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Experiment 5

Experiment 5. Thermocalorimetry of Acid-Base Reactions. 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

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Experiment 5

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  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 H++OH- 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 = -CH2O*mass*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?

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