Enthalpy of a Reaction

1
Enthalpy of a Reaction
2
Experimental Objective

• Determine the enthalpy of reaction of the
  neutralization reaction between sodium hydroxide
  (strong base) and hydrochloric acid (strong acid)
• NaOH(aq) HCl(aq) ? NaCl(aq) H2O(l) DH ??

3
Procedure Overview

• Known quantities of reactant solutions reacted in
  Styrofoam cup calorimeter
• Temperature change of the reaction solution
  observed
• Enthalpy change of reaction determined from
  observed temperature change

4
Procedure Details Obtain Reactants

• Work in pairs
• Obtain 100 mL of NaOH record molarity
• Fill buret with NaOH (use proper technique)
• Obtain 100 mL of HCl record molarity
• Fill second buret with HCl (use proper technique)

5
Procedure DetailsPrepare Styrofoam cup
Calorimeter

• Place Styrofoam cup in beaker
• Stability
• Insulation (leave space between bottom of cup and
  beaker)

6
Procedure DetailsReact NaOH and HCl

• Deliver 20.00 mL of HCl from the buret directly
  into calorimeter
• Record temperature of HCl solution
• Deliver 20.00 mL of NaOH from the buret directly
  into calorimeter while stirring with thermometer
  (reaction is fast!!)
• Record maximum temperature of reaction solution
• Record actual initial and final buret readings

7
Procedure DetailsRepeat Procedure three times

• Rinse and dry thermometer
• Rinse and dry calorimeter
• Discard waste down drain
• Reactants dilute NaOH and dilute HCl
• Product NaCl solution
• Run water to flush sink drain

8
Clean-up and Waste Handling

• Discard waste down drain
• Reactants dilute NaOH and dilute HCl
• Product NaCl solution
• Run water to flush sink drain
• Return Styrofoam cup to end of the bench top
• Return burets to stockroom window

9
Data AnalysisCalculate the Enthalpy of Reaction

• Calculate heat transferred to solution
• total volume of solution volume of HCl plus
  volume of NaOH
• Calculate moles reacted
• Identify limiting reactant
• Moles of NaOH (volume NaOH) (molarity NaOH)
• Moles of HCl (volume HCl) (molarity HCl)
• moles of limiting reactant Moles reacted

10
Data AnalysisCalculate the Enthalpy of Reaction
cont.

• Calculate magnitude of enthalpy of reaction
• Assign () or (-) sign to enthalpy of reaction
• Reaction evolves heat - Exothermic DHlt0
• Reaction absorbs heat - Endothermic DHgt0

11
Data AnalysisCompare experimental and
theoretical values

• Calculate theoretical enthalpy of reaction using
  Hesss Law
• NaOH(aq) HCl(aq) ? NaCl(aq) H2O(l) DH ??
• DHoreaction S (moles of product)(DHof
  product) - S (moles of reactant)(DHof
  reactant)
• coefficients of the balanced chemical
  equation
• HCl(aq) DHof, -167.2 kJ/mole
• NaOH(aq) DHof, -469.6 kJ/mole
• NaCl(aq) DHof, -407.1 kJ/mole
• H2O(l) DHof, -285.83 kJ/mole
• Calculate percent error.