Thermochemistry. n Thermochemistry is the study of _________________ during chemical reactions.
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Transcript of Thermochemistry. n Thermochemistry is the study of _________________ during chemical reactions.
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Thermochemistry
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Thermochemistry is the study of _________________ during chemical reactions.
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Energy
Capacity to do ______________ or supply ______________
2 Types of Energy – ______________ - stored energy– ______________ - energy of motion
Energy is weightless, odorless, and tasteless
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Energy
Kinds of ______________ and their arrangements in a substance determines the amount of ______________ stored in that substance.
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Heat Energy
Energy ______________ from one object to another causing a temperature change.
It cannot be detected - only the ______________ by heat can be measured.
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Heat Energy
Thermometers measure the average ______________, not heat.
Heat flows from a ______________ object to a ______________ object.
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Heat Energy
All chemical reactions and changes in physical states have either an ______________ of heat or a ______________ of heat.
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Heat Energy
______________ - What we focus on. ______________ - everything else. Heat flow is given in relation to the
______________
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System and Surroundings
The ______________ includes the molecules we want to study (here, the hydrogen and oxygen molecules).
The ______________ are everything else (here, the cylinder and piston).
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Heat Energy
Law of Conservation of Energy - energy cannot be ______________ nor ______________, it just changes from one form to another.
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2 Processes of Heat Energy
______________ - a process in which the system ______________ heat.– Value is ______________.– Symbol - +H
______________ - a process in which the system ______________ heat.– Value is ______________.– Symbol - -H
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Units of Heat
Calorie - food calorie ______________ - quantity of heat
required to raise the temperature of 1 gram of water 1°C
Joule - more commonly used (J)– 1 C = 1000 cal– 4.184 J = 1 cal
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Heat Capacity
The amount of energy needed to ______________ the temperature of an object 1°C
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Specific Heat
Amount of ______________ needed to raise 1 gram of a substance 1°C– q = m • C • T– q = heat change– m = mass– C = specific heat T = change in temperature
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Calorimetry
Measurement of heat changes ______________ - heat content of a
system at constant pressure Heat released = Heat absorbed ______________ is the device used to
measure the change
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Calorimeter
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qlost = qgained
If q = m • C • T, then:
(m • C • T)lost = (m • C • T)gained
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How much heat is needed to warm 250 g of water (about 1 cup) from 22°C (about room temperature) to near its boiling point, 98°C? The specific heat of water is 4.18 J/g•ºC.
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Large beds of rocks are used in some solar-heated homes to store heat. Assume that the specific heat of the rocks is 0.082 J/g-K. Calculate the quantity of heat absorbed by 50.0 kg of rocks if their temperature increases by 12.0°C.
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In the laboratory, a student heats a 100 g sample of metal to 101.5ºC. The student then adds the metal to a calorimeter that has 100 mL of water at 22.5C. Calculate the specific heat of the metal if the final temperature of the metal-water mixture is 38.7C.
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Thermochemical Equations
An equation that includes the ______________.
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Reactant
Heat as a reactant will give a ______________ q value.
Heat as a reactant will be an ______________ process.
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Product
Heat as a product will give a ______________ q value.
Heat as a product will be an ______________ process.
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2H2O(l) 2H2(g) + O2(g) Hrxn = 572 kJ
572 kJ + 2H2O(l) 2H2(g) + O2(g)
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Al2O3(s) 2Al(s) + 3/2 O2(g) Hrxn = -1676 kJ
Al2O3(s) 2Al(s) + 3/2 O2(g) + 1676 kJ
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Heat of Reaction
Conditions are standard conditions.– 1 atm or 101.3 kPa and 25°C
Hrxn
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Heat of Combustion
Complete ______________ of 1 mole of a substance.
Hcomb
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Heat of Fusion
Heat ______________ by one mole of a solid to change it to a liquid.
Hfus
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Heat of Solidification
Heat ______________ by one mole of a liquid to change it to a solid.
Hsolid
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Hfus = -Hsolid
Same process just in opposite direction.
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Heat of Vaporization
Amount of heat needed to change 1 mole of a ______________ to a ______________.
Hvap
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Heat of Condensation
Amount of heat needed to change 1 mole of ______________ to a ______________.
Hcond
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Hvap = -Hcond
Same process just in opposite direction
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Heat of Solution
Change in heat when dissolving a ______________ in a solvent.
Hsoln
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Hess’s Law of Summation
Hess’s law states that “If a reaction is carried out in a series of steps, H for the overall reaction will be equal to the sum of the enthalpy changes for the individual steps.”
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If we know the individual reactions that make up the final equation, we can determine the Hrxn for the final equation.
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Individual Steps
C3H8 (g) 3C(graphite) + 4H2 (g)
3C(graphite) + 3O2 (g) 3CO2 (g)
4H2 (g) + 2O2 (g) 4H2O (l)
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C3H8 (g) 3C(graphite) + 4H2 (g)
3C(graphite) + 3O2 (g) 3CO2 (g)
4H2 (g) + 2O2 (g) 4H2O (l)
_______________________________
C3H8(g) + 5O2(g) 3CO2(g) + 4 H2O(l)
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Standard Heats of Formation
An enthalpy of formation, Hf, is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms.
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Standard Heats of Formation
Standard enthalpies of formation, Hf, are measured under standard conditions (25°C and 1.00 atm pressure).
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When we do not know the individual steps of a reaction, we can use the standard heats of formation to calculate H.
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients.
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C3H8(g) + 5O2(g) 3CO2(g) + 4 H2O(l)
H = [3(-393.5 kJ) + 4(-285.8 kJ)] - [1(-103.85 kJ) + 5(0 kJ)]
= [(-1180.5 kJ) + (-1143.2 kJ)] - [(-103.85 kJ) + (0 kJ)]
= (-2323.7 kJ) - (-103.85 kJ)
= -2219.9 kJ