Chapter 5 - Thermochemistry Flashcards
what is a system?
the part of the universe chosen for study
what are surroundings?
the part of the universe outside the system in which these interactions can be detected
what is work (w)?
work is done hen a force acts through a distance,
w = -Pext * ΔV
w = F*d
what is heat (q)?
energy transferred between a system and its surroundings as a result of a temperature difference, q = mcΔT
what is specific heat (c)?
quantity of heat required to change the temperature of a system by one degree for one gram of the substance (J/g*C)
what is an endothermic reaction?
the corresponding situation is a temperature decrease in an isolated system or a gain of heat from the surroundings by a non-isolated system
what is an exothermic reaction?
one that produces a temperature increase in an isolated system or, in a non-isolated system, gives off heat to the surroundings
what is a calorimeter?
a device for measuring quantities of heat
what is the 1st Law of Thermodynamics?
A system contains ONLY internal energy, ΔU = q + w; the energy of an isolated system is constant
- heat is absorbed by the system, q>0; work is done on the system, w>0
- heat is given off by the system, q
what is a state function?
any property that has a unique value for a specific state of system
what is enthalpy (H)?
the sum of the internal energy and the pressure-volume product of a system
H = U + PV, ΔH = ΔU + PΔV, ΔH =q(p)
what is standard enthalpy of formation?
pure elements in reference from is 0; the enthalpy change for the overall reaction is the sum of the standard enthalpy changes of the individual steps
ΔHf = ΣProducts - ΣReactants
what is molar heat capacity?
is the amount of heat needed to raise the temperature of 1mol of the substance by 1 Kelvin (1K), Using C all four factors can be expressed in one equation that describes the temperature change resulting from a heat transfer
q = nCΔT
what is expansion work?
A gas in a container with movable walls can be expanded, and this expansion results in a movement AGAINST THE FORCE EXERTED BY THE WALLS. Thus, work is done in moving the walls of the container
wsys = -Pext*ΔV wsys = -wsurr
what is the Zeroth Law of Thermodynamics?
if two systems, A and B are separately in thermal equilibrium with a third system, C, then they are in thermal equilibrium with each other
How are the system and surroundings related, via Law of thermodynamics?
Whatever the system loses, the surroundings gains, and vice versa
qsys = -qsurr wsys = -wsurr
what is heat measurement: calorimetry?
Isolated systems
ΔEsys = qsys + wsys
what is a bomb calorimeter?
a bomb calorimeter is to measure the heat evolved in a combustion reaction.
chemical energy is converted to thermal energy, and the temperature of the system rises
qrnx = -qcalorimeter (Where qcalorimeter = qbomb + qwater +…)
qcalorimeter = c*ΔT
what are thermochemical equations?
a chemical equation that shows the enthalpy relation between products and reactants
magnitude of enthalpy is directly proportional to the amount of reactant consumed
ΔHrxn = ΣVpProducts - ΣVrReactants
what is heat of fusion?
the heat absorbed when a solid melts (s -> l)
what is heat of vaporization?
heat is absorbed when a liquid vaporizes (l -> g)
what is an extensive property?
depends on the amount of material (eg. ΔH)
what is an intensive property?
doesn’t depend on the amount of material
what is a path function?
depend on the sequences of the steps that the system takes from the initial to the final state;
- heat
- distance
- and work
What is Hess’s Law?
If a reaction proceeds in steps, the overall change in ΔH is the algebraic sum of the ΔH’s of the individual steps
One-step vs. two-step gas expansion?
- ΔU is the same for the single and two stage expansion process because internal energy is a state function
- However, more work is done in the two-stage expansion because work is path function
what is a state function?
depends only on the initial and final state of the system; ΔH ΔS ΔG ΔP ΔU ΔT
