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HW Solutions
New Unit: Chemical Equilibrium

A. New Vocabulary:

(1) The Irreversible Reaction

(2) Steady State

(3) Reversible Reactions: Equilibrium

(1) The Irreversible Reaction

Most of the reactions we have considered so far have been irreversible, as implied by the one-way arrow: à

In such a reaction, none of the product molecules reacted again to produce the original molecules.

Non-technical examples

Burning wood is irreversible because no matter how hard you pray, it is unlikely that you'll get the wood back once it has turned into ashes.

The digestion of food is irreversible within your body.

Chemical examples:

1.         Neutralization:

            NaOH + HCl à H2O + NaCl

 

2.         Oxidation of iron:    2Fe + 1.5 O2àFe2O3

 

Text Box: A Slightly In-Depth Look Why are many reactions one-way affairs? There are basically two reasons: energy and entropy. One-way reactions often create products that have a more attractive arrangement between the nuclei and the electrons of the newly created molecules. At the same time they create more disorder, making it virtually impossible to revert to the original state. Let’s elaborate on each point. First the energy-issue: a reaction whose products are more stable and at the bottom of the energy well are like the large boulder that has just rolled down the mountain. It takes a lot of work to move a large mass against the force of gravity. Similarly it takes a lot of energy to overcome the electrostatic attractions within atoms and molecules. But why is disorder favoured? We sometimes personify Nature by saying she prefers chaos, but in reality it’s a probablility-issue. If you have 200 different issues of a magazine, what is the likelihood that a young, illiterate child will place them on a shelf and in chronological order? Very small. There are so many other places in the room or house where those magazines can end up, and even if the child wanted to stack them, he would have so many options that it would be a tremendous fluke if he happened to place them in chronological order. Similarly, if one well-ordered molecule becomes a bunch of other smaller molecules, the act of combining them back into the original becomes only one possible option out of millions.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(2) Steady State

An open system can be in a steady state if the input rate of a substance equals the output rate. An open system is one that loses a substance from one "opening" and then gets that same substance back from a totally different source.

Non-technical examples :

You open the valve that lets water out of a kiddie swimming pool. It flows out at a rate of one litre per second. But in the meantime, your hose is letting water into the pool at the same rate.

Chemical Steady State Examples from the Natural World:

If the volume of a lake is at steady state, then all the water that is lost through evaporation and usage by people and animals is returned at the same rate by streams, rain and animal and human waste.

The amount of nitrogen in the air is at steady state. Whatever is removed by bacteria as they turn it into nitrates is returned by other bacteria that convert ions in the soil back into nitrogen

(3) Equilibrium

Equilibrium is a state where the rate of the forward reaction equals the rate of the reverse reaction.  At the macroscopic level, the reaction seems to have stopped, but at the molecular level, reactions continue in both directions. Equilibrium occurs in a closed system, so that  no matter or energy can enter or leave.

Non-technical examples:

If you let a sealed flask of water sit for a while on the counter, the rate of evaporation will equal the rate of condensation so that the amount of H2O liquid will not change, and of course there will also be a fixed amount of water vapour.

H2O(l)  = H2O(g)

Your neighbour dumps snow on your driveway, and you retaliate by dumping snow on his driveway at the same rate.

At the macroscopic level (let’s say you viewed the whole thing  from an airplane) nothing seems to be going on. The amount of snow on the driveways seems constant. The whole area is white. But close up(microscopically), work is occurring; it’s just that its effects are canceling out.

Chemical Equilibrium Examples:

1.         2 NO2 = N2O4 + heat

Macroscopic observations:

At t=0, we have no N2O4 (invisible) and 2 moles/L of NO2. (red- brown gas). With time, the brown colour fades. While these changes are occurring, we have not yet reached equilibrium.

At equilibrium, the brown colour stops fading, and the concentrations of the two gases remain constant.

Microscopically, what's happening?

Every time 2 molecules of nitrogen dioxide (NO2) bond to form N2O4, some other molecule of N2O4 absorbs heat and decomposes into 2 molecules of nitrogen dioxide.

 

2.         A large crystal of iodine is dropped into CCl4. Describe what you would see before equilibrium is reached and after equilibrium is attained. Also explain what occurs at the molecular level once equilibrium is established.

Macrosopically, before equilibrium is reached, you see the CCl4 getting progressively more purple as more and more solid iodine dissolves. When equilibrium is attained, the purple colour becomes constant, and any left over pieces of solid iodine will not be shrinking any more.

Microsopically, after equilibrium is attained, the rate at which solid iodine molecules enter into solution will equal the rate at which the molecules stick together to create visible chunks of solid iodine.

How can you prove this? Well, if after equilibrium was reached, you replaced the “undissolved” crystal with a radioactive one, a change would occur after waiting a while. The solution would get progressively more enriched with  radioactive iodine, even though to the naked eye all would seem the same. The crystal would become less radioactive. If the reaction had been truly dead and over with, then no radioactive iodine should have appeared in the solution. Analogy: let’s say the snow that you were shoveling from your side was a little dirtier than your neighbour’s to begin with, then a close-up analysis would reveal that your snow is getting a little cleaner, even though the total amount of snow on either side did not change.

 

Text Box: A Slightly In-Depth Look Why does equilibrium occur in certain cases? Consider the example of water and ice at 0oC in a closed system. H2O(l) = H2O(s) + heat There are more attractions between hydrogen atoms and oxygen atoms in the solid state. Ice is energetically more stable than liquid water. Hence there seems to be an advantage to having the reaction move forward. Luckily for life though, freezing is not irreversible. The reverse reaction(melting) has the advantage of creating disorder or in other words of moving into states that are more probable because there are many possible arrangements for liquid molecules, and very few(nice orderly hexagons) for solid water. And so equilibrium is very easily attained.

 

 

 

 

 

 

 

 

 

 

Exercise 3.1

 

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Created:January/19/2001