lecture 4 Flashcards
what is a standard state
the most usual form at 1 bar pressure and a specified temperature (298.15K)
how is enthalpy in non standard conditions calculated
H = H* + correction
what is H*
enthalpy at standard conditions
1 bar pressure
298.15K temp
what is standard enthalpy change
triangle H*
enthalpy change for a process where the initial and final substances are in their standard states
standard state for o2
gas
standard state for h2o
liquid
standard state for C
solid graphite
what is the enthalpy of formation
triangle f H*
enthapy change associated with forming a molecule from its constituent elements in their standard states
what is the enthalpy of formation of ‘x’ in their standard states
0
what is the enthalpy of formation when smt is formed but their constituent elements were not in their standard states
not 0
it has a value
what is enthalpy of vaporization
triangle vH*
enthalpy change when smt goes from liquid to gas under standard conditions
what is enthalpy of combustion
triangle cH*
oxidation of ‘x’ with o2 gas to give co2(g) and h20 (l)
what is enthalpy of solution
triangle sH*
separation of + and - ions in water
infinite dilution limit
hess law definition
standard enthalpy of a given reaction can be obtained as the sum of standard enthalpies for a sequence of reactions when we start with the same rea and end with the same pro.
what is standard enthalpy change of a reaction
pro - rea
hess law broken down
rea
standard state elements
pro
what is a born haber cycle
closed path of transformations starting and ending at the same point
what temp were enthalpy changes recorded at
a couple but definitely not all
how can we determine the enthalpy of a substance at any given temp
use the substances specific heat capacity
equation to find enthalpy of substance at temp 1 when we know enthalpy at temp 0
H (T1) =
H (T0) + integral of Cp dT
integral limits =
T1 top
T0 bottom
what is kirchhoffs law
enthalpy change of reaction at temp 1 = enthalpy change of reaction at temp 0 + intgral of the specific heat capacity multiplied by dT)
integral: t1 top t0 bottom
the change in specific heat capacity of a reaction is
total specific heat capacity of products - total specific heat capacity of reactants
including the stoichiometry (molar ratio)
what does kirchhoffs law help us do
helps us calculate the enthalpy change for a reaction at any temp when we know the enthalpy change at one temp + the heat capacities of the rea and products
change in Cp
specific heat of pro - specific heat of rea
temp change
final temp - intial temp
actual heat capacity variations equation
H(T1) = H(T0) + integral Cp dT
integral upper limit = t1
integral lower limit = t0
area under graph = enthalpy change of the substance
plot of cp against temp
diff phases = diff heat capacities
how can we measure enthalpy change
use differential scanning calorimetry
DSC
how does DSC work
differential scanning calorimetry
- increase temp in a sample with a reference that doesnt change
- measure changes in heat capacity at a constant pressure
- enthalpy change is equal to the area under the curve when Cp is plotted against temp
