Standard Enthalpy Change of Formation and Hess's Law Flashcards
what is the standard enthalpy change of formation
- the enthalpy change measured at 100kPa at a specified temperature
- when one mole of a substance is formed from its elements in their standard states
what is the symbol for the standard enthalpy change of formation
delta fH-
what is the standard state of an element
- the form in which the element exists at a a specified temp and pressure
- usually 298K and 100kPa
what would be the equation for the enthalpy change of formation of gaseous carbon dioxide and why
- C(s, graphite) + O2(g) = CO2(g)
- because all the reacting elements have that from in room temp and pressure
- and one mole of carbon dioxide is formed
what is the equation for the standard enthalpy change of formation for ethanol
- 2C(s) + 3H2(g) + 1/2O2(g) = C2H5OH
why is it very important to include your state symbols of the elements reacting and the product when writing equation for delta fH-
- because the enthalpy change of formation for the same elements which produce the same product can be different depending on their states
- for example the enthalpy change of formation of gaseous ethanol is less than liquid ethanol for the same reactants
why is it impossible to directly determine the enthalpy change of formation of C(s) + 1/2O2(g) = CO(g) for example
- because it is impossible to burn carbon with oxygen to only form carbon monoxide
- most enthalpy changes of formation cant be determined experimentally because of this
what does hess’s law state
- the enthalpy change of a reaction is independent of the path taken in converting reactant to products
- provided the initial and final conditions are the same in each case
what is hess’s law a direct application of
the first law of thermodynamics about the conservation of energy
what does hess’s law now allow us to do
- despite us not being able to find the enthalpy change of formation for many reactions
- we can now calculate the delta fH- of them
the enthalpy changes of combustion of carbon and carbon monoxide can both be determined experimentally. they are: C(s) + O2(g) = CO2(g) = -394kLmol-1 and CO(g) + 1/2O2(g) = CO2(g) = -283kJmol-1. what are the two different methods you could use to calculate the enthalpy change of formation of carbon monoxide
- subtracting equations
- or constructing an enthalpy cycle using hess’s law
how would you work out the enthalpy change of formation of carbon monoxide using the subtracting method
- reverse the second equation above then add the first
- reversing gives CO2 = CO + 1/2O2 = +283kJmol-1
= as the first one is C + O2 = CO2 = -394kJmol-1 - then added is C + 1/2O2 = CO
- so 283 +(-394) = -111kJmol-1
how would you work out the standard enthalpy change of formation of carbon monoxide by constructing an enthalpy cycle using hess’s law
- 1 = C + O2 –(-394)– CO2 (3)
- 2 = CO + 1/2O2 –(-283)– CO2 (3)
- this means that going from 1 to 3 = -394 and going from 2 to 3 = -283
- so if you want to go from 1 to 2 to form carbon monoxide, you need to go find the enthalpy change of going from 1 to 2
- 3 to 1 is -394, so 1 to 2 to 3 must also be -394 according to hess’s law
- as 2 to 3 = -283, 283 - 394 = -111kJmol-1
- so the standard enthalpy change of formation for carbon monoxide is -111kJmol-1
what is the general formula for enthalpy cycles with combustion products
- from the top left, you go from the compound elements to the compound with delta fH- in between
- from the top left, you also go from compound elements to the combustion products
- with the sum of delta cH- of elements labeling the arrow
- then from the top right, you go to combustion products too
- with the sum of delt cH- of the compound labeling the arrow
what formula is derived if you simply want to calculate the standard enthalpy change of formation
delta fH- = sum of delta cH- of elements - sum of delta cH- of compounds
