Sample Problems Flashcards
How much PV work is performed by a system that expands its volume by given a volume and external pressure?
Use w = -(P x deltaV)
Negative work doesn’t make sense in this context
Liter-atmospheres is not a convenient unit of energy
How much PV work is performed by a system that expands its volume by 3.20 L against an external pressure of 0.952 atm?
What are the sign conventions for q, w, deltaEsystem?
POS q + POS w ==> POS internal energy of system
NEG q + NEG w ==> NEG internal energy of the system
POS q + NEG w ==> depends on the sizes (math)
NEG q + POS w ==> depends on the sizes (math)
What does it mean for q when its POS and NEG?
What it does mean for w when its POS and NEG?
For q:
+ means system gains heat
– means system loses heat.
For w:
+ means work done on system;
– means work done by system.
How do you derive w from external pressure?
What if the “system” is a chemical reaction? How does this relate to delta E reaction?
A manufacturer claims that its new dietetic dessert has “fewer than 10 Calories per serving.” To test the claim, a chemist at the Department of Consumer Affairs places one serving in a bomb calorimeter and burns it in O2 (the heat capacity of the calorimeter = 8.151 kJ/K). The temperature increases 4.937 oC. Is the manufacturer’s claim correct?
When 1.010 g of sucrose (C12H22O11) undergoes combustion in a bomb calorimeter, the temperature rises from 24.92 ºC to 28.33 ºC. Find DErxn for the combustion, if Ccal = 4.90 kJ/K.
Where would you use enthalpy?
- Reactions that do not involve gases.
- Reactions in which the number of moles of gas does not change.
- Reactions in which the number of moles of gas does change but q is >>> PDV.
A standard BBQ grill propane tank contains 13.2 kg of propane. How much heat (in kJ) would come from the complete combustion of all of the propane in the tank?
What is constant pressure calorimetry?
We can measure deltaHrxn of aqueous reactions in a “coffee-cup calorimeter”
deltaHrxn = qrxn = -qsolution
What is one way to find the mass for calculation qsoln?
What is the formula for qsoln?
density = mass/volume
mass = density x volume
then use the mass for the qsoln equation
qsoln = mass x specific heat x deltaT
How do you find deltaHrxn using qsoln and molar amount?
Step 1: find the limiting reactant
One way is to use the molarity and volume if the mass isnt given
(molarity) x (volume) = moles of substance
the lower one is the limiting reactant
Step 2: plug in -qsoln
deltaHrxn = -qsoln = (-qsoln)/(limiting reactant mooles amount) x (kj/10^3J) = FINAL IN JOULES
What is Hess’s Law?
the change in enthalpy for a stepwise process is the sum of the steps
How do you manipulate the equation of deltaHrxn?
Because deltaHrxn is a state function, what can you do with the equation?
IF the equation is reversed, so is the sign of deltaHrxn?
How do you find standard enthalpy of formation?
How do you use stoichiometry to find the delta Hrxn from the deltaHformation values?
How do you find the limiting reactant given the mass of two compounds?
How do you use the limiting reactant to calculate the amount of heat that can be formed by reacting the two mass compounds?
Finding the limiting reactant
Find the delta Hrxn from the deltaHformation values?
then take the original mass of the limiting reactant x molar mass x stoichiometric ratio x deltaHrxn
