SCH4U1 Name: ______
Molar Enthalpy of Solvation and Neutralization
Purpose: To test the validity of the Hess’ Law by determining the enthalpy change for three chemical reactions.
Materials: calorimeter 100 mL graduated cylinder
thermometer scoopula
electronic balance watch glass
sodium hydroxide pellets 1.0 mol/L NaOH solution
0.50 mol/L HCl solution 1.0 mol/L HCl solution
Background: This experiment will measure and compare the quantity of energy absorbed or released in three chemical reactions. Three reactions will be compared:
1) NaOH (s) Na+(aq) + OH- (aq) ∆H1 = ?
2) NaOH(s) + H+(aq) + Cl- (aq) H2O + Na+ (aq) + Cl- (aq) ∆H2 = ?
3) Na+(aq) + OH- (aq) + H+(aq) + Cl- (aq) H2O + Na+(aq) + Cl- (aq) ∆H3 = ?
Method:
CAUTION: SODIUM HYDROXIDE IS CORROSIVE TO THE SKIN AND MAY CAUSE BLINDNESS IF IT GETS IN YOUR EYES. WEAR EYE PROTECTION AT ALL TIMES.
Part 1: Determination of the Enthalpy Change of Reaction 1
1) Using the graduated cylinder, measure out 200 mL of tap water
and place it in the calorimeter. Stir carefully with a thermometer until a constant temperature is reached (about room temperature). Measure this temperature as precisely as possible and record it.
2) Weigh a clean dry watch glass. Then weigh out approximately 4.00 g of sodium hydroxide pellets on the watch glass. Do this as quickly as possible since sodium hydroxide is very hygroscopic (it absorbs moisture when in contact with the open air).
3) Put the sodium hydroxide pellets into the water in the calorimeter. Place the thermometer into the solution and stir gently but continuously until the sodium hydroxide is dissolved. Record the highest temperature reached. Before proceeding to reaction 2, discard the solution and rinse the cup thoroughly with water.
Part 2 - Determination of the Enthalpy Change of Reaction 2
1) Repeat steps 1, 2 and 3 of Part 1, but substitute 200 mL of 0.50 mol/L HCl solution for the tap water in step 1.
2) Discard the solution and rinse the cup thoroughly with water.
Part 3 - Determination of the Enthalpy Change of Reaction 3
1) Using the graduated cylinder, measure 100 mL of 1.0 mol/L HCl into the calorimeter. Rinse out the graduated cylinder with water and then measure 100 mL of 1.0 mol/L NaOH. Both of these solutions should be at, or slightly below, room temperature. Check this with the thermometer (rinse and dry the thermometer before changing from one solution to the other). Record the average temperature of the two solutions.
2) Add the sodium hydroxide solution to the hydrochloric acid solution in the calorimeter. Mix quickly and record the highest temperature reached.
Observations:
Prepare this chart in your lab book.
Part 1 / Part 2 / Part 3Initial temperature of solution (Ti)
Final temperature of solution (Tf)
Temperature change (∆T = Tf – Ti)
Total Volume of Solution
Mass of sodium hydroxide
Volume of 1.0 mol/L NaOH
Volume of 1.0 mol/L HCl
Analysis:
1. Calculate the molar enthalpy change for reactions 1, 2 and 3 in terms of kJ/mol NaOH.
2. Using the experimental molar enthalpy results from reactions 1 and 3, determine the theoretical enthalpy change for reaction 2 using Hess’ Law (use the additivity of heats method which is the type that examines the individual step equations).
3. Calculate the percent error for your experimental result for reaction 2. Use your result from question 1 as the experimental value and your result from question 2 as the theoretical value.
Discussion:
1. State three possible sources of experimental error in this experiment. Be specific.
Do NOT state forms of human error (e.g. errors in measurement) as experimental error.
2. Suppose you used 8.00 g of NaOH in reaction 1:
a) How would this have affected the change in temperature?
b) How would this affect the number of kJ produced in the new experiment 1?
c) What effect would this change have on the resulting molar enthalpy value (DH1)?
Conclusion
1. Write a conclusion by restating the purpose and stating the result of your experiment.