AP Chemistry – Class 33

Dates

• Periods E/ F – Wednesday, November 7th

Details from Quiz

• no need to write full sentences
• mole fractions and ratios – leave as decimals

Lab Takeaway #1:  Delta E = 0

• the system (substances dissolved in water) and the surroundings (water) exchange energy, facilitated by the calorimeter
• we measure the surroundings, not the system

Lab Takeaway #2: Delta H = Q (@ constant pressure)

• enthalpy is difficult to grasp (H = E + PV)
• most processes (and all in this class) are at constant pressure
• we want the DH of a process so we use the Q of the process, which is the opposite of the Q of the surroundings

Lab Takeaway #3: Q = mcDT

• simple formula, powerful implications
• “c” is the specific heat capacity  and is particular to a substance
• units follow the equation, not the other way around
• water is difficult to heat, lots of Q needed to make DT big

Lab Takeaway #4: DH is proportional to moles

• Finding heat flow, and therefore DH, shows that it is proportional to the moles of substance used
• the DH for a process can be treated the same as we would treat other molar quantities

New Idea: Work on a PV graph

• W = -PDV only applies if Pext is constant
• On a P vs V graph the work is area under the curve
• can determine the work using the graph
• practice the idea

Homework

• We will work on problems handed out during next class
• Students should finish by the end of the next class