Energetics (3.1.4-Phys.) Flashcards
Define ΔH
Enthalpy change at constant pressure.
Define ΔH° (standard)
Enthalpy change at 100kPa and 298K.
Define ΔH°f (standard, formation)
Enthalpy change when 1 mole of product is formed from its elements in their standard state at 100kPa and 298K.
Define ΔH°c (standard, combustion)
Enthalpy change when 1 mole of substance is completely combusted with all reactants and products in their standard states at 100kPa and 298K.
State Hess’ Law
This law states that the total enthalpy change is independent of the route taken.
Define Mean Bond Enthalpy
Enthalpy required to break a particular covalent bond averaged over a range of compounds.
How do you calculate ΔH using Hess’ Law?
Beneath the equation given, write out all elements present and draw arrows from the compounds to the elements to represent the formation and breaking of bonds.
Where do you change the signs of the ΔH°fs given?
Because these are formation enthalpies, you would change the sign if you are breaking bonds.
i.e. If you go from SO₂ to S₂.
How do you know when you are calculating ΔH°fs?
When you have been given values of ΔH°c.
How do you calculate ΔH°f?
You can calculate it similarly to ΔH, except you are putting the products of combustion underneath the equation instead of elements.
Why is O₂’s ΔH equal to 0?
By definition, as it is an element in its standard state.
What is the equation to calculate energy in calorimetry and what does each letter mean?
Q = mcΔT
Q-energy, m-mass of water, c-capacity of water (4.18), ΔT- change in temp.
What is the equation used in calorimetry to calculate ΔH?
ΔH= Q/n n= number of moles
How do you know what moles to use in your equation when calculating ΔH?
With two possible reactants, you use the smaller number of moles as the bigger one would likely be in excess.
What is the equation used to calculate bond enthalpy?
∑bonds broken − ∑bonds formed
