Lecture Fourteen Flashcards
What is the equation for change in internal energy?
ΔU = q + w Where q = heat (ΔH) and w = work. In general: - Positive sign indicates a gain in energy by the system from its surroundings. - Negative sign indicates a loss of energy from the system to its surroundings. Other equations to remember - that make the above equation make sense: w = P * A * Δh = P * ΔV
What is the equation for enthalpy?
H = U + pV ΔH = ΔU + pΔV
How can ΔHc and ΔU be measured using different types of calorimeters?
Bomb calorimeter (measures ΔU):
q reaction = -Ccalorimeter *ΔT.
Measures ΔU at a constant volume.
Where ΔT is the observed chage in temperature and the calorimeter heat capacity, C, is measured from the observed ΔT from combustion of a standard compound (benzoic acid).
‘Coffee cup’ calorimeter (measures ΔH):
ΔH = q subscript p.
Measures ΔH at a constant pressure.
Note that ΔH does not equal ΔU.
The difference is that constant pressure conditions allow a volume to change. Most significant different when a gas is involved.
In calorimetry, what equation can be used to calculate the change in heat?
The specific heat capacity of a substance, C, is teh heat required to raise the temperature of a unit of mass of a substance by 1K. Unit - J/K/g.
Heat change (in J) = mass * C * ΔT.
Exaplin standard enthalpy.
ΔH values for chemical processes depends on a lot of factors including pressure, concentration and temperature.
Chemists have agreed ona set of standard states to make it easier to report and compare heat of reactions.
Most thermochemical equations are written for reactants and products with:
Pressure of 10^5 Pa (gasses).
Concentration of 1M (solution).
A temperature of 25 degrees C is often specified as well, but this is not part of the definition of standard states in thermochemistry.
The use of standard states is denoted by the superscript symbol ^o.
Explain the standard enthalpy of formation, ΔH subscript f ^o.
The energy change when a molecule forms from its constituent elements in their standard state.
The enthalpy of formation, ΔH subscipt f ^o, is the enthalpy change that accompanies the formation of one mole of a substance in its standard state from its consitient elements int ier standard states.
What is Hess’s law?
For some reactions, it si experimentally challenging to determine the ΔH subscript r value for the reaction.
However in many cases we can use some very simple arithmatic to help determine this value indirectly.
This is known as Hess’ law.
How can you calculate the enthalpy for a reaction?
