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My Packs: (UWO) (CHEM 1302) > Chapter 2: 2.2 Enthalpy > Flashcards

Chapter 2: 2.2 Enthalpy Flashcards

1
Q

Define and give equation for:

Enthalpy

A

A thermodynamic state function defined by:
H = E + PV

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2
Q

State the function and variables for:

Enthalpy

A

H = E + PV
* E is the internal energy
* P is the pressure
* V is the volume of the system

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3
Q

True or False:

Absolute enthalpy of a system can be measured

A

False, only changes in enthalpy (ΔH) can be measured

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4
Q

What is the constant of many physical processes and chemical reactions? What isn’t?

A

Constant (atmospheric) pressure
Non-constant volume

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5
Q

State the formula for:

Change in enthalpy at constant pressure

A

ΔH = ΔE + PΔV

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6
Q

What does the ΔH of a system equal at constant pressure?

A

The heat (qp) flowing into or out of a system

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7
Q

Define:

Latent heat

A

The energy change associated with physical processes that involve a change of state (phase)

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8
Q

Define and list:

Endothermic processes

A

Processes the consume energy (+ΔH)
* Melting (fusion)
* Boiling (vapourization)
* Sublimation

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9
Q

Define and list:

Exothermic processes

A

Processes that release energy (-ΔH)
* Condensation
* Freezing
* Deposition

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10
Q

True or False:

In latent heat, the temperature changes and the state also changes

A

False, in latent heat, the temperature DOES NOT change while the state changes

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11
Q

State the relation between:

Q values for reverse and forward processes

A
  • Same magnitude
  • Opposite sign
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12
Q

How is enthalpy change under standard conditions denoted?

A

ΔH^o

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13
Q

What does the superscript “o” in ΔH^o mean?

A

Standard conditions

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14
Q

Unless otherwise stated, in the study of thermodynamics what is standard condition?

A
  • 1 atm (101.325 kPa)
  • 25 degrees Celsius (298.15 Kelvin)
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15
Q

State the formula and notation for:

Enthalpy of a reaction

A

Sum of absolute molar enthalpies of the and those of the reactants
* ΔHrxn = ΣH (products) - ΣH (reactants)

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16
Q

Explain:

Endothermic vs. Exothermic reactions in terms of enthalpy

A
  • Exothermic reactions releases heat into the surroundings, system loses energy, ΔH < 0
  • Endothermic reactions absorbs heat from the surroundings, ΔH > 0
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17
Q

What are the two possible units for reaction enthalpies? Explain when each is used

A
  1. kJ: The enthalpy change for the reaction involving the numbers of moles equal to the stoichiometric coefficients of the balanced equation as written
  2. kJ/mol: The enthalpy change for the reaction per 1 mole of a particular substance that is either stated or implied by the context
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18
Q

Define:

Calorimetry

A

An experimental technique used to study the heat flow of physical and chemical processes

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19
Q

What apparatus is used to allow processes to occur in calorimetry?

A

A calorimeter

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20
Q

For an isolated calorimeter system:

How do we write the formula?

A

q surr = -q sys = - (q rxn + q sol + q cal) = 0
* This all turns into qrxn = -qsol if the calorimeter does not absorb/release heat

Since this is isolated

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21
Q

When the calorimeter absorbs/releases heat:

How do we write the formula for q rxn?

A

q rxn = - (q cal + q sol)

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22
Q

What needs to be calculated when the calorimeter absorbs/releases heat?

A

q cal = C cal * ΔT

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23
Q

What is C cal? What is the process of determining it called?

A
  • Heat capacity of the calorimeter (calorimeter constant)
  • Calorimeter calibration
24
Q

What are the 2 common methods of calorimeter calibration?

A
  1. Adding a known amount of hot water with known initial temperature
  2. Carrying out a chemical reaction, with a known heat of reaction
25
Q

In calorimeter calibration:

How does the water method calibrate the calorimeter?

A
  • No chemical reaction takes place
  • Measure the temperature at which the water and calorimeter are at equilibrium
  • q water = - q cal
26
Q

In calorimeter calibration:

How does a reaction calibrate the calorimeter?

A
  • The heat of the reaction is absorbed by calorimeter
  • q cal = -q rxn
27
Q

Define:

Bomb calorimeter

A

Typically used to measure the heat liberated from gaseous or combustion reactions

28
Q

State the formula for:

Amount of heat produced by the chemical reaction in a bomb calorimeter

A

q rxn = - (q cal + q water) = - (C cal + c water * m water)ΔT

29
Q

Define:

Standard enthalpy of formation

A

The enthalpy change for the reaction in which 1 mole of a substance under standard conditions (1 atm and 298.15 K) is formed from the constituent elements in their reference states under standard conditions

30
Q

Define:

Molar Enthalpy of Reaction

A

The amount of energy per 1 mole of a particular reactant or product

31
Q

For standard enthalpy of formation, state:

  • Notation
  • Other names
  • Type of reaction involved
A
  • ΔH^o f
  • Standard heat of formation
  • Formation reaction
32
Q

Define:

Standard state

A

The state of a substance under standard conditions

33
Q

Define:

Reference state

A

Refers to elements; form of the element which is the most stable under standard conditions

34
Q

What is it called when an element can exist in two or more different forms?

A

Allotropes

35
Q

State:

ΔH^o f of element in the reference state under standard conditions

A

0

36
Q

True or False:

The standard enthalpy of formation of an element is always zero under standard conditions

A

Not necessarily.
The standard enthalpy of formation of an element under standard conditions is NOT zero if the element is not in reference state

37
Q

State:

Hess’s Law of Thermochemistry

A

The enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps of the reactions

38
Q

How are standard enthalpies of formation used to calculate the enthalpy change of a reaction? State the formula

A

The reactants are hypothetically “unformed” in a reaction that is the reverse of formation, after which the resulting elements combine to form the products
* ΔH^o rxn = ΔH^o f (products) - ΔH^o f (reactants)

39
Q

State formula for:

Hess’s Law when:
1. Multiple products or reactants are present
2. For a generic reaction: aA + bB –> cC + dD

A
  1. ΔH^o reaction = ΣΔH^o f (products) - ΣΔH^o f (reactants)
  2. ΔH^o rxn = [cΔH^o f (C) + dΔH^o f (D)] - [aΔH^o f (A) + bΔH^o f (B)]
40
Q

What is an extensive property?

A

A property where stoichiometric coefficients matter (for example, ΔH)

41
Q

Define:

Heat of combustion

A
  • ΔH^o for a given combustion reaction
  • The energy released when one mole of a fuel in its standard state undergoes complete combustion with oxygen to form products in their standard state
42
Q

What is the heat of combustion also known as?

A

Standard enthalpy of combustion

43
Q

What are the common products of complete combustion? Incomplete combustion?

A
  • CO2 and H2O
  • CO
44
Q

Define:

Bond enthalpy

A

The energy required to break one mole of a specific type of bond (single, double, or triple) between two atoms, provided the reactants and products are BOTH gases

45
Q

When bonds form between atoms in gas phase:

A

Energy is always released (exothermic)

46
Q

When bonds are broken to form atoms of gases:

A

Energy is always required (endothermic)

47
Q

Define:

Total Bond Enthalpy (TBE)

A

The energy required to break all of the bonds in one mole of a gaseous compound

48
Q

What is TBE also known as?

A

Enthalpy of atomization

49
Q

What is the general concept of TBE?

A

One mole of gaseous compound –> Gaseous atoms

50
Q

How is the average bond enthalpy determined?

A

Determined by dividing the TBE of a molecule which contains only one type of bonds by the total number of those bonds

51
Q

List:

The 2 methods for determining TBEs

A
  1. Determining the ΔH^o using the standard enthalpy of formation (ΔH^o f) data
  2. Summing up the enthalpies of all the bonds in the molecule, using the average bond enthalpies
52
Q

True or False:

The is a small difference between calculating TBE using standard enthalpy of formation and average bond enthalpies

A

True

53
Q

Which method of calculating TBE is more accurate?

A

Using standard enthalpy of formation

54
Q

What is another method we can determine ΔH for gas-phase reactions?

A

ΔH rxn = TBE (reactants) - TBE (products)

55
Q

When can we calculate ΔH using the TBE of reactants and products?

A

Only when ALL reactants and products are in GAS-PHASE

My Packs: (UWO) (CHEM 1302) (15 decks)

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