Transcript Exp 6: Calorimetry

Plan for Tues/Wed, 4/5 Nov 08
• Turn in Exp 5 Report and Exp 6 Pre-lab
• Today: Exp 6, Calorimetry
• Purpose:
– PART A: To calibrate (determine the heat capacity of) a constantpressure calorimeter by monitoring the heat transferred between two
samples of water at different temperatures.
– PART B: To use your calibrated calorimeter to determine the heat of
reaction of two acid-base neutralization reactions:
HCl(aq) + NaOH(aq)  H2O(l) + NaCl(aq)
HNO3(aq) + NaOH(aq)  H2O(l) + NaNO3(aq)
• THIS EXPERIMENT WILL BE WRITTEN UP AS A FORMAL LAB
REPORT.
• NO LAB NEXT WEEK.
Tips for success
TECHNIQUE
• The Vernier systems are EXPENSIVE.
• BE CAREFUL when placing the the LabQuest probes in your calorimeter…they
are sharp and can pierce the bottom of your styrofoam cup.
• MAKE SURE to keep your probe tip completely submerged in the
water/solution, but DO NOT let it touch the bottom of the cup.
• DON’T FORGET TO SAVE YOUR DATA.
PROCEDURE / DATA ANALYSIS
• In your calculations, BE VERY CAREFUL with your plus and minus signs!!!
• Also, BE VERY CAREFUL with your significant figures!!!
WASTE
• All solutions go down the sink, accompanied by 10-20 seconds of tap water.
• You may throw your internal cup in the trash. Return the external cup to the
stockroom.
SAFETY
• HCl, HNO3, and NaOH are toxic and corrosive!!
• Wipe down your entire work area before you leave!!
• Wash your hands with soap and water!!
Constant P Calorimetry
• Most strong acid / strong base
neutralizations are exothermic:
NaOH + HX  H2O + NaX; DH < 0
• The heat evolved in these reaction
increases the average Ek of the water
molecules, leading to an increase in the
temperature of the solution.
• In a perfectly insulated calorimeter, the
heat lost by the reaction is exactly equal
to that gained by the solution.
 q rxn   q solution
• However, some of the heat evolved in
the reaction is absorbed by the
calorimeter, so we have to account for
that too:
 q rxn    q solution  q calorim eter

A. Calibrating the
Calorimeter
Monitor the heat transferred from a
sample of hot water to a sample of
cold water and to the calorimeter
Cold water
(Ti = ~25oC)
Hot water
(Ti = ~60oC)
50 mL
energy flow
50 mL
Lukewarm
water
Tf = ?? oC
A. Calibrating the Calorimeter (cont)
The heat lost by the hot water is equal to the heat
gained by the cold water and the calorimeter:
 q h o t    q co ld  q cal 
  q cal    q h o t  q co ld 
Finding qhot and qcold:
q hot  s  m hot  D T  s   V hot d hot   T f  Ti 
q cold  s  m cold  D T  s   V cold d cold   T f  Ti 
s = 4.184 J/(goC)
Vhot, Vcold: the volumes
you measured
dhot, dcold: From density
of H2O calculator
Tf: From regression
analysis of temp data
A. Calibrating the Calorimeter (cont)
The mathematical formula for the heat gained by
the calorimeter is:
q cal  C cal D T
 C cal 
q cal
DT
What you just
calculated in the
calibration trial.
Same as the DT for
the cold water in the
calibration trial.
Now that we have determined the heat capacity of
the calorimeter (aka the “calorimeter constant”), we
can experimentally determine DH for our acid-base
reactions…
B. DH for Acid-Base Rxns
NaOH + HCl  H2O + NaCl
NaOH + HNO3  H2O + NaNO3
DH < 0
DH < 0
• The heat evolved in the reactions is absorbed by the solution and the
calorimeter.
 q rxn    q so ln  q cal 
q rxn    q so ln  q cal 
q rxn    s so ln m so ln D Tso ln  C cal D Tso ln

   s so ln m so ln  C cal  D Tso ln
ssoln = 4.184 J/(goC)
msoln = msoln + cal - mcal
Ccal : From calibration trial
D Tsoln  T f  Ti
Tf: from regression
analysis
Ti: avg of initial
soln temps
B. DH for Acid-Base Rxns (cont)
NaOH + HCl  H2O + NaCl
NaOH + HNO3  H2O + NaNO3
DH < 0
DH < 0
• The enthalpy of reaction in each case is reported as kJ per mole of
water produced.
• This means you will have to perform a limiting-reactant calculation
based on the moles of NaOH and the acid.
THIS means n = M*V calculations!
DH 
q rxn
# of m oles of H 2 O produced