Handout 1 Flashcards
Equation for Internal Energy
delta U= q + w
Mechanical Work
F x ds
Expansion against constant pressure
w=- definite integral Pex dV
Free expansion ISOTHERMAL IRREVERSIBLE (using volume)
w= -definite integral (V2,V1) pexdV = =pexdeltaV
Isothermal REVERSIBLE
w=-nRTdefinite integral (V2,V1)dV/V= -nRTln(V2/V1)
HEAT, q equation in terms of irreversible expansion
=q-pex(V2-V1)
What does the heat, q, equation lead to at constant volume?
delta U= qv
What does the heat, q, equation lead to at constant pressure?
expression for enthalpy…
where H= U+ pv
delta H=qp
What is a state function?
any quantity whose value is independent of its history
What is delta U for perfect gas?
delta U=0
What are units for heat capacity?
JK-1 (mol-1)
Constant volume heat capacity (INTEGRAL)
Cv= (delta U/T )v where dpv = dU
Constant pressure heat capacity (INTEGRAL)
Cp= (delta H/T)p where dpq= dH
What is experimental method for measuring internal energy change?
bomb calorimeter. Constant volume. Leads to delta U=qv
How do we get to calorimeter equation?
delta U reaction = Ccal delta T/n
heat of combustion warms calorimeter. T equalises. n moles of compound to delta T.
How can you define heat capacity in terms of a ratio?
heat capacity= dq/dt
How do you measure enthalpy change experimentally?
DSC…. Differential Scanning Calorimeter
Describe what happens in DSC thermogram.
Cp= heat flow/heat rate Excess heat to keep sample and reference at same T. C= qp, ex/ delta T Know H=qp, ex Therefore Cpex = delta H/ delta T
What is standard molar reaction enthalpy?
Change in enthalpy when reactants are converted to products at 1 bar and a specified temperature.
Why can we get enthalpy change from products and reactants?
Because enthalpy is a state function
What is standard enthalpy of formation?
Change in enthalpy when a substance is formed from its elements in their reference states at 1 bar and a specified temperature.
What is Kirchoff’s Law?
Hm(T2) = Hm(T1) + Cpm (T2-T1)
Also put in delta r Hm
Can get delta Cpm as you would get enthalpy change (products - reactants)
What relationship does Kirchoff’s law make use of? And what does it assume?
Cp= delta H/ delta T
Cp independent of T
