7-EnthalpyOfReaction

General Chemistry LaboratoryUniversity of Kentucky

Determining the Enthalpy of a

Chemical Reaction

Pre-Lab

Bring completed pre-lab to classUse net ionic equations

Include pre-lab results in introduction section of report

Results of pre-lab assignment are your accepted valuesUse only for determining percent error

Enthalpy & Heat

Enthalpy (H) - Amount of heat change occurring during reactionExothermic (heat released, DH < 0)Endothermic (heat absorbed, DH > 0)

Heat (q) - Amount of energy transferred

q=H under constant pressure

Calorimetry

Measurement of heat changes Performed in a calorimeter which

prevents heat loss from the systemConstant volume calorimeterConstant pressure calorimeter

Calorimeter

Used to prevent the escape of heat

One version - Styrofoam cup in a beaker with a cardboard lid

q and DH

qUnits of JoulesExtensive propertyValue only true for the experimental

amounts of reactants and products DH

Units of Joules/mole Intensive propertyValue based on the moles given by the

coefficients of balanced equation

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

Hmolesq

D#

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 mixedFinal temperature: Maximum

temperature in data

q = msDT

Finding Enthalpy

Mix reactants Measure temperature change Calculate heat change Calculate change in enthalpy (DH)

Experimental Results

Enthalpy values for Reactions 1, 2, and 3

Two experimental values for reaction 3Value from running the reactionUsing Hess’ law from reactions 1 & 2

Accepted values are those from pre-lab

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

Hess’s Law : Example

DH (kJ/mole) C(gr) + O2(g) CO2(g), DH0

rxn1= -393.5 S(rh) + O2(g) SO2(g), DH0

rxn2= -296.4CS2(l) + 3O2(g) CO2(g) + 2SO2(g) DH0

rxn3= -1073.6

Does this add up to: C(gr) + 2S(rh) CS2(l)?

Determine the enthalpy of the reactionshown using the three reactions given.C(gr) + 2S(rh) CS2(l)

C(gr) + 2S(rh) CS2(l)

C(gr) + O2(g) CO2(g) S(rh) + O2(g) SO2(g) CS2(l) + 3O2(g) CO2(g) + 2SO2(g) Therefore, rewrite equations C(gr) + O2(g) CO2(g) 2S(rh) + 2O2(g) 2SO2(g) 2SO2(g) + CO2(g) CS2(l) + 3O2(g) C(gr) + 2S(rh) CS2(l)DH0

rxn = DH0rxn1 + 2DH0

rxn2 - DH0rxn3

= 87.3 kJ/mol

2 x-1 x

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

Experiment Tips

Push collect before mixing solutions Values of DT will vary for each of the

three partsPress “Autoscale” button in LoggerPro

to expand graph Don’t wait until the last minute to

complete calculationsAllow time to get help, if needed