Dynamic+Equilibrium

Many chemical reactions go to completion. However, some reactions don’t go to completion since when the products are produced they react with each other to reform the reactants. These reactions are called reversible reactions. Eg. NH 3  (g) + HCl (g) NH 4 Cl (s)

The sign is for a reversible reaction with the forward arrow pointing to NH 4 Cl indicating the forward reaction and the backward arrow pointing to NH 3  (g) + HCl (g) indicating the reverse reaction.

//If the reversible reaction happens in a closed system it will reach **dynamic equilibrium**. // __//A closed system is an environment where neither energy nor matter can be gained/lost. //__

**Dynamic equilibrium** is when the rate of the forwards reaction is equal to the rate of the reverse reaction. (thus the concentration of reactants and products remains constant)

//__ Note: __// When a reaction reaches equilibrium it does not stop. This can be explained as follows: Initially the forwards reaction occurs rapidly but the rate decreases as the concentration of the reactants decreases (since they are used up). The reverse reaction doesn’t occur initially but as the products are formed its rate increases. Thus a point is reached, where the rate of the reactions is equal and their concentrations become constant.



Eg. The decomposition of dinitrogen tetroxide. N 2 O 4  (g) 2NO 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> (g)

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">It can be seen that the reaction reaches dynamic equilibrium when the rate of the forwards and reverse reactions are equal as seen in the second diagram. At equilibrium the concentrations of reactants and products remains constant.

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Once a reaction reaches equilibrium its macroscopic properties such as colour remain constant (this is because the concentrations of the reactants and products is constant so the colour or other visible properties of the mixture would not change)

//** (Remember that dynamic equilibrium occurs only when the reversible reaction occurs in a closed system since if the system is open, the products will escape and therefore cannot react back to form the reactants.) **//

By the end of this lesson you should be able to:
 * Know that a reversible reaction in a closed system reaches a state of dynamic equilibrium.
 * Define dynamic equilibrium


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