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CHEMISTRY - Thermochemistry > Enthalpy > Flashcards

Enthalpy Flashcards

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1
Q

What are the three types of Molecular Motion & Explain how they operate?

A 
1. Translational Molecular Motion
Molecule moves from one place to 
another
2. Rotational Molecular Motion
Molecule spin around its center of mass
3. Vibrational Molecular Motion

Atoms within the molecule move back &
forth very slightly. Solid phase has mainly vibrational. Liquid & Gas have all three types (in gas, can overcome
attractive forces)

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2
Q

What is the First Law of Thermodynamics?

A

Thermal NRG can be transferred (called heat)
Objects cannot possess heat, it flows.

NRG released = NRG absorbed
heat lost = heat gained

When NRG is transferred for 1 object to another, there is a
change in thermal energies therefore a change in temp, in both
objects.

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3
Q

What does Temp change of a substance depend on ?

A

Temp change (ΔT) of a substance depends on:

  1. amt. of heat (q ) transferred into and out of a system
  2. the mass of the substance (m)
  3. heat capacity of substances (c)
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4
Q

What is the second Law of Thermodynamics?

A

NRG can only be converted from one form to

another.

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5
Q

What are the 2 types of energy changes?

A
  1. Endothermic (endergonic)
    For an endothermic reaction (chemical system absorbs heat),ΔH (the enthalpy change) is positive.
  2. Exothermic (exergonic)
    For an exothermic reaction (chemical system releases heat), ΔH (the enthalpy change) is negative.
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6
Q

What are the two types of heat capacities?

A

a. specific heat capacity (S.H.C) J/g°C or
kJ/kg°C
b. volumetric heat capacity J/L°C (if given
a volume)
REM. 1 g = 1 mL (Only for Water!)

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7
Q

Define thermochemistry & calorimetry

A

The study of energy changes by a chemical system during a chemical reaction is called thermochemistry.

Calorimetry is the technological process
of measuring energy changes of an
isolated system called a calorimeter. Recall that an isolated system does not exchange matter or energy with its outside
environment. No calorimeter is 100% sealed and insulated,
so they only approximate an isolated system.

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8
Q

Define Heat, Thermal energy and identify the different symbols within a thermal energy formula.

A

Heat refers to the form of energy that is transferred
from an object at a higher temperature to an object at
a lower temperature.

Thermal energy is the total kinetic energy of the
entities of a substance.

Q = mcΔt
Q = quantity of thermal energy (J)
m = mass (g)
c = specific heat capacity (J/g·°C) 
Δt = temperature change (°C) 
The S.I. unit for energy is the joule (J).
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9
Q

Define S.H.C and the values of different substances in S.H.C form

A

The specific heat capacity of a
substance is the quantity of energy
required to raise one gram of a
substance by one degree Celcius.

The change in temperature of the water is
used to determine the quantity of heat energy
released or absorbed by the chemical system.

Substance     Value in S.H.C
Silver - 0.235
Copper - 0.385
Iron - 0.449
Aluminum - 0.903
Water - 4.19
Ethanol - 2.42
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10
Q

What are the different systems that Enthalpy changes can occur?

A
  1. open system : NRG and matter can flow in & out

2. closed system : NRG flows in & out, but not matter total NRG of a system is sum of all Ek & Ep

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11
Q

Define Enthalpy and Enthalpy change

A

The enthalpy (H) of a system is the sum of the kinetic and potential energy within it.

WE CANNOT MEASURE ENTHALPY DIRECTLY!

We can calculate the quantity of heat that is released or absorbed by the surroundings of a chemical system by measuring a change in temperature of the surroundings.

ΔH = Hproducts – Hreactants

An enthalpy change, ΔH, is the difference between the enthalpy of the products and the enthalpy of the reactants for a system under constant pressure.

ΔH = Q (system) (calorimeter)

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12
Q

Define potential energy and draw hydrogen bonding and covalent bonding

A 
Potential energy (stored energy in chemical bonds) includes: 
•covalent and/or ionic bonds between the entities (intramolecular)
•intermolecular forces between entities

The change in potential energy of the chemical system equals the change in kinetic energy of the surroundings.

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13
Q

Define Enthalpy of reaction & Indicate meaning of symbols within formula and Molar enthalpy of reaction

A

Enthalpy of reaction (or enthalpy change of reaction) refers to the energy change for a whole chemical system when reactants change to products.

Δr H = n Δr Hm
Δr H = enthalpy of reaction (kJ)
n = chemical amount (mol)
Δr Hm = molar enthalpy of reaction (kJ/mol)

Molar enthalpy of reaction is the enthalpy change in a chemical
system per mole of a specific chemical in a system at constant pressure.

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14
Q

What are the different types of changes that are undergone?

A

Phase Changes
- Involve relatively little NRG

Chemical Changes
- Usually require more NRG enthalpy changes are greater
due to NRG in covalent & ionic bonds (Ep)
- Can be endo- or exo- thermic
- If NRG released from system is exothermic .’. (-) potential NRG decreases
- If NRG absorbed from system is endothermic .’.(+) potential NRG increases

Nuclear Changes

  • Involve huge amts of NRG due to changes in nuclear (Ep)
  • Changes in subatomic structure of atom (E=mc2)
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15
Q

What are the associated Energy vocabulary for endothermic and exothermic energy changes.

A 
Exothermic        Endothermic
condensation    boiling
cooling               heating
solidifying          melting
sublimation (solid)   sublimation (gas)

fusion = NRG req’d to melt
same # as solidification

vaporization   = NRG req'd to vaporize
# same as condensing
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16
Q

Define Molar Enthalpy and formuals for molar enthalpy of vaporization and fusion

A

Molar Enthalpy (H) kJ
mol
amt of NRG req’d to change state of one mole of a specific
pure substance
H vap = molar enthalpy of vaporization
ΔHvap = nHm vap

H fus  =  molar enthalpy of fusion
ΔHfus  =   nHm fus
ΔH        =  Enthalpy change  (kJ) 
 n          =   # moles 
ΔHm vap   = heat of vaporization (kJ/mol) 
ΔHm fus    =  heat of fusion (kJ/mol)
17
Q

Define Enthalpy Constants & Changes.

A 
Enthalpy changes are always reported per unit quantity of 
substance. Either as:
molar enthalpy (J/mol)
              or
specific enthalpy (J/g)

molar enthalpy constants may represent endothermic or exothermic changes.
Endothermic enthalpy changes positive
Exothermic enthalpy changes negative

Eg.) Ice melts, system gains NRG from surroundings
Hmelting = +6.03 kJ/mol

and because of law of conservation of NRG…

Hfreezing = -6.03 kJ/mol
(water lost heat to freeze)

18
Q

What is the pattern of potential NRG Diagrams of phase changes

A

Melting 1mol ice

tbu…

19
Q

Two types of Calorimeter measurments

A
  1. Bomb Calorimeter (High Tech)
Consists of : Insulated Container
Thermometer
Bomb
Water
Cup holding sample
  1. Foam Cup Calorimeter (Low Tech)

Consists of : Stirrer
Styrofoam cups

If the system is exothermic (the water in the calorimeter absorbs heat)
Hsys is negative

If the system is endothermic (the water in the calorimeter loses heat)
Hsys is positive

20
Q

Heat Capacity using Bomb Calorimeters

A

Used for combustion i.e.) to figure out calories or joules of food used to measure enthalpies of foods, fuels, explosives, etc …

21
Q

What are the 4 methods to express Energy changes?

A
  1. Mothar Enthalpies of Reaction
  2. Enthalpy Changes (deltaH)
  3. Energy Terms in Balanced Equations
  4. Potential Energy Diagrams
22
Q

Method 1: Molar Enthalpies of Reaction, ΔrHm

A

When reactants and products are in their
standard state, they are at a pressure of
100 kPa, an aqueous concentration of
1.0 mol/L. and liquids and solids are in
their pure state.

23
Q

Method 1: Molar Enthalpies of Reaction, ΔrHm 2

A

molar enthalpies of rxn are the amt. of NRG change when one mole of a substance reacts at constant pressure.
This is expressed as Hf (formation) or Hc (combustion)
to express the reaction clearly the following must be shown:
a. the chemical formula
b. the sign + or -
c. the units kJ
mol
d. the # value

24
Q

What are the 2 types of Molar Enthalpy Reaction?

A

Formation Reaction
Δf Hm° = –239.2 kJ/mol
CH3OH

C(s) + 2 H (g) + O (g) CH OH(l)

When 1 mol of methanol is formed from its elements when they are in their standard states at SATP, 239.2 kJ of energy is released

Combustion Reaction Δc Hm° = –725.9 kJ/mol
CH3OH

The complete combustion of 1 mol of methanol releases 725.9 kJ of energy.

Note that the above reactions are balanced for one mole of the compound

25
Q

What are the different formulas for temperature and phase change?

A

Most times situations involving transfers of NRG systems can undergo a series of phase & temperature changes

If temp. change  q = mcΔt or q = vcΔt
If phase change  ΔHvap / fus = nHvap / fus

To get total (ΔEtotal) add all eq’ns together Eg

26
Q

Define Hess Law and it’s formula

A

some reactions cannot be measured directly using simple calorimetry the temp, change is independent of time and route in which the temp, changed. If several reactions occur in diff. directions but the initial reactants and final products are the same, the net enthalpy change is the same.

Hess’ Law (Predicting ΔH r)
This law helps us to determine the ΔH r for rxns that are impossible to measure. If standard enthalpy changes ( ΔH°r ) are not available for a particular rxn, then it is possible to add separate rxns, and thus, the enthalpy changes for each rxn to obtain the total ΔH°f for the rxn.

ΔH net = ΔH 1 + ΔH 2 + ΔH 3 + …

ΔH net = ΣΔH r

27
Q

Hess’ discovery works if the following 2 rules are applied. What are the 2 rules?

A
  1. If a chem. rxn is reversed, then the sign of ΔHr is also reversed.
  2. If the coefficient of a chem. eq’n are changed multiplying or
    dividing by a constant factor, then the ΔHr, is altered in the same
    way.
    eg): C (diamond)  C (graphite)

because the rxn is too slow (it may take 1000s years ) can’t take diamond in a constant-pressure calorimeter and measure the heat change.
Must use an indirect approach (Hess’ Law)

28
Q

Hess’ law components and examples

A

We know that enthalpy changes for following two rxns can be
measured:
(a) C (diamond) + O2 (g)  CO2 (g) ΔH°r = -395.4 kJ
(b) C (graphite) + O2 (g)  CO2 (g) ΔH°r = -393.5 kJ
Reversing eq’n (b) get…
CO2 (g)  C (graphite) + O2 (g) ΔH°r = +393.5 kJ
Next add eq’n (a) & (c) to get desired eq’n
C (diamond) + O2 (g)  CO2 (g) ΔH°r = -395.4 kJ
(c) CO2 (g)  C (graphite) + O2 (g) ΔH°r = +393.5 kJ
____________________________________________________

29
Q

Hess’ Law components and examples continued…

A

C (diamond)  C (graphite) ΔH r = - 1.9 kJ
thus: (for formation)
ΔH°f (C,Diamond) = - ΔH°r/moI = -1.9 kJ
mol
Problem:
What is the heat of hydrogenation of acetylene at SATP?
1C2H2 (g) + 2H2 (g)  1C2H6 (g) ΔH°f= ?

The thermochemical eq’n for the combustion of acetylene is:
1. 2C2H2 (g) + 5O2 (g)  4CO2 (g) + 2H2O (l) ΔH1 = -2602 kJ
Ethane, another hydrocarbon fuel, reacts as follows:
2. 2C2H6 (g) + 7O2 (g)  4CO2 (g) + 6H2O (1) ΔH2 = -3123 kJ
hydrogen & oxygen combine to form:
3. 1H2 (g) + ½ O2 (g)  1H2O (1) ΔH3 = -286 kJ

30
Q

Hess’ Law example 2

A

SOLUTION:
Want to add eq’ns (1), (2), & (3) in a way as to obtain the eq’n
from the question and to have everything else cancel out.
NOTE: the eq’ns are manipulated so that there is one mole of the
initial reactant and one mole of the req’d. We therefore now
sometimes break the rule that there can be no fractions in
balanced eq’ns.
(5) 1C2H2 (g) + 5/2 O2 (g)  2CO2 (g) + 1H2O (l) ΔH5 = -2602 kJ/ 2
= -1301 kJ
(6) 2H2(g) + O2(g)  2H2O(1) ΔH6 = 2(-286 kJ)
= -572 kJ
(7) 2CO2(g) + 3H2O(g)  C2H6(g) + 7/2O2(g) ΔH7 = 3123 kJ/ 2
= +1561 kJ

31
Q

Hess’ Law example 3 cont.

A

Therefore:
1C2H2 (g) + 2H2 (g)  1C2H6 (g) ΔH°r = - 312 kJ

When we reverse an eq’n we reverse the sign for the enthalpy
change value. We add the enthalpy changes after making all
appropriate mole and sign adjustments.
ΔH net = ΣΔH r
ΔH net = - 312 kJ
The heat of hydrogenation of acetylene is .’. - 312 kJ

32
Q

Define Reference Energy State

A

When comparing enthalpy changes for formation reactions of different compounds, we must choose a reference energy state.
It is convenient to set the enthalpies of elements in their most stable form at SATP to be zero. As an arbitrary convention, for the sake of simplicity, all other enthalpies of compounds are measured relative to that reference energy
state.

33
Q

Define Standard Enthalpy of Formation

A

A formation reaction always begins with elements, so any standard enthalpy of formation reactions are measured from the reference energy state of zero.

34
Q

Define Thermal Stability

A

Thermal stability is the tendency of a compound to resist decomposition when heated.
The lower (i.e. more negative) the value of a compound’s standard molar enthalpy of formation, the more stable it is.
Δf Hm° = – 280.7 kJ/mol
SnO

Δf Hm° = – 577.6 kJ/mol
SnO2

Tin(IV) oxide has a greater thermal stability than tin(II) oxide.

35
Q

What is the standard enthalpy of change reaction & what is it’s formula?

A

The standard enthalpy change of a reaction is the sum of the
standard enthaplies of formation of the products minus the sum
of the standard enthalpies of formation of the reactants.

ΔrH° = ΣnΔfPHm° – ΣnΔfRHm°

36
Q

Predicting ΔHr Using Formation Rxns (formula of e.o.r) & (example)

A 
We can use enthalpies of formation (ΔHf) to calculate enthalpies 
of reaction (Hr) using the formula:

ΔH°r = ΣnH°f (product) - ΣnH°f (reactants)

Example:
What is ΔH°C for propane from ΔHf values of products &
reactants:
1C3H8 (g) + 5O2 (g)  3CO2 (g) + 4H2O (l)

   REACTANTS                          PRODUCTS ΔH°r   =   [(3 mol CO2) (H°f (CO2)) + (4 mol H2O) (H°f (H2O))]  
                -   [ (1 mol C3H8) (H°f (C3H8) + (5 mol O2) (H°f (O2)) ]
37
Q

Predicting ΔHr Using Formation Rxns continued…

A

Because O2(g) is the most stable form of the element,
H°f (O2) = 0.
(reference NRG state) is in standard state.

The other enthalpies See Back of Text or p. 4 & 5 of Data
Booklet
ΔH°r = [(3 mol CO2) (-393.5 kJ) + (4 mol H2O) (-285.8 kJ)]
mol mol
- [ (1 mol C3H8) (-103.8 kJ) + (5 mol O2) (0 kJ)]
mol mol
ΔH°r = (- 2323.7 kJ) - ( - 103.8 kJ)
ΔH°r = - 2219.9 kJ

38
Q

Enthalpy change for combustion

A

Enthalpy change for combustion of C3H8 = the sum of the enthalpy changes for converting the reactants into the standard states of the elements and then forming the products from the elements

CHEMISTRY - Thermochemistry (1 decks)

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