Free Energy and Temperature •Free energy decreases (becomes more negative) as temperature • At low T, G m for solid phase is lower than that of liquid or vapour, so the solid phase is prevalent •As we increase T to T fus and higher, the liquid state has a lower G m , so it is the phase that prevails •As we increase T further to T b , the gas
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Free Energy and Temperature Free energy decreases (becomes more negative) as temperature At low T, G m for solid phase is lower than that of liquid or.
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Free Energy and Temperature
•Free energy decreases (becomes more negative) as temperature
•At low T, Gm for solid phase is lower than that of liquid or vapour, so the solid phase is prevalent
•As we increase T to Tfus and higher, the liquid state has a lower Gm, so it is the phase that prevails
•As we increase T further to Tb, the gas phase has the lowest value of Gm
7.13: Gibbs Free Energy of Reaction
• To determine the spontaneity of a reaction, we use the change in the Gibbs Free Energy, G, or the Gibbs Free Energy of Reaction
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G = nGm Products∑ − nGm Reactants∑ We’ve seen something like this before somewhere…
Standard Gibbs Free Energy of Formation, Gf°
Gf° = The standard Gibbs Free Energy of reaction per mole for the formation of a compound from its elements in their most stable form.
Gf°is for the formation of 1 mole of product– Different amounts of reactants may be used…Be vigilant!
Gf°: What Does it Mean?
• Compounds with Gf° > 0 are Thermodynamically Unstable
• Compounds with Gf° < 0 are Thermodynamically Stable
Gibbs Free Energy and Nonexpansion Work
• we = ‘Extra work’– Nonexpansion work is any kind of work other than that done
against an opposing pressure
• Stretching a spring, moving a rope, importing a sugar molecule into a cell are all examples of nonexpansion work
• All cellular processes are examples of nonexpansion work
• How are the Gibbs Free Energy and we related?
Gibbs Free Energy and we
G = we
• If we know the change in free energy, we know how much nonexpansion work can be done
• What does this mean?– Let’s look at the combustion of glucose.
G° and the Combustion of Glucose
• The G° of the reaction is -2879 kJFor 1 mole of glucose, we get 2879 kJ of energy
OrFor 180 g of glucose, we get 2879 kJ of energy
• To make one mole of peptide bonds, 17 kJ of work must be done.– If we get 2879 kJ of energy from one mole of glucose, we
should be able to make 170 moles of peptide bondsOne molecule of glucose will provide enough
energy to add 170 amino acids to a growing protein (in actuality, you can only add 10 amino acids)
C6H12O6 (s) + 6 O2 (g) --> 6 CO2 (g) + 6 H2O (l)
The Effect of Temperature on G°
• Remember that H° or S° is the sum of the individual enthalpies or entropies of the products minus those of the reactants
• If we change the temperature, both are affected to the same extent, so the H° and S° values don’t significantly change
• This is not the case with G°. Why?
G° = H° - TS°
The Effect of Temperature on G°
The Effect of Temperature on G°
The Effect of Temperature on G°
The Effect of Temperature on G°
Thermodynamics Review
Let’s Look at some of the most important equations we’ve covered over the past 2 chapters…
The First Law of Thermodynamics
• Up until now, we have only considered the changes in the internal energy of a system as functions of a single change: either work or heat
• However, these changes rarely occur singly, so we can describe the change in internal energy as:
U = q + w (The 1st Law)
• The change in internal energy is dependent upon the work done by the system and the heat gained or lost by the system
Heat Capacity
q = CT = mCsT
nCmT
System: Solution and chemicals that reactSystem: Solution and chemicals that react
Surroundings: Cup and the world around it!Surroundings: Cup and the world around it!
Assumptions: We use 2 cups to prevent energy Assumptions: We use 2 cups to prevent energy transfer to the surroundings (we assume that it transfer to the surroundings (we assume that it works as designed)works as designed)
Expected Changes:Expected Changes:
i)i) As the chemical reaction occurs, the potential As the chemical reaction occurs, the potential energy in the reactants will be released as heat or energy in the reactants will be released as heat or the solution can supply heat to allow formation of a the solution can supply heat to allow formation of a product with a higher potential energyproduct with a higher potential energy
ii)ii) The solution will absorb or release energy during The solution will absorb or release energy during the reaction. We will see this as a temperature the reaction. We will see this as a temperature changechange
qqrr + q + qsolutionsolution = 0 = 0
Coffee Cup CalorimeterCoffee Cup Calorimeter
We place 0.05g of Mg chips in a coffee cup We place 0.05g of Mg chips in a coffee cup calorimeter and add 100 mL of 1.0M HCl, and calorimeter and add 100 mL of 1.0M HCl, and observe the temperature increase from 22.21observe the temperature increase from 22.21°C °C to 24.46°C. What is the to 24.46°C. What is the ΔΔH for the reaction?H for the reaction?