Standard free energies of reactions?
All reactions have a definable standard free energy. This is ∆G°,
which can be determined from the standard free energy of formation of reactants and products.
What are the standard conditions for free energy?
Standard free energy of a reaction, ∆G°, is defined for reactions under standard conditions, which are 25°C, 1 atm, and 1.0 M concentration of solutes.
What molecule has an exception from standard conditions?
An exception is for protons (H+) because the pH is always close to 7.0, where the concentration is ~ 1 x 10-7 M. Then, the expression is ∆G°’
When is a reaction spontaneous?
A reaction is spontaneous when the change in free energy is negative. These reactions are exergonic (release heat).
How are free energy and catalysis similar - and what makes them diferent?
Catalysis reduces activation energy of a reaction, thereby speeding it up. Catalysis does not affect the direction of the reaction.
- The DIRECTION of the reaction is what the change in free energy tells us.
- And, it is the difference in free energy between reactants and products regardless of energy trajectory.
How do we deal with free energy when it is not under standard conditions?
For the reaction aA + bB -> cC + dD
DG = DG* + RTln ([C]^c[D]^d) / ([A]^a[B]^b)
where ∆G° is the standard free energy change, R is the gas constant, T is the temperature (in Kelvin), ln is the natural log and A, B, C and D are the concentrations of the reactants and products.
Values of ln indicate what? ><= 1
ln (value more than 1) => positive
ln = 1 => 0
ln (value less than 1) => negative
So what can we learn from the ln value in relation to concentrations and free energy?
If there are greater concentrations of products than reactants, then when we take the natural log (ln) of that, the value will be positive. The reverse is also true.
* If there are equal concentrations of products and reactants, when the ln of the value is obtained, it will be 0. So, the actual ∆G value will be equal to the standard one.
Suppose a reaction had a ;positive change in free energy not. How could we change the conditions to make the change in free energy negative and then allow the reaction to proceed?
hint: the reaction products are in the numerator and reactants in the denominator
We could increase the concentrations of the reactants, making the products/reactants ratio very low. The ln value of that would be strongly negative. If that term is negative and a greater absolute value than ∆G°, then the ∆G for the reaction with those concentrations of reactants would be a negative value.
How do we apply the equilibrium to free energy?
When ∆G = 0, the reaction is in equilibrium. So, there is no net change in the amounts of reactants or products.
* When would that happen?
0 = ∆G° + RT ln [C]c[D]d
[A]a[B]b
∆G° = - RT ln [C]c[D]d
[A]a[B]b
- This would happen at different concentrations of products and
reactants, depending on the value of ∆G° of a specific reaction.
How can we then simplify the equation further with equilibrium (Keq)?
So, ∆G° = - RT ln [C]c[D]d /
[A]a[B]b
And, we know that the equilibrium constant is expressed as follows:
Keq = [C]c[D]d / [A]a[B]b
Therefore, ∆G° = - RT ln Keq
- Using this equation, we can easily determine ∆G° of a reaction
from its Keq and vice versa. We can therefore determine quite
a lot about a reaction.
What does the free energy difference determine?
The free energy difference (∆G) between A and B determines
whether the reaction will proceed in the forward or reverse direction.
How can concentration affect free energy difference?
If the concentration [A] is raised, then the reaction will tend to counter that,
according to Le Chatelier’s principle.
- When sufficient, this can make the
direction going away from the added
material exergonic, shifting the direction of a reaction.
Metabolism and concentration changes?
In metabolism a large concentration of A (left side) can be produced in our systems, therefore favouring the transformation of A into B (forward direction)
Examining a reaction?
Consider the reaction CH3COOH CH3COO- + H+
T = 25 °C and the ionization constant is 1.8 x 10-5.
Calculate ∆G° and ∆G°’.
Determine whether the forward reaction as shown is spontaneous.
Using ∆G° = - RT ln Keq, under standard conditions
and with R = 8.315 J, T = 25 °C = 298 K, and Keq = 1.8x10-5 mol·K
we obtain:
∆G° = - (8.315 J)(298 K) ln (1.8x10-5)
mol·K
∆G° = 27.1 kJ/mol
Therefore, it is not spontaneous.
Yet, using ∆G°’ = - RT ln [H+], with [H+] = 10-7 at pH of 7
∆G°’ = - 12.9 kJ/mol
Therefore, it is spontaneous.
Example of a coupled reaction?
Consider the reaction
Glucose-1-P + UTP + H2O <=> UDP-glucose + PPi with ∆G°’ = 0
How does it come about?
PPi + H2O 2 Pi with ∆G°’ = -33.5 kJ
Therefore,
Glucose-1-P + UTP + H2O UDP-glucose + PPi ∆G°’ = 0
PPi + H2O 2 Pi ∆G°’ = -33.5 kJ
Net: ∆G°’ = -33.5 kJ
- Due to the highly exergonic hydrolysis of pyrophosphate (PPi), and the linkage of the two reactions, the formation of UDP-glucose is spontaneous.
What do coupled reactions produce?
Coupled reactions produce a net change in free energy.
Example of coupled reactions?
Given the reactions
A + B → C + W, ∆G = 10 kJ/mol
and W + X → Y + Z ∆G = -15 kJ/mol,
the upper reaction will not occur spontaneously on its own.
But, if the lower reaction is also taking place, it will drive the upper
reaction through the coupled product/reactant, W. (The net ∆G value will be -5.)
ATP coupled reaction?
For example, ATP hydrolysis is highly exergonic. (It has a very negative
value of ∆G). So, when it is coupled to endergonic reactions, which are
not spontaneous, it can allow them to proceed.
The role of ATP as a driving force in making endergonic reactions occur?
The hydrolysis of ATP is highly exergonic.
- In other words, the products of hydrolysis (forward reaction here)
have far less free energy (G) than the ATP molecule has.
- The magnitude of the negative ∆G value of ATP hydrolysis reflects this
difference (-30.5 kJ/mol)
- It signifies that the hydrolysis is highly favoured in terms of energy
change. And, ATP hydrolysis can be used to drive endergonic (unfavoured) reactions by reaction coupling.
Therefore, ATP…?
ATP provides energy to drive reactions forward.
What do coupled reactions have to do with metabolism?
- The oxidation of glucose using combustion to yields CO2 and water
is a catabolic reaction that releases energy. But, this process is unsuitable for our metabolism. - So, cells (including ours) engage in a plethora of chemical reactions
that allow energy to be transformed, stored and used in various ways. - Each of these reactions occurs spontaneously in one direction,
depending on its ∆G, and the response of specific reactions to
conditions and reaction direction in vivo are expressions of that.
Example of an endergonic reaction getting coupled to exergonic ATP reactions?
Pi + glucose <=> G6P + H2O
where change in free energy = 13.8
ATP + H2O <=> ADP + Pi
ATP is endergonic DG = -30.5
The net = -16.7
Therefore, the reaction is able to proceed spontaneously
Definition of a metabolic pathway?
A metabolic pathway is a sequence of reactions in which the product of one
reaction is the substrate for the next, with each reaction catalyzed by an
enzyme or enzyme complex.
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Lecture 174
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Lecture 237
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Lecture 358
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Lecture 454
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