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Chapter 7
Energy, Rate, and Equilibrium
Petr Vanýsek, Instructor
Denniston
Topping
Caret
6th Edition
1 November 2011
Enthalpy
• Enthalpy – represents heat energy
• Change in Enthalpy (Ho) – energy difference
between the products and reactants of a chemical
reaction “heat of”
• Energy released, exothermic reaction, enthalpy
change is negative
– In the combustion of CH4, Ho = -211 kcal (“per mole”
• Energy absorbed, endothermic, enthalpy change is
positive.
– In the decomposition of NH3, Ho= +22 kcal
Spontaneous and
Nonspontaneous Reactions
• Spontaneous reaction - occurs without
any external energy input
• Most, but not all, exothermic reactions
are spontaneous
• Thermodynamics is used to help predict
if a reaction will occur
• Another factor is needed, Entropy
Spontaneous and Nonspontaneous
Reactions
So is positive
So is negative
Spontaneous and
Nonspontaneous Reactions
Are the following processes exothermic or
endothermic?
– Fuel oil is burned in a furnace
– C6H12O6(s)
2C2H5OH(l) + 2CO2(g)
H= -16 kcal
2HNO3(l) + 18.3 kcal
– N2O5(g) + H2O(l)
Entropy
• The second law of thermodynamics – the
universe spontaneously tends toward increasing
disorder or randomness
• Entropy (So) – a measure of the randomness of
a chemical system
• High entropy – highly disordered system, the
absence of a regular, repeating pattern
• Low entropy – well organized system such as a
crystalline structure
• No such thing as negative entropy
Entropy of Reactions
So of a reaction = So(products) - So(reactants)
• A positive So means an increase in
disorder for the reaction
• A negative So means a decrease in
disorder for the reaction
• (yes, there is negative change of
entropy)
Processes Having Positive
Entropy
Phase change
Melting
Vaporization
Dissolution
All of these processes have a positive So
Entropy and Reaction
Spontaneity
• If exothermic and positive So…
SPONTANEOUS
• If endothermic and negative So…
NONSPONTANEOUS
• For the other two cases, it depends on the
relative size of Ho and So
Greatest Entropy
• Which substance has the greatest
entropy?
– He(g) or Na(s)
– H2O(l) or H2O(g)
Free Energy
also known as the Gibbs Free Energy or just Gibbs Energy
• Free energy (Go) – represents the
combined contribution of the enthalpy and
entropy values for a chemical reaction
• Free energy predicts spontaneity of
chemical reactions
Go = Ho - TSo
– Negative Go…Always Spontaneous
– Positive Go…Never Spontaneous
T in Kelvin
Gibbs Free Energy and Reaction
Spontaneity
• Need to know both H and S to predict the
sign of G, making a statement on reaction
spontaneity
• Temperature also may determine direction of
spontaneity




H +, S - : G always +, regardless of T
H -, S + : G always -, regardless of T
H +, S + : G sign depends on T
H -, S - : G sign depends on T
Experimental Determination of
Energy Change in Reactions
• Calorimetry – the
measurement of heat energy
changes in a chemical reaction
• Calorimeter – device which
measures heat changes in
calories
• The change in temperature
is used to measure the loss
or gain of heat
Heat Energy in Reactions
• Change in temperature of a solution, caused by a
chemical reaction, can be used to calculate the
gain or loss of heat energy for the reaction
– Exothermic reaction – heat released is absorbed
– Endothermic reaction – reactants absorb heat from
the solution
• Specific heat (SH) – the number of calories of
heat needed to raise the temperature of 1 g of
the substance 1 oC
Q  ms  Ts  SH s
Heat Energy in Reactions
• Specific heat of the solution along with the total
number of g solution and the temperature
change permits calculation of heat released or
absorbed during the reaction
• S.H. for water is 1.0 cal/goC
• To determine heat released or absorbed, need:
– specific heat
– total number of grams of solution
– temperature change (increase or decrease)
Calculation of Heat Energy in
Reactions
• Q is the product
– ms is the mass of solution in the calorimeter
 Ts is the change in temperature of the solution from
initial to final state
– SHs is the specific heat of the solution
• Calculate with this equation
Q  ms  Ts  SH s
– Units are: calories = gram x ºC x calories/gram - ºC