Scientific Tutor

What is the reaction quotient?

The reaction quotient is often explained in a very confusing manor. However, it is something quite simple. The reaction quotient allows you to compare the current concentrations or pressures of the reactants and products to the eventual equilibrium they will achieve. This gives you a hint as to which way or how the equilibrium will shift. Very similar to Le Chatelier’s Principle. How do we figure out the reaction quotient? The same way we figure out the equilibrium.

 

The reaction quotient is represented by a Q.

 

The formula for the reaction quotient is:

Q =	[ products ]
	[ reactants ]

 

Notice it is the exact same formula as the equilibrium.

 

What is the reaction quotient for a chemical equation we know and have worked with before?

N_2(g) + 3 H_2(g) <—-> 2 NH_3(g)

 

Q =	[ NH3 ]2
	[ N2 ] [ H2 ]3

 

Again, it is the exact same as the equilibrium formula. So what is the difference between the equilibrium constant (K) and the reaction quotient (Q)? The reaction quotient is usually used before the chemical reaction has achieved equilibrium. Another way to say that is the reaction quotient is usually used before the concentrations of all the chemicals have stabilized.

 

Examples: What is the reaction quotient (Q) for the problems below?

 

2 H₂O_(g) <—-> 2 H_2(g) + O_2(g)

Given that the chemical equation above has not yet achieved equilibrium, what is the reaction quotient if the concentration of H₂O is 0.3M, the concentration of H₂ is 5M, and the concentration of O₂ is 10M.

Answer:

Q =	[ H2 ]2 [ O2 ] =	[ 5 ]2 [ 10 ] =	Q = 2780
	[ H2O ]2	[ 0.3 ]2

 

MgBr_2(aq) + 2 NaI_(aq) <——> MgI_2(aq) + 2 NaBr_(aq)

After the chemical equation above reaches equilibrium additional NaBr is added. Directed after the NaBr is added, the concentration of MgBr₂ is 0.2M, the concentration of NaI is 0.4M, the concentration of MgI₂ is 0.5M, and the concentration of NaBr is3M. What is the reaction quotient?

Answer:

Q =	[ MgI2 ] [ NaBr ]2 =	[ 0.5 ] [ 3 ]2 =	Q = 1400
	[ MgBr2 ] [ NaI ]2	[ 0.2 ] [ 0.4 ]2

 

VIDEO Determining the Reaction Quotient Demonstrated Example 1: Given that the chemical equation below has not yet achieved equilibrium, what is the reaction quotient if the concentration of N₂O₃ is 0.7M, the concentration of N₂ is 1.2M, and the concentration of O₂ is 0.3M

2 N₂O_3(g) <——> 2 N_2(g) + 3 O_2(g)

 

What is the reaction quotient expression for this chemical formula?

Answer:

Q =	[ N2 ]2 [ O2 ]3
	[ N2O3 ]2

 

How do we fill in the concentrations?

Answer:

Q =	[ 1.2]2 [ 0.3]3
	[ 0.7]2

 

What is the solution?

COMPLETE ANSWER:

Q =	0.079
	1

 

PRACTICE PROBLEMS: What is the reaction quotient (Q) for the problems below?

 

3 CaCl_2(aq) + Al₂(SO₃)_3(aq) —-> 3 CaSO_3(aq) + 2 AlCl_3(aq)

Given that the chemical equation above has not yet achieved equilibrium, what is the reaction quotient if the concentration of CaCl₂ is 12M, the concentration of Al₂(SO₃)₃ is 2M, the concentration of CaSO₃ is 3M, and the concentration of AlCl₃ is 1M.

Answer:

Q =	[ CaSO3 ]3 [ AlCl3 ]2 =	[ 3 ]3 [ 1 ]2 =	Q = 7.8 * 10-3
	[ CaCl2 ]3 [ Al2(SO3)3 ]	[ 12 ]3 [ 2 ]

 

H₂SO_4(aq) <—–> H_2(aq) + S_(s) + 2 O_2(aq)

After the chemical equation above reaches equilibrium additional S is added to the reaction mixture. Directed after the S is added, the concentration of H₂SO₄ is 0.4M, the concentration of H₂ is 0.1M, the concentration of S is 0.03M, and the concentration of O₂ is 0.8M. What is the reaction quotient?

Answer: (DON ‘T include S because it is a solid)

Q =	[ H2 ] [ O2 ]2 =	[ 0.1] [ 0.8]2 =	Q = 0.16
	[ H2SO4]	[ 0.4]