measuring changes in energy etc Flashcards
(73 cards)
change in E for system with constant volume
if no change in volume work = 0 therefore change in E = qv (which means q at constant volume)
what would you use to measure change in heat with constant volume
bomb calorimeter
heat capacity and relationship to q
heat capacity (C) is the heat required to raise the temperature of obj by one degree (extensive, depends on size of system)
q = C(change in T)
molar heat capacity and equation
heat capacity/mol or heat required to raise one mole of something temp by one degree
q = n(Cn)*change in T
where Cn is molar heat capacity (heat capacity divided by moles) and n is number of moles
specific heat capacity and equation
Csp heat capacity per unit mass
q = mCsp * change in temp
where m is the mass of the object
measuring energy under constant pressure
use enthalpy (H) J or kJ
enthalpy definition
total heat content of a system - measure of heat energy released or absorbed when bonds are broken or formed during a reaction
H = E + PV
enthalpy = internal energy of the system + pressure*volume
good for reactions completed at constant pressure, when the only type of work is expansion work
change in enthalpy in constant pressure processes
change in H = change in internal energy of system + Pressure * change in volume of the system
at atmospheric pressure where pressure is constant and Psys = Pext,
heatflow qp = change in H
what is molar enthalpy and what type of variable is it
Hm = H/n where n is number of moles
intensive state variable - ratio of two extensive variables
what is molar enthalpy and what type of variable is it
Hm = H/n where n is number of moles
intensive state variable - ratio of two extensive variables
what enthalpy is extensive vs intensive
enthalpy in kJ is extensive
molar enthalpy in kJ/mol is intensive
heat of reaction
qp
heat transfer resulting from transformation of reactants to products at the same temp and pressure
for constant pressure processes change in H = qp
or change in enthalpy = heat transfer from transformation of reactants to products
what would you use to measure heat at constant pressure
what thing and what equation
constant pressure calorimetry with a coffee cup calorimeter
change in enthalpy H = qp(heat of reaction) = Cp (heat capacity at constant pressure) * change in temp
bomb vs constant pressure calorimetry
BOMB
measures change in temp related to heat transfer, w constant volume
qv = mCsp*change in temp
Work = 0 as volume is constant
determines the state function internal energy
qv = change in E
CONSTANT PRESSURE
measures change in temp corresponding to heat transfer
work = work, volume is NOT constant
determines the state function enthalpy
qp = change in enthalpy
enthalpy vs heat transfer
enthalpy - state function, change indicates the amount of heat transferred between system and surroundings at constant pressure
change in internal energy = sum of heat transferred and work done
what type of function is H enthalpy
a state function, independent of steps
what type of function is H enthalpy
a state function, independent of steps
hess’s law
total enthalpy change during course of chemical reaction is independent of sequence of steps taken
state function
heat of formation
change in enthalpy measured upon forming one mole of substance from elements in their standard states
generally measured in kJ/mol (heat transferred when one mole of substance is formed)
standard state
pressure of 1 bar and a purity based on phase/state of aggregation
for gas P –> 1 bar
liquid or solid –> pure substance
solution —> 1M solute and pure liquid solvent
what is carbon’s standard state
graphite
what is carbon’s standard state
graphite
STP and SATP
standard temp and pressure, T = 273.15 K/0 degress C and P = 1 bar
standard ambient temp and pressure, T = 298.15 K, 25C and P = 1 bar
equation for change in H (enthalpy) of a reaction
change in H reaction = sum of each products Hf(heat of formation)coefficient - sum of each reactants Hfcoefficient
