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 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

Hazards Acids & Bases – – –

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?

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

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? – –

What will we measure? Heat of reaction = q –Is it mass dependent? –Does the temperature change –Is this measurement dependent on anything else?

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 C H 2 O 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 H 2 O has an extremely high C and addition of things makes it smaller. Why is C H 2 O so high? –

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?

Goal: To Determine Affect of Limiting Reagent on  H  H =  H products -  H reactants 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 -  H 2 O Because HA gives H + and B gives OH - (LeChatelier)

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  H reaction /mol H 2 O

Example 25mL 3MHCl mixed with 25mL 3M NaOH HCL + NaOH  NaCl + H 2 O 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  H reaction /mol H 2 O to be the same for all of the rections? – –

One to watch out for Na 2 CO 3 + H 2 O  ??? Na 2 CO 3 is diprotic Na 2 CO 3  2Na + (aq) + CO 3 2- (aq) CO 3 2- (aq)  HCO OH - HCO H 2 O  H 2 CO 3 + OH - H 2 CO 3 is not real stable in H 2 O H 2 CO 3  CO 2(g) + H 2 O (make good observation) (could affect your values) How many moles of acid will Na 2 CO 3 react with?

Before Analyzing Experimental Data Calc Standard  H/mol H 2 O for the general reaction: H + (aq) + OH -  H 2 O (l) Balance each reaction Determine limiting reagent for each reaction Determine moles of H 2 O made in each cup

Analyzing Experimental Data All calculations are listed in handout  H rxn = -q cal contents = -C H 2 O *mass*  T Then calc  H/mol H 2 O made in each cup Then average the  H/mol H 2 O for 7 reactions with standard deviation For comparison to standard value in discussion, remember that  H =  H products -  H reactants 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

Experimental Design Variables Do your amounts allow for you to produce a large enough  T to give reliable results?