MHS Chemistry
Hess’s Law

Introduction:

If            Reaction # 1 + Reaction #2 = Reaction #3
Then     DEnthalpy of #1 + DEnthalpy of #2 = DEnthalpy of #3

Specifically in this lab:
    Reaction #1: The dissolving of solid sodium hydroxide in water.
    NaOH(s)  ®  Na+(aq) + OH-(aq)

    Reaction #2: The reaction of sodium hydroxide solution with hydrochloric acid solution.
    H+(aq)  +  OH-(aq)®  H2O(l)

    Reaction #3: The reaction of solid sodium hydroxide and hydrochloric acid solution.
    NaOH(s)  +  H+(aq)®  Na+(aq)  +  H2O(l)

Verify for yourself that Reaction #1 + Reaction #2 = Reaction #3.
 
 
 
 
 
 

Materials:
Styrofoam cups solid sodium hydroxide 250 mL beaker
thermometers 0.50 Molar NaOH solution weighing trays
100 mL graduated cylinder 0.25 Molar HCl solution goggles
balance 0.50 Molar HCl solution apron

Safety:


Procedure:
Reaction 1: The dissolving of sodium hydroxide.

  1. Put 200 mL of distilled water into the Styrofoam cup. Measure and record its temperature.
  2. Use a weighing boat to carefully measure and record the mass of about 2 grams of solid sodium hydroxide, NaOH. Do this as quickly as possible because it will quickly absorb moisture.
  3. Transfer the solid sodium hydroxide into the water in the cup. Stir gently with the thermometer until the solid is dissolved and record the highest temperature reached.
  4. Discard the solution by carefully pouring it down the drain followed by plenty of water. Rinse the cup thoroughly with water.
Reaction 2:  The reaction of aqueous sodium hydroxide with hydrochloric acid.
  1. Carefully measure 100 mL of 0.50 mol/L hydrochloric acid solution into the Styrofoam cup and 100 mL of 0.50 mol/L sodium hydroxide solution into a 250 mL beaker.
  2. Record the temperatures and volumes of each solution.
  3. Add the sodium hydroxide to the acid solution in the Styrofoam cup. Stir the mixture with the thermometer and record the highest temperature reached.
  4. Discard the solution by pouring it down the drain. Rinse the cup and beaker thoroughly.
Reaction 3:  The reaction of solid sodium hydroxide and hydrochloric acid.
  1. Put 200 mL of 0.25 mol/L hydrochloric acid into the Styrofoam cup. Measure and record its temperature.
  2. Use a weighing boat to carefully measure and record the mass of about 2 grams of solid sodium hydroxide, NaOH. Do this as quickly as possible because it will quickly absorb moisture.
  3. Transfer the solid sodium hydroxide into the solution in the cup. Stir gently with the thermometer until the solid is dissolved and record the highest temperature reached.
  4. Discard the solution by pouring it down the drain. Rinse the cup thoroughly.


Data:
Reaction 1:
 
Initial temperature of H2O __________
Mass of empty weighing tray __________
Mass of weighing tray and NaOH(s) __________
Final temperature of H2O __________

Reaction 2:
 
Initial temperature of HCl(aq) __________
Volume of HCl(aq) __________
Initial Temperature of NaOH(aq) __________
Volume of NaOH(aq) __________
Final temperature of mixture __________

Reaction 3:
 
Initial temperature of HCl(aq) __________
Mass of empty weighing tray __________
Mass of weighing tray and NaOH(s) __________
Final temperature of mixture __________

Calculations:
Show all your work on a separate sheet of paper. Copy the answers onto this page.

Recall that:
Heat absorbed by H2O = (mass of H2O) (Dtemp of H2O) (4.184 Joules/goC)
            q = mDTCp

Assume that all the solutions in this lab are dilute enough so their specific heat capacities and densities are the same as water.
 
 
Reaction 1
Moles of NaOH used __________
Temperature change __________
Heat released by the reaction __________
Heat released per mole of NaOH dissolved __________
NaOH(s)  ®  Na+(aq) + OH-(aq)      DH = ? __________

 
Reaction 2
Moles of NaOH used __________
Moles of HCl used __________
Temperature change __________
Heat released by the reaction __________
Heat released per mole of H2O produced __________
H+(aq) + OH-(aq)  ® H2O(l)       DH = ? __________

 
Reaction 3
Moles of NaOH used __________
Moles of H+ used (= moles of HCl used) __________
Temperature change __________
Heat released by the reaction __________
Heat released per mole of H2O produced __________
H+(aq) + NaOH(s)  ® Na+(aq) + H2O(l)       DH = ? __________

Summary:

If            Reaction # 1 + Reaction #2 = Reaction #3

Then     DH of #1 + DH of #2 = DH of #3

Does your data agree?
 
 
 
 

[Hess's Law score sheet][Chapter 12 notes][MHS Chem page]