Thermodynamics: the first law Flashcards
Define the internal energy of a system
Internal energy (U) is made up of kinetic energy and rotational/vibrational/potential energies
Define a change in the internal energy of a system
ΔU = Ufinal - Uinitial = q + w
q = heat transfer
w = work done
Describe how the signs are affected by the direction of energy transfer
Energy entering the system is positive and energy exiting the system is negative
Define work (w)
Energy transfer through orderly motion of the atoms eg a moving piston
Define heat (q)
Energy transfer through random thermal motion of the atoms, eg a hot metal block
State the name for an isolated system and its value of q
An isolated system is said to be adiabatic, q = 0
Describe the boundary between a system and its surroundings if heat transfer takes place
The boundary must be diathermic (ie must allow heat transfer)
Describe volume work, w
If a gas expands quickly, it will cool down.
If a gas is compressed quickly, it will heat up.
work (J) = force (N) x distance (m) = -pressure (Nm-2) x volume change (m3)
Ie w = -pexΔV where pex is the external pressure
Describe the special case of free expansion
pex = 0
w = 0
This the case for expansion into a vacumm
Describe a reversible change in thermodynamics
A reversible change is one that can be reversed by an infinitesimal modification of a variable.
The system is in equilibrium with the surroundings before and after the change
For the piston example, this means p = pex
Describe the reversible expansion of an ideal gas at constant temperature
By integrating p over ΔV and substituting p for the ideal gas equation we get w.
note: if we plot p against V, w is the area under the curve
Define enthalpy
Enthalpy is the heat obtained from a chemical reaction
ΔH = ΔU + pΔV
Define and give examples of state functions
A state function is a property that only depends on the current state and not the path taken to get there.
p,T,V,U,H,S and G are all state functions. w and q are not. w and q are path dependent variables
Describe general functions of U and H
U = f(V,T)
H = f(p,T)
Define heat capacity, C
C is the amount of heat needed to raise the temperature of a substance by one degree
Define a calorie
A calorie, or cal, is defined as the amount of heat needed to raise the temperature of 1g of water by 1 degree
Define C at constant volume
Define C at constant pressure
Go through the steps of finding the difference between Cv and Cp
Give Cp in terms of Cv
Cp = Cv + nR
State the equipartition principle
For each ‘degree of freedom’ we get ½RT per mole
State how many degrees of freedom an ideal gas has and relate this to U
An ideal gas has 3 degrees of freedom
U = 3/2 RT
Define πT in thermochemistry and specifically for an ideal gas
πT is internal pressure. For an ideal gas it is equal to 0 as there are no interactions
