Thermochemistry. The study of heat changes in chemical reactions Exothermic: reactions that release...
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Transcript of Thermochemistry. The study of heat changes in chemical reactions Exothermic: reactions that release...
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Thermochemistry
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Thermochemistry
• The study of heat changes in chemical reactions
• Exothermic: reactions that release heat• Endothermic: reactions that absorb heat• Enthalpy: the heat content of a system at
constant pressure
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Units of Heat
• 1 Calorie = 1 kilocalorie = 1000 calories
• 1 J = 0.239 cal
• 4.186 J = 1 cal
Food Calorie
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Heat Capacity
•The heat capacity of an object depends on:–mass –chemical composition.
•The amount of heat needed to increase the temperature of an object exactly 1°C is the heat capacity of that object.
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Specific Heat
q = mcΔTWhere:
q = heat (energy)m = massc = specific heatΔT= temperature change
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Specific Heat of Substances
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Why is the sand hot and the water cool at the beach?
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Jet Stream
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Enthalpy
• The heat content of a system at constant pressure is the same as a property called the enthalpy (H) of the system.
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Calorimetry• The measurement of
heat changes for physical and chemical processes
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Calorimetry
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CalorimetryTwo Key Equations
1. qreaction = -qsurroundings
qr = -qs
2. q = mcΔT = ΔHWhere: m = mass
c = specific heatΔT= temperature
change
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Calorimetry Example
1. 50 mL of water is placed in a beaker. A piece of calcium is weighed and placed in the cup. The temperature change of the water is measured. Calculate the heat released to the water by the calcium. Note: c = 4.184 J/g°C
2. Calculate the heat released per mole of calcium based on the previous results
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Thermochemistry
• Exothermic reaction: Heat is a productC3H8 + 5O2 → 3CO2 + 4H2O + 2043 kJ
ΔH = -2043kJ
H
Reaction Progress (t)
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Thermochemistry
• Endothermic reaction: Heat is a reactant
C + H2O + 113 kJ → CO + H2
ΔH = +113kJ
H
Reaction Progress (t)
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Enthalpy (ΔH)• Heat content of a system at constant pressure
• ΔH = Hproducts - Hreactants
• ΔH◦ is the standard enthalpy reported at 25 degrees C and 1 atm.
Sign of Sign of ΔΔH H ProcessProcess HeatHeat
++ endothermicendothermic absorbedabsorbed
-- exothermicexothermic releasedreleased
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Standard Heats of Formation
•For a reaction that occurs at standard conditions, you can calculate the heat of reaction by using standard heats of formation.
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Standard Heats of Formation
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Entropy and Reaction Tendency
•There is a tendency in nature to proceed in a direction that increases the randomness of a system.
•A random system is one that lacks a regular arrangement of its parts.
•This tendency toward randomness is called entropy.
•Entropy, S, can be defined in a simple qualitative way as a measure of the degree of randomness of the particles, such as molecules, in a system.
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Standard Entropy Changes for Some Reactions
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Entropy
•In a solid, the particles are in fixed positions, and we can easily determine the locations of the particles.
•In a liquid, the particles are very close together, but they can move around. Locating an individual particle is more difficult. The system is more random, and the entropy is higher.
•In a gas, the particles are moving rapidly and are far apart. Locating an individual particle is much more difficult, and the system is much more random. The entropy is even higher.
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Entropy
• Absolute entropy, or standard molar entropy, of substances are recorded in tables and reported in units of kJ/(mol•K).
• Entropy change, which can also be measured, is defined as the difference between the entropy of the products and the reactants.
• An increase in entropy is represented by a positive value for ∆S, and a decrease in entropy is represented by a negative value for ∆S.
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Free Energy•Processes in nature are driven in two
directions: toward least enthalpy and toward largest entropy.
•As a way to predict which factor will dominate for a given system, use this equation:
∆G0 = ∆H0 – T∆S0
•This combined enthalpy-entropy function is called the free energy, G, of the system; it is also called Gibbs free energy.
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Relating Enthalpy and Entropy to Spontaneity
∆G0 = ∆H0 – T∆S0
A spontaneous reaction is one that goes to completion unaided
Example: rusting, ice melting