Chapter 5: Thermochemistry Flashcards
internal energy (E)
sum of heat added/released from a system
(sum of all kinetic and potential energies in the system)
ΔE= q+w
ΔE= Efinal-Einitial
q (+)
head is transferred TO system FROM surroundings
-endothermic
q (-)
heat leaves system
-exothermic
w (+)
work is done ON system BY surroundings
w (-)
work is done ON surroundings BY system
enthalpy (H)
-flow of heat into/out of system under CONSTANT PRESSURE
H= E+PV
ΔH=ΔE+PΔV
ΔHrxn= Hproducts-Hreactants
-ΔH
exothermic
+ΔH
endothermic
+ΔE
net GAIN of energy by system
-ΔE
net LOSS of energy by system
When HEAT is added to a system, internal energy ____. ______.
increases
When volume increases, work is ___.
negative
When volume decreases, work is ___.
positive
q soln
- heat gained/lost by solution
- ΔT increase- exothermic (q rxn0)
enthalpy of formation
- the enthalpy change for the formation of a compound from its constituent elements
- ∆Hf
- f means substance is formed from its constituent elements
standard enthalpy change
- the enthalpy change when all the reactants and products are in their standard states
- ∆Hº
- º means standard-state conditions
standard state (of a substance)
Its pure form at 1 atm and 298 K/25 degrees celsius
standard enthalpy (heat) of formation
- change in enthalpy for the reaction that forms one mole of the compound from its elements
- all substances are in their standard states
- ∆Hºf
kinetic energy
the energy of motion
potential energy
energy stored by its position relative to other objects
force
any push or pull exerted on an object
heat
the energy transferred from a hotter object to a colder one
first law of thermodynamics
1) Energy is conserved
state function
function whose value depends ONLY on the present state of the system
non-state function
function whose value depends on the path the system took to reach the present state
pressure-volume work
The work involved in the expansion or compression of gases
The change in enthalpy (∆H) ALSO equals ____
- qp
- the heat qp gained or lost at constant pressure
State functions (examples)
1) enthalpy (H)
2) internal energy (E)
3) pressure
4) volume
Non-state functions examples)
-qp
enthalpy of reaction
- the enthalpy change that accompanies a reaction
- ∆Hrxn or ∆H
- also called ‘heat of reaction’
calorimetery
the measurement of heat flow
heat capacity
- The amount of heat required to raise its temperature by 1 K/1 °C
- determines the temperature change when an object absorbs a certain amount of heat
- C
molar heat capacity
heat capacity of one mole of substance (Cm)
specific heat capacity/ specific heat
heat capacity of one gram of a substance (Cs)
bomb calorimeter
studies combustion reactions
- Substance is placed in a small cup inside insulated vessel called ‘bomb’
- Heat released when combustion occurs is absorbed by water and other components of calorimeter, causing water temperature to rise
- Will measure change in water temp.
Hess’s law
If reaction is carried out in a series of steps, ∆H is the sum of the enthalpy changes of individual steps. (because enthalpy is state function)
