Scientific Understanding

**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 = | [ NH_{3} ]^{2} |

[ N_{2} ] [ H_{2} ]^{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 H_{2}O_{(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 H_{2}O is 0.3M, the concentration of H_{2} is 5M, and the concentration of O_{2} is 10M.

Answer:

Q = | [ H_{2} ]^{2} [ O_{2} ] = |
[ 5 ]^{2} [ 10 ] = |
Q = 2780 |

[ H_{2}O ]^{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 MgBr_{2} is 0.2M, the concentration of NaI is 0.4M, the concentration of MgI_{2} is 0.5M, and the concentration of NaBr is3M. What is the reaction quotient?

Answer:

Q = | [ MgI_{2} ] [ NaBr ]^{2 }= |
[ 0.5 ] [ 3 ]^{2 }= |
Q = 1400 |

[ MgBr_{2} ] [ 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 N_{2}O_{3 } is 0.7M, the concentration of N_{2 } is 1.2M, and the concentration of O_{2} is 0.3M

**2 N**_{2}**O**_{3(g)}** <——> 2 N**_{2(g)}** + 3 O**_{2(g)}

What is the reaction quotient expression for this chemical formula?

Answer:

Q = | [ N_{2 }]^{2} [ O_{2 }]^{3} |

[ N_{2}O_{3 }]^{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)} + Al_{2}(SO_{3})_{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 CaCl_{2} is 12M, the concentration of Al_{2}(SO_{3})_{3} is 2M, the concentration of CaSO_{3} is 3M, and the concentration of AlCl_{3} is 1M.

Answer:

Q = | [ CaSO_{3 }]^{3} [ AlCl_{3} ]^{2 }= |
[ 3 ]^{3} [ 1 ]^{2 }= |
Q = 7.8 * 10^{-3} |

[ CaCl_{2 }]^{3} [ Al_{2}(SO_{3})_{3 }] |
[ 12 ]^{3} [ 2 ] |

H_{2}SO_{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 H_{2}SO_{4} is 0.4M, the concentration of H_{2} is 0.1M, the concentration of S is 0.03M, and the concentration of O_{2 } is 0.8M. What is the reaction quotient?

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

Q = | [ H_{2 }] [ O_{2 }]^{2 }= |
[ 0.1] [ 0.8]^{2 }= |
Q = 0.16 |

[ H_{2}SO_{4}] |
[ 0.4] |