Thermochemistry

Question 1

What does thermochemistry study

Answer 1

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

Question 2

Is it possible to define any state function as molar state function

Answer 2

Yes. Ex: molar volume, molar internal energy(Um=U/n)

Question 3

When is it satisfied that ΔU=q when w=0

Answer 3

a)transformation occurs at constant volume
b)the only type of work is volume work

Question 4

What is enthalpy

Answer 4

It describes thermal effects in transformation at constant pressure

Question 5

What isnformula for enthalpy

Answer 5

H=U+pV

Question 6

Is enthalpy a state function and if so what kind?

Answer 6

It is. It’s extensive property

Question 7

What is fundamental property of enthalpy in transformation?

Answer 7

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

Question 8

What is molar enthalpy

Answer 8

Hm=enthalpy of 1 mol of pure substance.

Question 9

What kind of property is molar enthalpy

Answer 9

Intensive one

Question 10

What is formula for molar enthalpy

Answer 10

Hm(T,p)=H(T,p,n)/n

Question 11

Does enthalpy of ideal gasses depend on pressure

Answer 11

No

Question 12

Explain dependance of enthalpy for liquids and solids on pressure

Answer 12

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

Question 13

What is specific molar heat

Answer 13

Cp=coefficient that determines temperature increase for a pure substance

Question 14

What is formula for specific molar heat

Answer 14

Cp=(dHm/dT)p=const.

Question 15

What is unit of measure of Cp

Answer 15

Joule/(K*mol)

Question 16

What are properties of specific molar heat

Answer 16

1. Its function of p and T
2. For ideal gases it doesnt depend on p
3. Depends weakly from pressure in liquids and solids
4. Temperature dependance only manifests itself only for large temp.changes
5. It strongly depends on the phase and the type of substance

Question 17

What does cp allow us to predict

Answer 17

Change in temperature of pure substance caused by exchange of certain quantity of heat(q)(p=const.and only work is volume work)

Question 18

What is formula for dHm/ΔHm if cp remains constant over the temo.range

Answer 18

dHm=c(p)dT
ΔHm=(Hm2-Hm1)=c(p)ΔT
ΔT=T2-T1

Question 19

What is formula for q when c(p) is constant

Answer 19

q=nΔHm=nc(p)*ΔT

Question 20

What is latent heat

Answer 20

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

Question 21

What is proces of melting ice(solid->liquid)

Answer 21

Ice fusion

Question 22

What js formula for Δ(fus)H

Answer 22

Δ(fus)H=Hm(H2O(l),Tfus,p)-Hm(H2O(s),Tfus,p)

Question 23

Do Tfus and Δ(fus)H depend on p

Answer 23

Yes

Question 24

What behavior does function of molar enthalpy display in phase transitions

Answer 24

Discontinuous

Question 25

What is standard state of chemical compound

Answer 25

Pure substance at the standard pressure of 1bar p⊝=1bar

Question 26

What are standard thermodynamic quantities for a compound J

Answer 26

Value assumed in the standard state by 1 mol of the compound Ex. Vm[H2O(l),25°C]=18.06*10^(-6)m³

Question 27

What do standard state of chemical compound and standard thermodynamic quantities depend on

Answer 27

Temperature, type of compound J and the phase

Question 28

What is standard enthalpy of a reaction

Answer 28

ΔrH⊝(enthalpy difference) between reagents and products

Question 29

What conditions have to be satisfied for standard enthalpy of a reaction

Answer 29

1. Moles of the reagents are equal to the stoichiometric coefficients in the reaction 2. Reaction goes to completion 3. Reagents and products are in the standard state at the same temperature

Question 30

What if formula for standrd enthalpy of a reaction

Answer 30

ΔrH⊝=Σv(J)Hm⊝(J,T)

Question 31

What is v(J)

Answer 31

Stoichiometric coefficients of products that are taken with + sign for products, and - sign for reagents

Question 32

What are properties of standard enthalpy of a reaction

Answer 32

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

Question 33

What is Hess law

Answer 33

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

Question 34

What is standard enthalpy of formation

Answer 34

•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)

Question 35

Considering Hess law and enthakpy of formation, what is the new formula?

Answer 35

ΔrH⊝=Σv(J)Hm⊝(J)=Σv(J)ΔfH⊝(J)

Question 36

What is formation enthalpy for pure elements

Answer 36

0

Question 37

What is exothermic process

Answer 37

**Exothermic process**=when system looses energy; ΔrH⊝<0(reaction heat is negative)

Question 38

What is endothermic process

Answer 38

**Endothermic process**=system absorbs heat; ΔrH⊝>0(reaction heat is +)

Question 39

If temperature dependence can be neglected, what is ΔrH⊝(T-T0)

Answer 39

ΔrCp⊝(T-T0)