Solid potassium fluoride is dissolved in water in a perfect calorimeter. When 10.00 grams of potassium fluoride is dissolved in 120.0 grams of water initially at a temperature of 20.00 ºC, the temperature of the solution increases to 25.61 ºC. Calculate ΔHº for the reaction below in kJ/mol. KF (s) ➝ K+ (aq) + F – (aq) ΔHº= ? [You may assume that no heat escapes the calorimeter and that all solutions have the same specific heat capacity as pure water (4.184 J/g·ºC)] • Was energy released by the salt into the water or was energy absorbed by the salt from the water? Think about how the temperature of the water changed. What does this tell you about ΔH for the reaction taking place? Is the process of dissolving the salt endothermic or exothermic? What is the sign of ΔH? ΔH = ___ Energy does not escape the calorimeter. So, if energy is released by the reaction it must be take into (absorbed) by the water. If energy is absorbed by the reaction, it must have been taken out of the water. We must account for all of that energy. Remember, with energy (q), the sign tells us direction of the transfer. The magnitude of the energy change is the same, but its sign is different depending on whether we are looking at the water (solution) or the salt (reaction). qreaction = –qsolution. How do we relate energy (q) to temperature change (ΔT)? They are related though a property called the Specific Heat Capacity of the substance. Specific Heat Capacity is given the symbol "Cp" or sometimes just "S.H.". q = mass·Cp·ΔT or q = mass·S.H.·ΔT So, we can relate the temperature change for the solution to "q" for the chemical reaction taking place in the solution. When we measure the temperature in our calorimeter, we are measuring the temperature change for the solution, ΔTsolution. qsolution = mass·Cp·ΔTsolution and from above: qreaction = –qsolution The two equations above can be combined in order to directly relate the heat change for the reaction to the measured temperature change for the solution qreaction = –mass·Cp·ΔTsolution • With a "perfect" calorimeter, we assume that the calorimeter itself absorbs no heat and does not allow any energy to escape. Thus, all of the energy is transferred to or from the contents of the calorimeter. What is the "total mass" of the contents of the calorimeter? _______ • We need to know the specific heat capacity of the solution in the calorimeter. If the solution is sufficiently dilute (not much salt compared to water) then the specific heat capacity of the water with the salt dissolved in it (the solution) will be very close to that of pure water. For water Cp = 4.184 J/g·ºC • ΔTsolution was measured. What is the value of ΔTsolution in this experiment? ΔTsolution = _______ Now can calculate qreaction from the mass, Cp, and ΔT. Answer in Joules. qreaction = _____ J? • This value is NOT ΔH for the reaction.. ΔH is the quantity of heat transeferred per mol of salt used. This experiment did not use 1 mole of salt. It only used 10.00 grams of KF. How many moles of KF were actually used? Moles of KF = _____ moles of KF • ΔH is expressed as qreaction/(moles of salt used). It is usually expressed in units of kJ/mol. Calculate ΔH for the reaction. ΔH = _____ kJ
Thermochemistry
Thermochemistry can be considered as a branch of thermodynamics that deals with the connections between warmth, work, and various types of energy, formed because of different synthetic and actual cycles. Thermochemistry describes the energy changes that occur as a result of reactions or chemical changes in a substance.
Exergonic Reaction
The term exergonic is derived from the Greek word in which ‘ergon’ means work and exergonic means ‘work outside’. Exergonic reactions releases work energy. Exergonic reactions are different from exothermic reactions, the one that releases only heat energy during the course of the reaction. So, exothermic reaction is one type of exergonic reaction. Exergonic reaction releases work energy in different forms like heat, light or sound. For example, a glow stick releases light making that an exergonic reaction and not an exothermic reaction since no heat is released. Even endothermic reactions at very high temperature are exergonic.
Solid potassium fluoride is dissolved in water in a perfect calorimeter. When 10.00 grams of potassium fluoride is dissolved in 120.0 grams of water initially at a temperature of 20.00 ºC, the temperature of the solution increases to 25.61 ºC.
Calculate ΔHº for the reaction below in kJ/mol.
KF (s) ➝ K+ (aq) + F – (aq) ΔHº= ?
[You may assume that no heat escapes the calorimeter and that all solutions have the same specific heat capacity as pure water (4.184 J/g·ºC)]
• Was energy released by the salt into the water or was energy absorbed by the salt from the water? Think about how the temperature of the water changed. What does this tell you about ΔH for the reaction taking place? Is the process of dissolving the salt endothermic or exothermic? What is the sign of ΔH?
ΔH = ___
Energy does not escape the calorimeter. So, if energy is released by the reaction it must be take into (absorbed) by the water. If energy is absorbed by the reaction, it must have been taken out of the water. We must account for all of that energy. Remember, with energy (q), the sign tells us direction of the transfer. The magnitude of the energy change is the same, but its sign is different depending on whether we are looking at the water (solution) or the salt (reaction).
qreaction = –qsolution.
How do we relate energy (q) to temperature change (ΔT)? They are related though a property called the Specific Heat Capacity of the substance. Specific Heat Capacity is given the symbol "Cp" or sometimes just "S.H.".
q = mass·Cp·ΔT or q = mass·S.H.·ΔT
So, we can relate the temperature change for the solution to "q" for the
qsolution = mass·Cp·ΔTsolution
and
from above: qreaction = –qsolution
The two equations above can be combined in order to directly relate the heat change for the reaction to the measured temperature change for the solution
qreaction = –mass·Cp·ΔTsolution
• With a "perfect" calorimeter, we assume that the calorimeter itself absorbs no heat and does not allow any energy to escape. Thus, all of the energy is transferred to or from the contents of the calorimeter.
What is the "total mass" of the contents of the calorimeter? _______
• We need to know the specific heat capacity of the solution in the calorimeter. If the solution is sufficiently dilute (not much salt compared to water) then the specific heat capacity of the water with the salt dissolved in it (the solution) will be very close to that of pure water. For water Cp = 4.184 J/g·ºC
• ΔTsolution was measured. What is the value of ΔTsolution in this experiment?
ΔTsolution = _______
Now can calculate qreaction from the mass, Cp, and ΔT. Answer in Joules.
qreaction = _____ J?
• This value is NOT ΔH for the reaction.. ΔH is the quantity of heat transeferred per mol of salt used. This experiment did not use 1 mole of salt. It only used 10.00 grams of KF. How many moles of KF were actually used?
Moles of KF = _____ moles of KF
• ΔH is expressed as qreaction/(moles of salt used). It is usually expressed in units of kJ/mol. Calculate ΔH for the reaction.
ΔH = _____ kJ
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