Chemical Equilibrium 1

1. What is meant by a closed system?
The mass of the system remains constant. In other words, no substances can enter or escape the chemical system.

2. What is meant by the term reversible reaction?
The reactants react forwardwise, but the instant the products are formed, they re-react to become reactants.

3. How are reversible reactions indicated?
They are indicated by a 2-way arrow, or two arrows pointing oppositely.

or


4. What do the two way arrows indicate about a reaction?
The indicate that the reaction is reversible.
They DO NOT indicate if the reaction is in equilibrium.

5. What is meant by the term chemical equilibrium?
The rate of the forward reaction equals to the rate of the reverse reaction.

6. What is meant by a "shift" in the chemical equilibrium?
One of the rates, either the forward or reverse, has become faster than the other, essentially creating a "resultant rate". That direction is called the shift.
A forward shift means that the forward reaction is faster than the reverse reaction.

7. Name three factors that can cause a shift.
  • concentration (solutions)
  • pressure (gases)
  • temperature (all reactions)

8. What is meant by the Equilibrium Constant Kc?
When a reaction has reached equilibrium, the concentrations are all constant. These concentrations are put together in an expression, to calculate a number.
This number is called the equilibrium constant.
The equilibrium constant does not have units

9. What does the Equilibrium Constant tell us about the equilibrium?
It indicates the extent of the forward shift, and hence the yield (quantity of products).
A high number indicates a high yield (a lot of products).

10. How is the equilibrium constant calculated?
It is calculated from the Equilibrium Expression. (Law of Mass Action).
The Equilibrium Constant Kc is a ratio of the concentration of products divided by reactants, where each concentration is raised to the balancing number of the equation.

The balanced equation:


The Kc expression:

Here is a specific example:


The Kc expression:

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