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Chemical reactions: what drives them?

Really the same thing that drives all spontaneous processes such as a ball rolling down a hill.

Minimize energy (ball rolling down hill minimizes energy, roles to state of lower potential energy)

 

With chemical reactions, the energy criteria we used is called free energy

free energy is the energy made available for work by a process. 

  aA + bB -> cC + dD

A,B,C,D all have a free energy (we can talk about the free energy of a compound just like the potential energy a book on a shelf)

DG = change in free energy during a chemical reaction

DG = Gproducts – Greactants 

If DG < 0 free energy is given off, the reaction is spontaneous (exergonic)

If DG > 0 free energy is needed to drive it.  nonspontaneous, endergonic

 

What goes into DG?  What makes a reaction spontaneous?

Many reactions we think of as spontaneous give off heat.  Giving off heat lowers “energy” (DH < 0)

  Demo: combustion of ethanol in a flask.  gives off heat flames

So one criteria is the evolution of heat

What about a dye dissolving in water?   This does not give off any heat but is still spontaneous.

Must be more than heat involved.

The initial drop of dye is very ordered.  All of the dye molecules are in one place.  As they dissolve the disorder of the system increases.  

Two things that contribute to the spontaneity of a chemical reaction

1. evolution of heat

2. increase in disorder

Heat - exothermic vs. endothermic

DH = heat evolved or consumed by a chemical reaction

DH < 0 exothermic, gives off heat (favorable)

DH > 0 endothermic, reaction absorbs heat (unfavorable)

Entropy

  DS­ = Sproducts - Sreactants

  DS > 0 increase in disorder (favorable)

DS < 0 decrease in disorder (unfavorable)

Some notes on entropy:

1.  Greater disorder (less regular arrangement of particles, greater entropy: e.g.)

2. entropy increases when a substance is divided into parts:  reactions that form more molecules of products than there were reactants tend to occur with an increase in entropy (DS > 0). 

Examples

 S(gas) > S (liquid) > S(solid); ice melting occurs with an increase in entropy

Remember animation of solid/liquid/gas

dye dissolving in water results in a less orderly arrangement where all of the dye molecules are no longer all together. 

2NO2(g) -> N2O4(g)  entropy decreases, fewer products than reactants
Note:  NO2 is the brown of smog.

NaCl(s) -> NaCl­(aq) entropy increase, in aqueous solution the sodium and chloride ions are not pack together in an organized manner like in the solid.  They “break apart” to form sodium ions and chloride ions in solution.

Mg(s) + Cl2(g) ->MgCl2(s)  entropy decreases a gas is converted into a solid with more order (solids are organized).

Free energy puts together heat and entropy:

  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.

 

A closer look at DH

CH4 (g) + 2 O2(g) -> CO2(g) + 2H2O(g)  (H2O is a gas because of the high  temperature)

 

CH­4 (g) + 2 O2(g)

CO2(g) + 2H2O(g)

 

reactants store energy in bonds

products store energy in bonds

 

4 C-H bonds break

2 C-O bonds form

 

2 O-O bonds break

4 O-H bonds form

 

638 kcal / mol stored in bonds

425 kcal / mol stored in bonds

net change: DH = -213 kcal / mol (heat given off)

 

Free energy diagram

 

 

 

 

Important: for this reaction the entropy change is small so the free energy is dominated by heat (DH).