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G = free energy

Change in free energy for some process (like a chemical reaction) is related to the change in entropy and heat given off or absorbed by the process:

  DG = Gproducts – Greactants = DH – TDS

  Important:  T has to be in Kelvin and is always positive!!!! 

DG < 0   (-)
Gproducts < Greactants  
(decrease in energy - downhill)
exergonic

reaction spontaneous - it proceeds “forward” with the formation of more products.  Free energy is given off

DG < 0   (+)
Gproducts < Greactants  
(increase in energy - uphill)

energy needs to be added to drive the reaction

DG = 0

equilibrium (later)

   

DH

DS

DG = DH - TDS

 

-

+

- (reaction will go)

 

+

-

+, reaction needs input of energy to go

 

-

-

DG,  Can be (+) or (-), depends on relative size of DH and DS

IF reaction is spontaneous, the heat given off is sufficient to overcome the unfavorable decrease in entropy

+

+

IF reaction is spontaneous, the reaction is entropy driven – the increase in disorder is sufficient to drive the reaction despite the fact that it absorbs heat from its surroundings.

Chemical reactions are driven by the evolution of heat and an increase in disorder

DH = heat evolved (negative) or consumed (positive), related to the chemical potential energy stored in bonds.  This term describes the energy involved in making and breaking bonds

DS = amount disorder increase (positive) or decreases (negative)

Free energy diagrams:

 

Rates of chemical reactions:

We left something out of the diagram above

if bonds are broken in the reactants and formed in the products, the initial breaking of bonds means energy must be added to the system:

 

Activation energy - amount of energy required to overcome the "hill" between the products and reactants. 

-The rate of a chemical reaction is determined by the activation energy.

Reaction rate:  how fast a reaction proceeds - the number of atoms, ions, or molecules that react in a given time to form products

high activation energy - reaction rate slow

low activation energy - reaction rate fast

 

Ex: lighting a match (started by burning phosphorus).  Need to add a little energy from friction to get the reaction rolling.

 

Reaction rates  depend on

1. the activation energy for the particular reaction being considered.

2. Temperature - increasing temperature increases the rate of a reaction

Why?

-At higher temperatures, molecules have more kinetic energy:
-With more energy, molecules move faster and collide more often
-With more energy in the molecules/atoms, collisions are more effective at promoting reaction

Basically, there is more energy available at higher temperatures to overcome the activation energy.  (the faster we shoot a marble out of a cannon, the higher the hill it can climb)

3. Concentration - related to how close together molecules are,  the more concentrated, the closer they are. 

Increase in concentration => increase in frequency of collisions => increased reaction rate

4. particle size - surface area

For reactions involving solids, the more exposed surface there is, the faster the reaction.

5.  the presence of a catalyst

Catalyst: a substance that speeds up a reaction by providing reactants with a lower activation energy pathway (a shortcut).

Catalysts are neither consumed or created during a chemical reaction and consequently, do not appear as a reactant or product.

A catalysts helps others do their work.

Ex: Demo: H2O2 - hydrogen peroxide 

2H2O2(aq)--> 2H2O(l) + O2(g)  very slowing essentially no reaction

Now add a catalyst - KI (potassium iodide) - reaction much faster.

Demo: coke can with added sugar.

A common type of catalysts are the enzymes in your body.  Many enzymes work by holding the reactants in the right orientation to react.

Ex: train tracks  S + 3T -> TS3  through random collisions this is harder to achieve, but if we organize all of the S tracks in a T first, the reaction will proceed more quickly.

Reversible reactions and chemical equilibrium:

Up until know, we have mainly considered reactions as converting reactants to products completely.

Many reactions are reversible and do not result in complete conversion to the products but rather a population of both products and reactants remain.

In reversible reactions, reactants are continually converted to products and products to reactants resulting in what is termed a dynamic equilibrium

chemical equilibrium - the dynamic state in which the forward and reverse reactions are taking place at the same rate - but there is no change in the net amounts of reactants and products.  

Demo: balls in bucket example - balls are continually moving from left to right and right to left but at any given instant there is an equal number on each side. 

Another example of dynamic equilibrium:  Consider people going up and down on escalators between two floors.  If the rate people move down is the same as people move up then the average number of people on each floor will remain the same.  People, however, are still moving up and down between floors.  It is a dynamic equilibrium.  Note that an equal  does not mean that the number of people on each floor is necessarily the same.

A double arrow represents equilibrium:

aA + bB

cC + dD

An example is the reaction of NO2 (the brown stuff in smog) to form N2O4

N2O4(g)

2NO2(g)

 

At equilibrium, the amount of products and reactants can be quantitatively described by an equilibrium constant.

For a general reactions

aA + bB

cC + dD

the equilibrium constant is given by:

[C]c[D]d

K =

------------

[A]a[B]b

where the brackets [ ]  indicate the concentration (in units of mol / L) of a species.

Ex.: 

N2O4(g)

2NO2(g)

 

[NO2]2

K =

------------

[N2O4]

 

Shifting the equilibrium -

LeChatelier's principle:  If a system in dynamic equilibrium experiences a change in conditions, the system changes to relieve the stress.

1. Adding or removing products/reactants:

Add products --> makes more reactant

Remove products --> more products made

2. changing temperature - adding heat to an endothermic reaction will shift the equilibrium toward products.  Adding heat to an endothermic reaction will shift the equilibrium toward reactants.