Thermochemistry help: What is the difference between.... ? Please clarify and explain.
As the title says:
- What is the difference between enthalpy, molar enthalpy, and change in enthalpy?
- What are the variables that correspond to them (
#DeltaH, DeltaH_"r"# )?
- How is enthalpy different from thermal energy?
- What formulas can I use to solve for each variable?
Thank you in advance!
As the title says:
- What is the difference between enthalpy, molar enthalpy, and change in enthalpy?
- What are the variables that correspond to them (
#DeltaH, DeltaH_"r"# )? - How is enthalpy different from thermal energy?
- What formulas can I use to solve for each variable?
Thank you in advance!
1 Answer
DISCLAIMER: LONG ANSWER!
Generally only a change in a thermodynamic quantity has a physical meaning to us. The first two terms as absolute quantities relative to absolute zero are more conceptual, rather than something you'd have to calculate.
Enthalpy as a concept is the heat flow
#q# at constant pressure:
#DeltaH = q_P# where
#q_P# denotes constant pressure for heat flow#q# .Enthalpy always contains units of
#"J"# (joule), a unit of energy. The scenario this entails is fairly general. This becomes a good starting point for a derivation.Often, units of enthalpy are reported in
#"kJ/mol"# , so we also have molar enthalpy as a concept:
#DeltabarH = (DeltaH)/n = q_P/n# where
#n# is the mols of whatever object the enthalpy is referring to.This would be used in, e.g., latent heat of phase transition calculations, which are at constant temperature and pressure.
And you can see that all these equations refer only to the change in enthalpy,
#DeltaH# , not the absolute enthalpy,#H# .This is because it is a state function, so we only care about what starting state it was in and what it became, not what happened in between.
There is the relationship
#H = E + PV# where
#E# is the internal energy,#P# is the pressure, and#V# is the volume.But this equation is generally manipulated by considering changes in
#H# ,#E# ,#P# , and#V# . As it is, this equation is not physically useful.
As before,
#DeltaH# gives the change in enthalpy, which has the following properties:
- Since regular enthalpy is extensive, the larger the system, the greater the enthalpy.
- Enthalpy is due to heat flow at constant pressure.
- It can be related to molar enthalpy by division of the mols of the substance to which it refers. The change in molar enthalpy is intensive because the extensivity has been divided out through the mols.
Thus, the main variables that correspond to
#DeltaH# and#DeltaH_"rxn"# are#(i)# stoichiometric coefficients,#(ii)# heat flow#q# , and#(iii)# mols#n# .The latter can be calculated in two contexts:
- From average bond enthalpies
#DeltaH_"rxn"^@ = sum_(k=1)^(N) (H_f - H_i)_k#
#= barul|stackrel(" ")(" "sum_(i=1)^(N) DeltaH_("broken",i) - sum_(j=1)^(M) DeltaH_("made",j)" ")|# where each change in enthalpy on the right-hand side is for bonds broken or bonds made. Breaking bonds is always endothermic, and making bonds is always exothermic.
The drawback with this is that it is more approximate than the next method, as it uses average values without considering what different atoms are around.
- From enthalpies of formation
#DeltaH_"rxn"^@ = sum_(k=1)^(N) (H_f - H_i)_k#
#= barul|stackrel(" ")(" "sum_P nu_P DeltaH_(f,P)^@ - sum_R nu_R DeltaH_(f,R)^@" ")|#
#nu# is a stoichiometric coefficient from the balanced chemical reaction.#R# and#P# denote each reactant or product, respectively. So, this is basically a subtraction of two separate sums.Other quantities related are the change in entropy
#DeltaS# and Gibbs' free energy#DeltaG# , as well as temperature#T# (these likewise can be calculated from a similar equation to the one right above):
#barul|stackrel(" ")(" "DeltaG = DeltaH - TDeltaS" ")|# And this can be written for
#DeltaH_"rxn"^@# as well at#25^@ "C"# and#"1 atm"# :
#DeltaG_"rxn"^@ = DeltaH_"rxn"^@ - TDeltaS_"rxn"^@# General Chemistry textbooks should have standard quantities, denoted
#DeltaG_f^@# ,#S^@# , and#DeltaH_f^@# , tabulated in the appendix.
Enthalpy is specified as the change in state of a system due to the flow of thermal energy from a system into its surroundings and vice versa, at constant pressure.
At nonconstant pressure, heat flow
#q# is then the flow of thermal energy.And it becomes important to define your system and your surroundings to properly identify the sign of your change. and thus the direction of the flow of thermal energy.
Formulas are given above, but you should be able to start from concepts to derive them, except for
#H = E + PV# and#DeltaG = DeltaH - TDeltaS# .
