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 constant-pressure 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 successTECHNIQUE• 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!!

• Most strong acid / strong base neutralizations are exothermic:

NaOH + HX H2O + NaX; H < 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.

• However, some of the heat evolved in the reaction is absorbed by the calorimeter, so we have to account for that too:

rxn solutionq q

Constant P Calorimetry

rxn solution calorimeterq q q

Hot water (Ti = ~60oC)

50 mL

Cold water (Ti = ~25oC)

50 mL

Lukewarm water

Tf = ?? oC

energy flow

A. Calibrating the Calorimeter

Monitor the heat transferred from a sample of hot water to a sample of cold water and to the calorimeter

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:

hot cold calq q q cal hot coldq q q

Finding qhot and qcold:

hot hotq s m T hot hot f is V d T T

cold coldq s m T cold cold f is V d T T

s = 4.184 J/(goC)

Vhot, Vcold: the volumes you measured

dhot, dcold: From density of H2O calculatorTf: From regression analysis of temp data

A. Calibrating the Calorimeter (cont)The mathematical formula for the heat gained by the calorimeter is:

cal calC Tq

calcalC

T

q

What you just calculated in the calibration trial.

Same as the T 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 H for our acid-base reactions…

NaOH + HCl H2O + NaCl H < 0

NaOH + HNO3 H2O + NaNO3 H < 0

• The heat evolved in the reactions is absorbed by the solution and the calorimeter.

B. H for Acid-Base Rxns

soln calrxnq q q soln calrxnq q q

soln soln soln cal solnCrxnq s m T T

soln soln cal solnCs m T

ssoln = 4.184 J/(goC)

msoln = msoln + cal - mcal

Ccal : From calibration trial

soln f iT T T

Ti: avg of initial soln temps

Tf: from regression analysis

NaOH + HCl H2O + NaCl H < 0

NaOH + HNO3 H2O + NaNO3 H < 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!

B. H for Acid-Base Rxns (cont)

2# of moles of H O producedrxnqH