Hess Law Flashcards
Why is it not always possible to measure enthalpy changes of reactions directly from experiments?
- reaction may have very high activation energy
- reaction rate may be too slow
- possibility of competing reactions
If it is not possible to measure enthalpy changes of a reaction directly from exp, what do we use?
Hess Law
Define Hess Law
- states enthalpy change for a chemical reaction is the same, independent route that is taken from reactants to products
In an enthalpy cycle, how do you work out enthalpy change if theres an indirect route?
Add step 1 and 2 to get to the same destination as the direct route
What do you do if the direction of an arrow changes in an enthalpy cycle?
Change sign (+ / -) before numerical value
What do you do if you are given more than one mole of a compound in a enthalpy cycle?
Multiply enthalpy value by number of moles there are of the compound
What is the equation to use with enthalpy of formation values?
Formation of products - formation of reactants
If arrow is not pointing down to elements, what do you do?
Change minus to a plus
Why is it not possible to calculate enthalpy of formation for carbon monoxide (CO g ) ?
Reaction represents incomplete combustion that cannot be stopped, goes to completion and forms CO2 (g)
How can we measure enthalpy change of formation of carbon monoxide ?
It is possible to measure enthalpy change of combustion for both carbon and carbon monoxide, so we take results from this using an enthalpy cyle
What is the equation to calculate enthalpy of combustion?
Combustion reactants - combustion products
The enthalpy change when reacting solids with water (hydration) can also be measured using Hess cycle, but why can this enthalpy change not be measured directly?
- difficult to measure temp risee of a solid
- impossible to react exact amount of water with the solid
- difficult to prevent the solid from dissolving in the water
What states are the halogens in?
F2 = g
Cl2 = g
Br2 = l
I2 = s
