The Measurement of Heat Transfer in Chemical Reactions Thermochemistry Calorimetry The Measurement of Heat Transfer in Chemical Reactions Thermochemistry branch of chemistry concerned with heat effects accompanying chemical reactions. Direct and indirect measurement of heat. Answer practical questions: why is natural gas a better fuel than coal, and why do fats have higher energy value than carbohydrates and protiens.

Conservation of Energy In 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, one form of which is heat, q In practice, the temperature is allowed to change and the heat that is lost or needed as the system returns to its original temperature is calculated.

q is signed 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 Heat of reaction in an isolated system produces a change in the thermal energy of the system. The causes a temperature change. In an non-isolated system the temperature remains constant and the heat is transferred to the surroundings. Top picture is CaO(s) + H2O(l) → Ca(OH)2 Bottome picture is Ba(OH)2·8H2O + 2NH4Cl(s)→BaCl2(s) + 2 NH3(aq) + 8 H2O(l)

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) DHrxn = 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 = msolncsolnDTsoln

Polystyrene Foam Cup Calorimeter A simple calorimeter Well-insulated; nearly isolated Measure temperature change of solution inside qrxn + qsoln = 0 Can be expanded to… nDHrxn + msolncsolnDTsoln = 0 Energy produced in the reaction + Energy absorbed by the medium = 0

Heat of Combustion of Fuels DHcombustion 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… nDHcomb + mwatercwaterDTwater  0 Energy produced by combustion + Energy absorbed by the water  0

Simple Combustion Calorimeter qrxn + qwater = 0 q = mcDT (used for solutions and water) q = nDH (used for reactions)

Notes for Lab This Week Total volume should be constant for all runs Accurate values for DH 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 Conclusion 2: Discussion 3: Data/Results and Experimental A: Introduction, Conclusion, Data/Results B: Discussion and Experimental