Thermochemistry and Thermodynamics
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Transcript of Thermochemistry and Thermodynamics
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Thermochemistry and ThThermochemistry and Thermodynamicsermodynamics
Chapter 5Chapter 5
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Terms and Definitions• Thermodynamics: Study of the exchange of heat, energy and work between a system and its surroundings.
• System: that part of the universe of interest (reaction vessel, etc.)
• Surroundings: rest of the universe.
open system
mass & energyExchange
closed system
energy
isolated system
nothing
water vapor
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State and State Functions
• State: that condition in which all variables are fixed and unvarying.
• State Functions: variables (properties) whose value depends only on the state of the system.
• Thermodynamics is concerned with how the state variables change during a change of state.
Work
• W is positive when work is done on the system by the surroundings; W is negative when the system does work and expend energy.
• Some types of work: a) Mechanical work — exert a force through a distance.
b) Work of expansion of a gas under constant pressure (PV work)
c) Electrical work, W = charge x voltage
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Enthalpy• The first Law of Thermodynamics: the total energy of the universe (or any isolated) system is constant. Energy can neither be created nor destroyed but can be converted from one form to another.
• Enthalpy (H): heat transferred between the system & surroundings carried out under constant pressure.
• Only the change in enthalpy can be measured: H = Hfinal - Hinitial
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Energy Exchange
• Endothermic: system absorbs heat from surroundings (H = +).
• Exothermic: system transfers heat to the surroundings (H = -).
• An endothermic reaction feels cold; an exothermic reaction feels hot.
Hproducts < Hreactants H < 0 (negative)
Exothermic
Hproducts > Hreactants H > 0 (positive)
Endothermic
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Enthalpy and the First Law of Thermodynamics
Because E = q + W
At constant pressure,
q = H and W = -PV
H = E + PV
Thermochemical Equations
6.01 kJ are absorbed for every 1 mole of ice that melts at 0oC and 1 atm.
H2O (s) H2O (l) H = 6.01 kJ
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Calorimetry
• Calorimetry: measurement of heat flow.
• Calorimeter: apparatus that measures heat flow.
• The heat capacity of a substance is the amount of energy required to raise the temperature of a substance by one degree Celsius.
The Specific Heats of Some Common Substances
qrxn = -Ccal T
Bomb Calorimetry: combustion processes
where Ccal is the heat capacity of the calorimeter (to be determined experimentally)
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Phase Changes• Fusion: Solid Liquid. Hfus (Molar Heat of Fusion) =Heat needed to convert one mole of a solid to a liquid at a particular T.
• Vaporization: Liquid Vapor.Hvap (Molar Heat of Vaporization) = Heat needed to convert one mole of a liquid to a gas at a particular T.
• Sublimation: Solid Vapor. Hsub (Molar Heat of Sublimation) = Hfus + Hvap.
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Hess’ Law • Hess’ law: When reactants are converted to products, H is the same whether the reaction takes place in one step or in a series of steps.
H1 = H2 + H3
Potential energy of hiker 1 and hiker 2 is the same even though they took
different paths
• State functions are properties that are determined by the state of the system, regardless of how that condition was achieved
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Heats of Reactions • The standard enthalpy of reaction (Hrxn) is the heat change of a reaction carried out at 1 atm.
• Standard enthalpy of formation (Hf) is the heat change that results when one mole of a compound is formed from its elements at a 1 atm.
• Hf of the most stable form of an element is zero.
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• Standard enthalpy of solution (Hsoln) is the heat generated or absorbed when a certain amount of solute dissolves in a certain amount of solvent.
Step 1 (lattice energy)
Step 2 (heat of hydration)
Hsoln
+
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Spontaneous processes• Spontaneous process is a process that occurs by itself (and the reverse does not occur by itself).
• Reversible processes the systems must be at equilibrium
heat + ice water at +10oC Spontaneous, irreversible
heat + ice water at -10oC Spontaneous, irreversible
heat + ice water at 0oC Reversible; equilibrium
Ice and water coexist at 0oC. Either process or can occur at equilibrium
spontaneous
spontaneous
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What contributes to spontaneity?
a) Exothermic processes (heat is evolved).
b) Any process which increases randomness and disorder – Entropy
Ice melting
Ink drops in water
Water evaporating
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Entropy• The thermodynamic quantity which describes randomness and disorder is called entropy (S).
• The Second Law of Thermodynamics: the entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process.
Spontaneous process: Suniv = Ssys + Ssurr > 0
Equilibrium process: Suniv = Ssys + Ssurr = 0
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1) Gas has more entropy than liquid, which has more entropy than solid.
2) Melting or vaporization increases entropy.
3) In reaction, increasing the number of moles of a gas increases the entropy
4) Dissolving or mixing increases entropy; precipitation decreases entropy
5) Increasing the temperature increases entropy
Entropy Changes in the System (Ssys)
• The entropy of reaction is the entropy change for a reaction carried out at 1 atm and 25oC. Srxn = nS(products) - mS(reactants)
Exothermic Process Ssurr > 0
Entropy Changes in the Surroundings (Ssurr)
Endothermic Process Ssurr < 0
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Gibbs Free Energy (G)• For a constant-T process, the change in Gibbs free energy (G) is
G = Hsys - TSsys
1) If G is negative, there is a release of usable energy the reaction is spontaneous.
2) If G is positive, the reaction is not spontaneous.
3) If G is zero, the reaction is at equilibrium.
• The free energy change of reaction (Grxn) is the free energy change for a reaction when it occurs under standard-state conditions.
Grxn = nGf(products) – mGf (reactants)
• The free energy change of reaction (Gf) is the free energy change that occurs when 1 mole of the compound is formed from its elements in their standard states.
* The most stale allotropic form at 25oC and 1 atm
Standard States
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Gibbs Free Energy and Phase Transitions
H2O(l) H2O(g)
G = 0 = H – TS
S = TH
= 40.79 kJ373 K
= 109 J/K
Gibbs Free Energy and Chemical Equilibrium
G = -RT lnK
Time
Rat
e
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Adenosine diphosphate
Example of the change on Gibbs Free Energy
ATP + H2O + Alanine + Glycine ADP + H3PO4 + Alanylglycine
Alanine + Glycine Alanylglycine G = +29 kJ K < 1
G = -2 kJ K > 1
Adenosine triphosphate