Calorimetry

Thermochemistry

The Measurement of Heat Transfer in Chemical Reactions

Conservation of EnergyIn interactions between a system and its surroundings the total energy remains constant:

Energy is neither created nor destroyed

qsystem + qsurroundings = 0

qsystem = -qsurroundings

Heats of Reaction

Heat of reaction, qrxn– The quantity of heat exchanged between a

system and its surroundings when a chemical reaction occurs within the system, at constant temperature.

The flame produces energy, oneform of which is heat, q

Heats of Reaction Exothermic reactions

– Produce heat, qrxn < 0

Endothermic reactions

– Absorb heat, qrxn > 0

Calorimeter

– A device for measuring quantities of heat

produced or absorbed in a chemical reaction

q is signed

Polystyrene Foam Cup Calorimeter A simple calorimeter

– Well-insulated; nearly isolated– Measure temperature change

of solution inside

qrxn + qsoln = 0

qrxn is extensive (units are J)

Hrxn = qrxn/n is intensive (units are J/mol)

Polystyrene Foam Cup Calorimeter A simple calorimeter

– Well-insulated; nearly isolated– Measure temperature change

of solution inside

qrxn + qsoln = 0

qsoln is extensive (units are J)

qsoln = msolncsolnTsoln

Polystyrene Foam Cup Calorimeter A simple calorimeter

– Well-insulated; nearly isolated– Measure temperature change

of solution inside

qrxn + qsoln = 0

Can be expanded to…

Energy produced in the reaction

Energy absorbed by the medium+ = 0

nHrxn + msolncsolnTsoln = 0

Heat of Combustion of Fuels

Hcombustion relates to the economy of the fuel as an energy source

Two ways to report:– qcombustion per gram of fuel

– qcombustion per mole of fuel

This week: Determine the amount of energy produced (per g and mol) for the combustion of– ethanol

– diesel fuel, C14H30 (similar to kerosene or lamp oil)

A Simpler Calorimeter A soda can and an “alcohol” burner.

– Not well insulated.

– Measure temperature change.

qrxn + qwater 0

Can be expanded to…

Energy produced by combustion

Energy absorbed by the water+ 0

nHcomb + mwatercwaterTwater 0

Simple Combustion Calorimeter

qrxn + qwater = 0

q = mcT (used for solutions and water)

q = nH (used for reactions)

Notes for Lab This Week Total volume should be constant for all runs Accurate values for H will not be determined…

…but relative values between two fuels will be

How will you know how many moles of fuel were burned?

How will you determine the quantity of heat absorbed by the water in the aluminum can calorimeter?

What Data Should I Collect? Mass of fuel burned Volume (or mass) of water heated Temperature change of water

This investigation, Author 1: Introduction and Conclusion2: Discussion3: Data/Results and Experimental

This investigation, AuthorA: Introduction, Conclusion, Data/ResultsB: Discussion and Experimental