u1: hess law Flashcards
1
Q
when is it impractical to use calorimetry
A
- reaction occurs violently
- reaction occurs too slowly (rust)
2
Q
how do we find heat lost/gained without the use of a calorimeter?
A
calculate ∆Hr using Hess’s law and accumulated data
3
Q
hess’ law
A
enthalpy change for a chemical/physical process is only dependent on initial and final conditions and is thus path independent
4
Q
what is the enthalpy change of a multi-step process
A
- sum of enthalpy changes in all individual steps
- ∆Hrxn = ∆Hstep 1 + ∆Hstep 2…
5
Q
algebraic representation of hess’s law
A
∆Htargetreaction = Σ∆Hknownreactions
6
Q
rules of hess’s law
A
- if you reverse a chemical reaction, you must reverse sign of ∆H
- if you multiply coefficients in a reaction, you must multiply ∆H by the same factor
7
Q
∆H°f
A
- standard enthalpy of formation
- the amnt. of energy absorbed or released when one mole of a compound is formed from its elements in their standard (most stable) state at SATP
8
Q
equation to find enthalpy change
A
∆Hrxn = Σ n∆H°f (products) - Σ n∆H°f (reactants)
9
Q
what is the value of standard enthalpy of formation (∆H°f)
of an element in its standard state?
A
zero
