UNIT 3: EQUILIBRIUM

Notes 3.1.1- 3.1.2

3.1.1 Reversible Reactions

Typically when we think of what happens during a chemical reaction we think of the reactants getting totally used up so that none are left and ending up with only products. Also, we generally consider chemical reactions as one-way events. You may well have learned during earlier science classes that this is one way to distinguish chemical change from physical changes - physical changes (such as the melting and freezing of ice) are easily reversed, but chemical changes cannot be reversed (pretty tough to un-fry an egg).

In this unit we will see that this isn't always the case. We will see that many chemical reactions are reversible under the right conditions.

Example if a Reversible Reaction We typically write a reaction that can go

in both directions by using a double arrow

2 NO2 (g)   N2O4 (g)

Because the reaction continues in both directions at the same time, we never run out of either NO2 or N2O4. NO2 is continually being used up to form N2O4, but at the same time N2O4 is forming more NO2

Example #2

When hydrogen gas is passed over heated iron oxide, iron and steam are produced:

(1)Fe3O4 (s) + 4 H2 (g) → 3 Fe(s) + 4 H2O(g)

The reverse reaction can occur when steam is passed over red-hot iron:

(2)3 Fe(s) + 4 H2O(g) → Fe3O4 (s) + 4 H2 (g)

We can write these two equations together as: (3)Fe3O4 (s) + 4 H2 (g)   3 Fe(s) + 4 H2O(g)

When we have a reversible reaction we need to be able to distinguish between which way the reaction is headed. In the previous reaction, we would say that in the forward reaction iron oxide and hydrogen gas (the reactants) produce iron and steam.

During the reverse reaction, iron reacts with steam to produce iron oxide and hydrogen gas.

It is important to understand the terminology, and to use the terms correctly.

3.1.2 Equilibrium

Here's another example of a reversible reaction - dissolving salt in a beaker of water, described by the following reaction:

NaCl(s)   NaCl(aq)

If you keep adding more and more solid salt, eventually you'll reach the point where no more salt dissolves, and the excess sits at the bottom of the beaker. Has the dissolving reaction stopped? It would appear so, but that's not the case

Our solution has reached the point of equilibrium, is that both the forward and reverse reactions are happening at the same rate. This cancels out any observable changes in our system.

At the same rate that solid NaCl dissolves, the dissolved salt is recrystallizing to form more solid NaCl.

NaCl(s) → NaCl(aq)

NaCl(aq) → NaCl(s)

Equilibrium Definition

Equilibrium is the state at which the rate of the forward reaction equals the rate of the reverse reaction.

At the point of equilibrium, no more measurable or observable changes in the system can be noted.

Equilibrium is dynamic - both forward and reverse reactions continue, even though the reaction appears to have stopped.

It is important for you to understand that equilibrium means the rates of the forward and reverse reactions are equal; it does not mean that there are equal amount of reactants and products present at equilibrium.

For example, the following reaction was at equilibrium, and concentrations of all reaction participants were measured:

H2 (g) + I2 (g)   2 HI(g)

At equilibrium:[H2] = 0.022 M

[I2] = 0.022 M

[HI] = 0.156 M For this particular reversible reaction, there is more

HI at equilibrium (0.156 M) than there is of H2 and I2 

In this reaction the product side is favored

In order for a reversible reaction to reach equilibrium, the reaction must be carried out in a closed system - no additional reactants can be added or products removed.

If, in our last example, the product HI was removed as it formed, the reaction would never reach the point of equilibrium

Steady State System

If reactants are constantly being added and products removed as they form, the system would appear to be at equilibrium but that would not be the case. This situation is called a steady state system.

A factory with an assembly line is a steady state system - new materials are constantly being added; finished products are removed. A campfire with wood being added to the fire is another steady state system.

Be careful not to confuse steady state with equilibrium.

Assignment

Practice problems 3.1.2- Reversible Reactions (do together)- next slide

Assignment 3.1.2- do and hand-in

1. Write reversible reactions for each of the following situations (be sure to balance your equations):

 a. Hydrogen iodide gas (HI) decomposes into its elements.

 

b. Hydrogen and nitrogen gases combine to form ammonia gas, NH3  2. Describe two different mixtures of starting materials that can be used to

produce the equilibrium  

A + B ↔ C + D    

3. If the system represented by the following equation is found to be at equilibrium at a specific temperature, which of the following statements is true? Explain your answers.

 H2O(g) + CO(g) ↔ H2 (g) + CO2 (g)

 a. All species must be present in the same concentration

  

b. The rate of the forward reaction equals the rate of the reverse reaction. 

c. We can measure continual changes in the reactant concentrations.  4. Would you expect the combustion of methane, CH4 with oxygen to form carbon

dioxide and water, to be a reversible reaction? 

Hint: Methane, or natural gas, is an important energy source. Considering this, what did you learn in the last unit that will help you predict whether or not the reverse reaction is likely to be spontaneous?