2. Pre-Lab Bring completed pre-lab to class Use net ionic equations Include pre-lab results in introduction section of report Results of pre-lab assignment are your accepted values Use only for determining percent error
3. Enthalpy & Heat Enthalpy (H) - Amount of heat change occurring during reaction Exothermic (heat released, DH < 0) Endothermic (heat absorbed, DH > 0) Heat (q) - Amount of energy transferred q=H under constant pressure
4. Calorimetry Measurement of heat changes Performed in a calorimeter which prevents heat loss from the system Constant volume calorimeter Constant pressure calorimeter
5. Calorimeter Used to prevent the escape of heat One version - Styrofoam cup in a beaker with a cardboard lid
6. q and DH q Units of Joules Extensive property Value only true for the experimental amounts of reactants and products DH Units of Joules/mole Intensive property Value based on the moles given by the coefficients of balanced equation
7. Relationship of q and DH WATCH the units!!! DH will be expressed as a negative or positive dependant on the direction of heat flow Negative: exothermic, heat released Positive: endothermic, heat consumed
8. q = heat, units of Joules m = mass, units of grams s= specific heat of water, units of DT = change in temperature, units of Initial temperature: Right before solutions are mixed Final temperature: Maximum temperature in data q = msDT
9. Finding Enthalpy Mix reactants Measure temperature change Calculate heat change Calculate change in enthalpy (DH)
10. Experimental Results Enthalpy values for Reactions 1, 2, and 3 Two experimental values for reaction 3 Value from running the reaction Using Hess’ law from reactions 1 & 2 Accepted values are those from pre-lab
11. Hess’s Law Enthalpy of a reaction does not depend on number of steps involved If the enthalpy of a set of reactions is known, use combination of reactions to find enthalpy
12. Hess’s Law : Example Determine the enthalpy of the reaction shown using the three reactions given. C(gr) + 2S(rh) CS2(l) DH (kJ/mole) C(gr) + O2(g) CO2(g),DH0rxn1= -393.5 S(rh) + O2(g) SO2(g),DH0rxn2= -296.4 CS2(l) + 3O2(g) CO2(g) + 2SO2(g)DH0rxn3= -1073.6 Does this add up to: C(gr) + 2S(rh) CS2(l)?
14. Experiment Tips Rinse and reuse styrofoam cups Save cups for next class Do not poke hole in cups with temperature probe Record exact volumes of each solution
15. Experiment Tips Push collect before mixing solutions Values of DT will vary for each of the three parts Press “Autoscale” button in LoggerPro to expand graph Don’t wait until the last minute to complete calculations Allow time to get help, if needed
