Chapter 2: 2.2 Enthalpy Flashcards
Define and give equation for:
Enthalpy
A thermodynamic state function defined by:
H = E + PV
State the function and variables for:
Enthalpy
H = E + PV
* E is the internal energy
* P is the pressure
* V is the volume of the system
True or False:
Absolute enthalpy of a system can be measured
False, only changes in enthalpy (ΔH) can be measured
What is the constant of many physical processes and chemical reactions? What isn’t?
Constant (atmospheric) pressure
Non-constant volume
State the formula for:
Change in enthalpy at constant pressure
ΔH = ΔE + PΔV
What does the ΔH of a system equal at constant pressure?
The heat (qp) flowing into or out of a system
Define:
Latent heat
The energy change associated with physical processes that involve a change of state (phase)
Define and list:
Endothermic processes
Processes the consume energy (+ΔH)
* Melting (fusion)
* Boiling (vapourization)
* Sublimation
Define and list:
Exothermic processes
Processes that release energy (-ΔH)
* Condensation
* Freezing
* Deposition
True or False:
In latent heat, the temperature changes and the state also changes
False, in latent heat, the temperature DOES NOT change while the state changes
State the relation between:
Q values for reverse and forward processes
- Same magnitude
- Opposite sign
How is enthalpy change under standard conditions denoted?
ΔH^o
What does the superscript “o” in ΔH^o mean?
Standard conditions
Unless otherwise stated, in the study of thermodynamics what is standard condition?
- 1 atm (101.325 kPa)
- 25 degrees Celsius (298.15 Kelvin)
State the formula and notation for:
Enthalpy of a reaction
Sum of absolute molar enthalpies of the and those of the reactants
* ΔHrxn = ΣH (products) - ΣH (reactants)
Explain:
Endothermic vs. Exothermic reactions in terms of enthalpy
- Exothermic reactions releases heat into the surroundings, system loses energy, ΔH < 0
- Endothermic reactions absorbs heat from the surroundings, ΔH > 0
What are the two possible units for reaction enthalpies? Explain when each is used
- kJ: The enthalpy change for the reaction involving the numbers of moles equal to the stoichiometric coefficients of the balanced equation as written
- kJ/mol: The enthalpy change for the reaction per 1 mole of a particular substance that is either stated or implied by the context
Define:
Calorimetry
An experimental technique used to study the heat flow of physical and chemical processes
What apparatus is used to allow processes to occur in calorimetry?
A calorimeter
For an isolated calorimeter system:
How do we write the formula?
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
When the calorimeter absorbs/releases heat:
How do we write the formula for q rxn?
q rxn = - (q cal + q sol)
What needs to be calculated when the calorimeter absorbs/releases heat?
q cal = C cal * ΔT
What is C cal? What is the process of determining it called?
- Heat capacity of the calorimeter (calorimeter constant)
- Calorimeter calibration
What are the 2 common methods of calorimeter calibration?
- Adding a known amount of hot water with known initial temperature
- Carrying out a chemical reaction, with a known heat of reaction
In calorimeter calibration:
How does the water method calibrate the calorimeter?
- No chemical reaction takes place
- Measure the temperature at which the water and calorimeter are at equilibrium
- q water = - q cal
In calorimeter calibration:
How does a reaction calibrate the calorimeter?
- The heat of the reaction is absorbed by calorimeter
- q cal = -q rxn
Define:
Bomb calorimeter
Typically used to measure the heat liberated from gaseous or combustion reactions
State the formula for:
Amount of heat produced by the chemical reaction in a bomb calorimeter
q rxn = - (q cal + q water) = - (C cal + c water * m water)ΔT
Define:
Standard enthalpy of formation
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
Define:
Molar Enthalpy of Reaction
The amount of energy per 1 mole of a particular reactant or product
For standard enthalpy of formation, state:
- Notation
- Other names
- Type of reaction involved
- ΔH^o f
- Standard heat of formation
- Formation reaction
Define:
Standard state
The state of a substance under standard conditions
Define:
Reference state
Refers to elements; form of the element which is the most stable under standard conditions
What is it called when an element can exist in two or more different forms?
Allotropes
State:
ΔH^o f of element in the reference state under standard conditions
0
True or False:
The standard enthalpy of formation of an element is always zero under standard conditions
Not necessarily.
The standard enthalpy of formation of an element under standard conditions is NOT zero if the element is not in reference state
State:
Hess’s Law of Thermochemistry
The enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps of the reactions
How are standard enthalpies of formation used to calculate the enthalpy change of a reaction? State the formula
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)
State formula for:
Hess’s Law when:
1. Multiple products or reactants are present
2. For a generic reaction: aA + bB –> cC + dD
- ΔH^o reaction = ΣΔH^o f (products) - ΣΔH^o f (reactants)
- ΔH^o rxn = [cΔH^o f (C) + dΔH^o f (D)] - [aΔH^o f (A) + bΔH^o f (B)]
What is an extensive property?
A property where stoichiometric coefficients matter (for example, ΔH)
Define:
Heat of combustion
- 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
What is the heat of combustion also known as?
Standard enthalpy of combustion
What are the common products of complete combustion? Incomplete combustion?
- CO2 and H2O
- CO
Define:
Bond enthalpy
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
When bonds form between atoms in gas phase:
Energy is always released (exothermic)
When bonds are broken to form atoms of gases:
Energy is always required (endothermic)
Define:
Total Bond Enthalpy (TBE)
The energy required to break all of the bonds in one mole of a gaseous compound
What is TBE also known as?
Enthalpy of atomization
What is the general concept of TBE?
One mole of gaseous compound –> Gaseous atoms
How is the average bond enthalpy determined?
Determined by dividing the TBE of a molecule which contains only one type of bonds by the total number of those bonds
List:
The 2 methods for determining TBEs
- Determining the ΔH^o using the standard enthalpy of formation (ΔH^o f) data
- Summing up the enthalpies of all the bonds in the molecule, using the average bond enthalpies
True or False:
The is a small difference between calculating TBE using standard enthalpy of formation and average bond enthalpies
True
Which method of calculating TBE is more accurate?
Using standard enthalpy of formation
What is another method we can determine ΔH for gas-phase reactions?
ΔH rxn = TBE (reactants) - TBE (products)
When can we calculate ΔH using the TBE of reactants and products?
Only when ALL reactants and products are in GAS-PHASE
