Thermochemistry Flashcards
What does thermochemistry study
Thermal effects connected to physical and chemical tranformations. It deals with:
•cooling/healing of pure substance(gas,liquid,solid)
•phase transitions of pure substances
•chemical reactions
Is it possible to define any state function as molar state function
Yes. Ex: molar volume, molar internal energy(Um=U/n)
When is it satisfied that ΔU=q when w=0
a)transformation occurs at constant volume
b)the only type of work is volume work
What is enthalpy
It describes thermal effects in transformation at constant pressure
What isnformula for enthalpy
H=U+pV
Is enthalpy a state function and if so what kind?
It is. It’s extensive property
What is fundamental property of enthalpy in transformation?
a)transf. is under const pressure(pext=p1=p2=const.)
b)all the work done is volume work
With all of this combined we come to the equation that
ΔH=q
What is molar enthalpy
Hm=enthalpy of 1 mol of pure substance.
What kind of property is molar enthalpy
Intensive one
What is formula for molar enthalpy
Hm(T,p)=H(T,p,n)/n
Does enthalpy of ideal gasses depend on pressure
No
Explain dependance of enthalpy for liquids and solids on pressure
It’s weal and can be neglected when working at 1atm.
There are contributions from intermolecular forces but because of low compressability, they depend very weakly on pressure
What is specific molar heat
Cp=coefficient that determines temperature increase for a pure substance
What is formula for specific molar heat
Cp=(dHm/dT)p=const.
What is unit of measure of Cp
Joule/(K*mol)
What are properties of specific molar heat
- Its function of p and T
- For ideal gases it doesnt depend on p
- Depends weakly from pressure in liquids and solids
- Temperature dependance only manifests itself only for large temp.changes
- It strongly depends on the phase and the type of substance
What does cp allow us to predict
Change in temperature of pure substance caused by exchange of certain quantity of heat(q)(p=const.and only work is volume work)
What is formula for dHm/ΔHm if cp remains constant over the temo.range
dHm=c(p)dT
ΔHm=(Hm2-Hm1)=c(p)ΔT
ΔT=T2-T1
What is formula for q when c(p) is constant
q=nΔHm=nc(p)*ΔT
What is latent heat
Heat absorbed by 1 mole of substance undergoing a phase transition at well defined values of (p,T) when the only form of work is volume work
What is proces of melting ice(solid->liquid)
Ice fusion
What js formula for Δ(fus)H
Δ(fus)H=Hm(H2O(l),Tfus,p)-Hm(H2O(s),Tfus,p)
Do Tfus and Δ(fus)H depend on p
Yes
What behavior does function of molar enthalpy display in phase transitions
Discontinuous
What is standard state of chemical compound
Pure substance at the standard pressure of 1bar p⊝=1bar
What are standard thermodynamic quantities for a compound J
Value assumed in the standard state by 1 mol of the compound
Ex. Vm[H2O(l),25°C]=18.06*10^(-6)m³
What do standard state of chemical compound and standard thermodynamic quantities depend on
Temperature, type of compound J and the phase
What is standard enthalpy of a reaction
ΔrH⊝(enthalpy difference) between reagents and products
What conditions have to be satisfied for standard enthalpy of a reaction
- Moles of the reagents are equal to the stoichiometric coefficients in the reaction
- Reaction goes to completion
- Reagents and products are in the standard state at the same temperature
What if formula for standrd enthalpy of a reaction
ΔrH⊝=Σv(J)Hm⊝(J,T)
What is v(J)
Stoichiometric coefficients of products that are taken with + sign for products, and - sign for reagents
What are properties of standard enthalpy of a reaction
a)it’s a function of T only
b)it has a meaning only if chemical reaction is specified: stoichiometric coefficient and phase of reagents and products
c)(in contrast to the enthalpy of the substance) it can be quantified through measurments of the heat of the reaction
What is Hess law
ΔrH(I)⊝+ΔrH(II)⊝=ΔrH⊝
•standard enthalpy of a reaction is the sum of standard enthalpies of the reaction into which overall reaction is devided
What is standard enthalpy of formation
•ΔfH⊝=equal to ΔrH⊝ for the reaction in which 1mol of pure substance is formed starting from chemical elements composing it in their reference state(stable form at p⊝ and T)
Considering Hess law and enthakpy of formation, what is the new formula?
ΔrH⊝=Σv(J)Hm⊝(J)=Σv(J)ΔfH⊝(J)
What is formation enthalpy for pure elements
0
What is exothermic process
Exothermic process=when system looses energy; ΔrH⊝<0(reaction heat is negative)
What is endothermic process
Endothermic process=system absorbs heat; ΔrH⊝>0(reaction heat is +)
If temperature dependence can be neglected, what is ΔrH⊝(T-T0)
ΔrCp⊝(T-T0)
