Equilibria

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Definitions: 

  • Dynamic equilibria: When the rate of the forward reaction is equal to the rate of the backwards reaction, where the concentration remains constant. 

Dynamic equilibria can only occur in a closed system, which is sealed so that nothing can be added or escape.

How dynamic equilibria is formed: 

As the reactants in the reaction get used up, their concentration decreases (by the forward reaction) - this causes the rate of the reaction to slow , down. This causes a higher concentration of products to be formed, therefore increasing the rate of the reverse reaction. As more products are turned back into reactants, this reaction also slows down - forming a cycle until eventually the and backward reaction occur at the same rate, causing the concentrations of reactants and products to stop changing and dynamic equilibrium is reached. 

- This is why a catalyst induces equilibrium faster. 

(http://jahschem.wikispaces.com/file/view/progress_curve_uni.gif/195816122/progress_curve_uni.gif)

Note: 

- The position of dynamic equilibrium is not always at a half-way point (not always an equal amount of products and reactants either side of the reaction). It may be in a positiob whereby there are mainly reactants with little product or vice versa. 

However - changing the conditions (temperature, concentration or pressure) of the reaction, will alternatively change the equilibrium position (by either increasing or decreasing the forward or backwards reaction) which therefore changes the amounts/concentrations of reactants and products. This change is not permanent as the reaction will eventually reach equilibrium again - via Le Chatelier's Principle. 

Le Chatelier's Principle: If any factor is changed which affects an equilibrium, the position of equilibrium will shift to oppose the change. 

Changing conditions: 

1. We make a change 

2. The equilibrium shifts (to the right or to the left) to do the opposite of that change. 

3. The concentration of of side (reactants or products) will increase and the other side will decrease.

e.g: 

1. Before the change to the system: 

(http://www.iconsdb.com/icons/preview/orange/square-xxl.png)                                (http://greensportsalliance.org/images/lightGreenSquare.gif)

   Reactants                                              Products

2. Immediately after the change, the rate of one reaction has temporarily increased:

(http://www.iconsdb.com/icons/preview/orange/square-xxl.png)                                   (http://greensportsalliance.org/images/lightGreenSquare.gif)  

   Reactants                                             Products 

3. The system reaches equilibrium again, the rate of the forward reaction is equal to the rate of the backwards reaction: 

(http://www.iconsdb.com/icons/preview/orange/square-xxl.png)                                   (http://greensportsalliance.org/images/lightGreenSquare.gif)

  Reactants                                             Products

This has overall changed the concentration of reactants and products. 

Le Chatelier's Principle

Changing the

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