carried out under
constant pressure or volume Cp/Cv
Heat capacities are intensive properties of substances: don’t depend on quantity
substances. (extensive properties do depend).
Enthalpy change: dH is heat transferred at constant pressure by chemical
reaction/process. It’s a state function: only depends on current state system, not how it
got there. dX=Xfinal-Xinitial.
Standard enthalpy change of fusion: dfusH: energy to melt 1 mol substance at
melting point at 1 bar. dvapH at boiling point. dfusH<dvapH.
Sublimation: solid goes directly to gas: dsubH=dfusH+dvapH
Hess’s law: the total enthalpy change for a chemical
reaction is independent of the path by which the reaction
occurs, provided the starting and finishing states are the
same for each reaction path.
Standard enthalpy change of formation: 1 mol compound
formed under standard conditions from elements in standard
states. Enthalpy of formation for element in standard
state=0. Standard state= most stable form
elements under standard conditions.
, Vi is stoichiometric coefficient: number of moles of reactant of product involved in
balanced thermochemical equation.
Standard enthalpy change of combustion dcH: heat released when substance burns at
constant pressure a.k.a. the heat of combustion. Variation of dcH: energy desity or
enthalpy change per unit mass of a compound in units kJ(or MJ)/g = enthalpy change on
burning 1 (k)g of compound.
Standard enthalpy change of a solution dsolH: when 1 mol substance dissolves in
large excess pure solvent at 1 bar, a.k.a. integral enthalpy change of solution.
Bond dissociation enthalpy D(A-B): enthalpy change per mol when particular
chemical bond, A-B, is broken under standard conditions in the gas phase.
Different quantities of energy needed to break
each bond because of the different electron
densities in the particular C-H bonds. The
mean bond enthalpy is the average of the
bond dissociation enthalpies.
These mean bond enthalpies are
approximated values calculated from a large
number of compounds.
Enthalpies of substances increase when the temperature increases. Enthalpy change =
heat transferred at constant pressure. If qp is energy used to heat substance in open
container from T1 to T2, enthalpy change is:
dH T 1→ T 2=H T 2−H T 1 =q p
P stands for constant pressure. dH is an enthalpy change of a substance when
temperature is raised form initial to final temperature, not an enthalpy change of a
reaction (like before).
qp
Cm , p= , so qp=Cp∗n∗dT =dH T 1 →T 2=H T 2 −H T 1, so for 1 mol:
n∗dT
qp=Cp∗dT =dH T 1 → T 2=H T 2−H T 1, so H T 2 =H T 1 +Cp∗dT
