2.2 Enthalpy Flashcards
What is enthalpy change?
heat transfer at a constant pressure
How can enthalpy be represented?
H = E + PV
where E is internal energy
Can enthalpy be measured?
no, ∆H, change in enthalpy can:
∆H = ∆E + P∆V
how are enthalpy change (∆H) and heat flow (q) related at constant pressure?
they are equal
∆H = q
What is latent heat?
energy change from phase/state change
What are endothermic phase changes?
fusion, vaporization, sublimation
What are exothermic phase changes?
freezing, condensation, deposition
What is fusion?
melting: solid to liquid
What is vaporization?
boiling: liquid to gas
What is sublimation?
solid to gas
What is freezing?
liquid to solid
What is condensation?
gas to liquid
What is deposition?
gas to solid
Does latent heat relate to temperature?
NO! only phase changes, not temperature
What does energy added through latent heat do if it doesn’t change temperature?
break intermolecular forces
How are q and ∆H related?
directly proportional
they are equal at CONSTANT PRESSURE
What is Latent Heat measured in?
kJ/mol
What is ∆H dependent on?
temperature and pressure
What are the thermodynamic standard conditions?
PRESSURE: 1 atm (101.325 kPa)
TEMP: 25ºC (298,15ºK)
What is ∆Hº?
enthalpy under standard conditions
How is ∆Hº represented?
∑H(products) - ∑H(reactants)
When is ∆Hº an endothermic reaction?
heat consumed/absorbed:
∑H(products) > ∑H(reactants)
∆H > 0
When is ∆Hº an exothermic reaction?
heat released:
∑H(reactants) > ∑H(products)
∆H < 0
How is ∆Hº(rxn) an extensive property?
the amount of matter in the sample matters
What are examples of extensive properties?
enthalpy, energy, volume
What are examples of intensive properties?
temperature, pressure
Why does ∆Hº(rxn) vary in the same reaction?
dependent on per mole of which substance
What is the relationship between ∆Hº in kJ and coefficient of stoichiometric equation?
proportional
What is calorimetry?
study of heat flow of physical and chemical processes
What is a calorimeter?
apparatus where heat flow reactions take place
What is the apparatus of a low cost calorimeter?
Styrofoam cup with Styrofoam lid, thermometer and stirrer
What does the system of the calorimeter contain?
heat flow of reactants, solvent, and calorimeter
How does heat flow in a calorimeter for an endothermic reaction?
heat from calorimeter, to the solvent to the reactants – reactants absorb/gain heat
How does heat flow in a calorimeter for an exothermic reaction?
heat from reactants, to solvent, to calorimeter
– heat lost from reactants and gained by calorimeter and solvent
How can water be used to find heat capacity of calorimeter for calorimeter calibration?
- add known amt of hot water with known temperature to calorimeter
- measure temperature when calorimeter and water reach thermal equilibrium
(mass of water, specific heat capacity of water, ∆T are known, so C(cal) can be calculated)
How can a chemical reaction be used to find heat capacity of calorimeter for calorimeter calibration?
- carry out chemical reaction with known heat of reaction (q(rxn)) inside calorimeter, measure temperature, and ensure there is no water or solution in the calorimeter
(qrxn and temperature change is known so C(cal) can be calculated)
When is a bomb calorimeter used?
measure heat liberated from gaseous or combustion reactions
What are the conditions of a bomb calorimeter?
takes place under constant volume so the measured heat transfer corresponds to ∆E
- total pressure change is approx. constant because the calorimeter is filled with a large excess of oxygen, so heat transfer recorded is equal to ∆H
How is ∆T measured in a bomb calorimeter?
change in temperature of the calorimeter and the water bath
What is the bomb calorimeter apparatus?
water bath surrounding heavy vessel with thermometer and stirrer
What is Standard Enthalpy of Formation (∆H˚f)?
∆H for the reaction where 1 mole of a substance under standard conditions is formed from the constituent elements in their reference states under standard condition
What is the formation reaction?
elements (reference state, standard condition) → 1 mole of a substance (standard condition)
What is a reference state?
form of an ELEMENT that is MOST STABLE under standard conditions
Do reference states need to be an atom?
NO; they can be molecules too
What is a standard state?
state of substance under standard condition
What are allotropes?
chemical elements existing in 2+ different forms
Ex. carbons allotropes are graphite, graphene, diamond, etc.
How do we predict the enthalpy change of reaction?
define a reference point (0) and measure the difference in enthalpy compared to the reference point
what is the standard enthalpy of formation of an element in its reference state under standard conditions?
arbitrarily set to 0
What is HESS’S LAW?
enthalpy change (∆H˚) for a reaction is the SUM of the ENTHALPY CHANGES (∆H˚f or ∆H˚) for the individual steps of the reaction
What is the standard enthalpy of formation of reactants to elements?
-∆H˚f(reactants)
What is the standard enthalpy of formation of elements to products?
∆H˚f(products)
How is Hess’s Law represented?
∆H˚(rxn) = ∑∆H˚f(products) - ∑∆H˚f(reactants)
Must coefficients be considered for Hess’s Law?
YES! remember enthalpy is an extensive property:
multiply coefficient with ∆H˚f
[c∆H˚f(C) + d∆H˚f(D)] - [a∆H˚f(A) + b∆H˚f(B)]
Why is Hess’s Law possible?
because it is a state function (path independent)
Ex. going from solid to vapour = solid to liquid to vapour
What is heat of combustion?
ENERGY released when ONE MOLE of a fuel is in its standard state undergoes COMPLETE combustion with oxygen to form product in their standard state
What are the typical products of complete combustion? incomplete combustion?
H2O (l) and CO2(g)
incomplete creates undesirable compounds like CO
What is heat of combustion measured in?
kJ/mol
What is bond enthalpy?
ENERGY required to BREAK 1 MOLE of a specific type of bond (single, double, or triple) between two atoms where the reactants and products are GAS
What type of reaction occurs when bonds are formed between atoms in the gas phase?
exothermic (energy is always released)
What type of reaction occurs to break bonds?
Endothermic (energy absorbed)
–> energy/heat input is required
–> both enthalpies are always positive
What is total bond enthalpy?
enthalpy of atomization
–> energy required to break ALL BONDS in 1 MOLE of a gaseous compound
1 mole of gaseous compound → gaseous atoms
How can AVERAGE BOND ENTHALPY be determined?
divide TBE of a molecule which contains only one type of bond by the total number of those bonds
What is the relationship between bond strength and bond order?
directly proportional: C≡C is stronger than C=C is stronger than C-C
What are the two methods of calculating total bond enthalpy?
- Hess’s Law (need ∆H˚f for all products and reactants) - more precise
- sum all average bond enthalpy (need all average bond enthalpies) - approximation
How can enthalpy change be estimated with average bond enthalpies only for GASES?
gaseous reactants hypothetically broken up into atoms and gaseous atoms form new bonds in different arrangements :
∆Hrxn = ∑enthalpies of bonds broken - ∑enthalpies of bonds formed
