Experiment 5

1
Experiment 5

• Thermocalorimetry
• of Acid-Base Reactions

2
Goals

• To determine the heat loss of a system during
  various acid-base reactions
• To use the heat loss to determine the enthalpy
  change (?H) for the systems
• To determine the limiting reagents for each
  system and the moles of water produced by each
• To determine the ?H per mole of water produces by
  each system
• To determine the average ?H per mole of water for
  our acid-base reactions

3
Hazards

• Acids Bases

4
What is Thermocalorimetry?

• Thermo heat
• Calorimeter a closed-system (no heat exchange
  with surroundings) device that is used to measure
  the internal energy change of a system
• What is our calorimeter?
• How are we going to determine the internal energy
  change?

5
Why Use a Styrofoam Cup?

• Insulator
• No heat exchange with surroundings
• Closed system (lids are styrofoam, too)
• Hole in lid allows for a small amount of heat
  exchange with the environment
• We assume perfect calorimeter
• Why closed system?
• Be sure to KEEP LIDS ON TO KEEP HEAT IN

6
The Reactions

• What types of reactions will be occurring in the
  calorimeter?
• Simple representation HA(aq)B(aq)?BH(aq)A-(aq)
  
• Chemical reactions that give off heat
  (exothermic)
• When you add an acid or base to water is there a
  reaction?
• Does it give off heat?
• Why?

7
What will we measure?

• Heat of reaction q
• Is it mass dependent?
• Does the temperature change
• Is this measurement dependent on anything else?

8
Heat Capacity, C

• Constant for a particular substance
• Tells how much heat (energy) is required to raise
  the temp. of a particular substance
• All of our solutions are aqueous (water)
• Therefore, we can use CH2O as an approximation of
  the C for all of our solutions
• C is actually slightly different because the
  solutions are not pure water
• The actual C is slightly lower because H2O has an
  extremely high C and addition of things makes it
  smaller.
• Why is CH2O so high?

9
Approximations

• Perfect calorimeter
• Heat capacity value
• Mass of each system
• We are not weighing the liquids
• We are measuring the volume
• Volume 25mL 25mL 50mL
• How do you change volume to mass?

10
Goal To Determine Affect of Limiting Reagent on
?H

• ?H ?Hproducts- ?Hreactants
• Is the amount of energy gained by the calorimeter
  the same as the amount of energy produced from
  the reaction?
• For all of our reactions HOH- ? H2O
• Because HA gives H and B gives OH- (LeChatelier)

11
Limiting Reagents

• Balance reaction equation (stoichiometry)
• Determine moles of each reactant
• Determine which one limits moles of water to
  produce
• Determine moles of water produced
• Calculate ?Hreaction/mol H2O

12
Example

• 25mL 3MHCl mixed with 25mL 3M NaOH
• HCL NaOH ? NaCl H2O
• You have Vol conc. You can find what?
• How many moles of water does this make?
• Because we know all of the reactions are going to
  produce water, do you expect the ?Hreaction/mol
  H2O to be the same for all of the rections?

13
One to watch out for

• Na2CO3 H2O ? ???
• Na2CO3 is diprotic
• Na2CO3 ? 2Na(aq) CO32-(aq)
• CO32-(aq) ? HCO3- OH-
• HCO3- H2O ? H2CO3 OH-
• H2CO3 is not real stable in H2O
• H2CO3 ? CO2(g) H2O (make good observation)
  (could affect your values)
• How many moles of acid will Na2CO3 react with?

14
Before Analyzing Experimental Data

• Calc Standard ?H/mol H2O for the general
  reaction H(aq) OH- ? H2O(l)
• Balance each reaction
• Determine limiting reagent for each reaction
• Determine moles of H2O made in each cup

15
Analyzing Experimental Data

• All calculations are listed in handout
• ?Hrxn -qcal contents -CH2Omass?T
• Then calc ?H/mol H2O made in each cup
• Then average the ?H/mol H2O for 7 reactions with
  standard deviation
• For comparison to standard value in discussion,
  remember that
• ?H ?Hproducts- ?Hreactants
• Compare experimental and actual using absolute
  error
• Write all of the calorimeter reactions in your
  notebook (if you need help, come see me)
• All calculations (for each) in your notebooks

16
Experimental Design Variables

• Do your amounts allow for you to produce a large
  enough ?T to give reliable results?