-To find the reaction quotient (Qc) the formula is:

                               

                                 Qc= initial concentration of products

                                          Initial concentration of reactants

 

*If there’s a coefficient, that will be the exponent of the concentration.

*The calculated reaction quotient will be compared to the given equilibrium constant to know the direction of the reaction.

• If the Qc is greater than the Kc, the direction of the reaction is from right to left to reach the equilibrium.

• If the Qc is just equal to Kc, the reaction is in equilibrium already.

• If the Qc is less than the Kc, the direction of the reaction is from left to right to reach the equilibrium.

Example:

• The constant equilibrium of HI is 53.4 at 250 degree Celsius. Suppose that the H2 is 0.243  moles, the I2 is 0.146 moles and the HI is 1.98 moles all in a 1.00 container. Calculate the reaction quotient and tell the direction of the reaction.

The first thing you have to do is write the chemical formula then balanced the reaction.

So the chemical formula is:

 

 

 

 

 

 

 

Put 2 before the HI and its ratio is 1:1:2.

Then the unit must be in mole per liter or molarity. The given unit is in mole so divide it to the liters so that the unit will be in mole per liter.

          0.243 moles divided by 1 L = 0.243 moles/L

          0.146 moles divided by 1 L = 0.146 moles/L

          1.98 moles divided by 1 L = 1.98 moles/L

 

After converting, use the formula that is given above. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Since the Qc is greater than the Kc the direction of the reaction is from right to left to reach the equilibrium.

-The constant equilibrium of NH3 is 1.2. The nitrogen is 0.20 M, the hydrogen is 9.2 x 10-4 M and the ammonia is 2.8 x 10-6 M. Compute the Qc.

 

 

 

 

 

 

Put 3 before H2 and 2 before NH3 to make it balance. The ratio of the reaction is 1:3:2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Qc < Kc The reaction quotient is less than the equilibrium constant, the reaction direction is from left to right to reach the equilibrium.

 

 

Exercise:

*Calculate the Qc and determine the direction of the direction of the reaction.

• The constant equilibrium of NH3 is 1.2. The nitrogen is 0.40 M, the hydrogen is 5.2 x 10-4 M and the ammonia is 6.8 x 10-4 M. Compute the Qc.

• The constant equilibrium of HI is 53.4 at 250 degree Celsius. Suppose that the H2 is 5.6  moles, the I2 is .28 moles and the HI is 1.98 moles all in a 1.5 L container.

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Answers:

• Qc = 8.2 x 10^12    (Qc > Kc)

The direction of the reaction is left to right.

• Qc = 2.5 (Qc < Kc)

The direction of the reaction is right to left.

 

Source:

Advance Chemistry Notes

 

 

Written by: Bernadeth Castilleja