Presentation on theme: "The Measurement of Heat Transfer in Chemical Reactions"— Presentation transcript:
1 The Measurement of Heat Transfer in Chemical Reactions ThermochemistryCalorimetryThe Measurement of Heat Transfer in Chemical ReactionsThermochemistry 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.
2 Conservation of Energy In interactions between a system and its surroundings the total energy remains constant:Energy is neither created nor destroyedqsystem + qsurroundings = 0qsystem = -qsurroundings
3 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, qIn 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.
4 q is signed Heats of Reaction Exothermic reactions Produce heat, qrxn < 0Endothermic reactionsAbsorb heat, qrxn > 0CalorimeterA device for measuring quantities of heat produced or absorbed in a chemical reactionq is signedHeat 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)2Bottome picture is Ba(OH)2·8H2O + 2NH4Cl(s)→BaCl2(s) + 2 NH3(aq) + 8 H2O(l)
5 Polystyrene Foam Cup Calorimeter A simple calorimeterWell-insulated; nearly isolatedMeasure temperature change of solution insideqrxn + qsoln = 0qrxn is extensive (units are J)DHrxn = qrxn/n is intensive (units are J/mol)
6 Polystyrene Foam Cup Calorimeter A simple calorimeterWell-insulated; nearly isolatedMeasure temperature change of solution insideqrxn + qsoln = 0qsoln is extensive (units are J)qsoln = msolncsolnDTsoln
7 Polystyrene Foam Cup Calorimeter A simple calorimeterWell-insulated; nearly isolatedMeasure temperature change of solution insideqrxn + qsoln = 0Can be expanded to…nDHrxn msolncsolnDTsoln = 0Energy produced in the reaction+Energy absorbed by the medium= 0
8 Heat of Combustion of Fuels DHcombustion relates to the economy of the fuel as an energy sourceTwo ways to report:qcombustion per gram of fuelqcombustion per mole of fuelThis week: Determine the amount of energy produced (per g and mol) for the combustion ofethanoldiesel fuel, C14H30 (similar to kerosene or lamp oil)
9 A Simpler Calorimeter + A soda can and an “alcohol” burner. Not well insulated.Measure temperature change.qrxn + qwater 0Can be expanded to…nDHcomb mwatercwaterDTwater 0Energy produced by combustion+Energy absorbed by the water 0
10 Simple Combustion Calorimeter qrxn + qwater = 0q = mcDT (used for solutions and water)q = nDH (used for reactions)
11 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 beHow 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?
12 What Data Should I Collect? Mass of fuel burnedVolume (or mass) of water heatedTemperature change of waterThis investigation, Author1: Introduction and Conclusion2: Discussion3: Data/Results and ExperimentalA: Introduction, Conclusion, Data/ResultsB: Discussion and Experimental