Download - Causes of Change
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Causes of ChangeThermochemistry
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Thermodynamics Thermodynamics is the study of the flow
or exchange of energy. System Surroundings
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The First Law of Thermodynamics
The law of Conservation of Energy The total amount of energy in the
universe is constant The change in energy of the system +
the change in energy of the surroundings equals zero
“You can’t get something for nothing”
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The movement of heat Heat is always transferred from
hot to cold
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Enthalpy Enthalpy (H) is the heat content of a
system at a constant pressure. H or the change in enthalpy is what we
calculate H = Hproducts - Hreactants
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Molar Heat Capacity The molar heat capacity of a substance
is the amount of energy needed to raise 1 mole of a pure substance by one degree Celcius or Kelvin. C = q / mol x T Just like specific heat, except you are
using moles instead of mass
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Molar Enthalpy The change in energy per mole is
referred to as Molar Enthalpy q/mol=H
So, let’s change the molar heat capacity equation . . . If C=q/mol x T, and q/mol = H, then C=H/T or H=CT Remember that T= Tfinal - Tinitial
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What does H mean? Chemical changes and changes in physical
states either release or absorb energy. Exothermic reactions release heat so heat is
a product, let’s look at water freezingH2O(l, 292K) H2O(s, 273K) + 459.8 J
T = final – initial = 273K – 292K = -19KH=CT=(24.2J/molK)(-19K)= -459.8J/mol
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What does H mean? Endothermic reactions absorb heat so it
feels cold, let’s look at ice meltingH2O(s, 273K) + 459.8 J H2O(l, 292K)
T = final – initial = 292K – 273K = 19KH=CT=(24.2J/molK)(19K)= 459.8J/mol
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Reaction Pathways Reaction pathways are used to illustrate the
amount of energy throughout the reaction Reactants must absorb some amount of
energy in order for a reaction to proceed (unless it is a spontaneous reaction) This amount of energy is referred to as
Activation Energy Once the energy is absorbed, the reaction
can proceed These compounds are now referred to as an
Activated Complex
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Exothermic Pathway In an exothermic
reaction, the reactants absorb the activation energy but the resulting products lose energy (H)
H= a negative number
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Endothermic Reaction In an endothermic
reaction, the reactants absorb the activation energy and so the resulting products have more energy (H)
H= a positive number
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So to review, Endo or Exo? A positive change in enthalpy would be
an endothermic reaction. A negative change in enthalpy would be
an exothermic reaction
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Enthalpy (cont.) What would happen if the temperature
of a sample decreased? The kinetic energy of the sample also
decreases! If the kinetic energy of the sample
decreases, the enthalpy (H) decreases
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Entropy The measure of the randomness or
disorder in a system is called entropy (S) and is measured in J/K Aha! What are the variables in entropy? You guessed it, energy and temperature!
The entropy change in a system is the measure of the entropy of the products minus the reactants ΔS = Sproducts - Sreactants
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What is disorder? Disorder is any system which is not
ordered a solid has very ordered particles a liquid has slightly less ordered particles a gas has very unorderly particles your locker is a very disorderly!
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• In any spontaneous change the entropy of the universe must increase
• ΔSsys + ΔSsurr > 0
• ΔSreaction = ΔSproducts - ΔSreactants
• “You can’t break even”
The Second Law of Thermodynamics
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Entropy
Randomness of the system
Increasing order
Increasing entropy
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Trends in Entropy What happens if . . .
Temperature increases, disorder increases Change in phase toward gas, disorder increases Concentration decreases, disorder increases # of products increases, disorder increases A solute is dissolved in a solvent, disorder
increases
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Chemical EntropyChemical Entropy
Lower Entropy Higher Entropy
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Entropy ProblemsWill the entropy change for each of the following be positive or negative?
Sugar dissolves in tea Air is pumped into a tire Acetone evaporates from nail polish CaCO3(s) CaO(s) + CO2(g)
N2(g) + 3H2(g) 2NH3(g)
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Gibb’s Energy Gibb’s Energy is the measure of the
available energy in a system, the energy available to do work often referred to at “free” energy
The change in Gibb’s Energy is, ΔG = Gproducts - Greactants
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Gibb’s is Not alone Gibb’s Energy is a function of enthalpy
(H), entropy (S) and temperature (K) ΔG = ΔH - TΔS
So, what happens when the temperature goes up? Kinetic energy increases, entropy
increases and temperature increases . . . Now what!
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Let’s Talk Spontaneous A reaction is spontaneous (proceeds
without any help) if the change in Gibb’s Energy is negative That would mean that the measure of the
temperature times the change in entropy is a large number
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This will help predictH S G Spontaneous?
- + - Yes at all temperatures
- - + / - Yes, at low temp.
+ + + / - Yes, at high temp.
+ - + No
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2NO2(g) 2N2(g) + O2(g) The change in enthalpy is negative What is the change in entropy?
Positive Is this spontaneous or not?
Yes, it is spontaneous
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H2O(l) H2O(s) Is the reaction endothermic or exothermic?
Exothermic Is the enthalpy positive or negative?
Negative Does entropy increase or decrease?
Decrease Is this spontaneous?
Only at low temperatures
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2NH3(g) N2(g) + 3H2(g) The change in enthalpy is positive. What is the change in entropy?
It increases. Is this reaction spontaneous?
Yes, as long as the temperature is high
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3O2(g) 2O3(g) What would you predict the change in
enthalpy would be? (endo or exo?) Endothermic, so a positive change
What do you predict the entropy would be? More ordered, so decreased
So . . . Spontaneous? No, not at any temperature
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Hess’s Law Well, Hess’s law states that the total
amount of energy lost or gained in a given reaction is the sum of each step in the reaction.
Some reactions must occur in 2 or more steps.
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Apply Hess’s Law Guess what is conserved according to
Hess’s Law?
Yeah! Energy!
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Add Each Step Let’s decompose water a little bit
3H2O(g) 3H2O(l) ΔH = -132 kJ
3H2O(l) 3H2(g) + (3/2)O2(g) ΔH = +858 kJ
The total change in enthalpy is 858 kJ - 132 kJ = +726 kJ