Reaction Quotient - Online Tutor, Practice Problems & Exam Prep

Understanding the reaction quotient ($Q$) is crucial for predicting the direction of a chemical reaction's shift to reach equilibrium. $Q$ is a measure of the ratio of product concentrations to reactant concentrations at a given moment, excluding solids and liquids, similar to the equilibrium constant ($K$), but not necessarily at equilibrium. When $Q$ is compared to $K$, if $Q$ < $K$, the reaction will shift right, increasing product concentration and decreasing reactant concentration to reach equilibrium. Conversely, if $Q$ > $K$, the reaction shifts left, increasing reactants and decreasing products. If $Q$ = $K$, the system is at equilibrium, and concentrations remain constant. This concept is essential for understanding chemical reactions' dynamic nature and predicting the outcome of changes in reaction conditions.

Calculating the Reaction Quotient

Hey everyone, so in this video we're going to take a look at the reaction quotient. Now, the reaction quotient itself is represented by the variable Q and it is a ratio of product to reacting concentrations at a particular time. Like the equilibrium constant K, it can be calculated by setting up an expression and ignoring solids and liquids.

So our equilibrium constant K looks at concentrations at equilibrium. Q looks at a particular time. That time could be at equilibrium or could be outside of equilibrium. So that's the distinction between the two. But both of them will ignore solids and liquids when setting up their expressions.

Click on the next video and let's take a look at how to set up one of these expressions using our reaction quotient Q.

Reaction Quotient Example

Here in this example question it states the formation of gaseous ammonia is displayed by the equation given below. Here we have one mole of nitrogen gas reacting with one mole of hydrogen gas to produce 2 moles of ammonia gas. What is the reaction quotient if the following amounts in moles of each component is placed in a 10 liter vessel?

All right, so here we have the moles of N₂, H₂, and NH₃ respectively. So remember we're going to set up an expression in the same way we would set up an expression for equilibrium constant K. But here it's going to be Q equals products over reactants. If we look, everything is in the gaseous state. There are no solids or liquids, so we're not going to ignore any of the compounds.

So here would be NH₃. Remember, your coefficient becomes your exponent or power, so this would be squared divided by N₂ times H₂. Remember, when it comes to these expressions, we typically use the units of molarity or atmospheres. Here they're giving us moles, and they're also giving us liters. With this information, we can figure out our molarity.

So you would divide each one of these moles by 10 liters, and when you do that you get our new molarities for each one. So here NH₃ will become 0.0529 molar. It'd still be squared divided by N₂, which is 0.0650, times H₂, which is 0.0330. When we punch that into our calculators, we'll get 1.3046 as our reaction quotient Q value.

If we look at the question, all these numbers have three significant figures, so I'm going to round this down to 1.30 as our final answer. So this will represent the value of our reaction quotient Q.

Comparing Q to K

So once you've learned how to calculate Q, you can compare it to your equilibrium constant K. Now here we're going to say once Q, once you've determined Q compared to K to determine which direction the chemical reaction will shift, and this is important, Q will always shift towards K in order to maintain equilibrium. Here we're going to say the balance chemical equation will shift in the same direction as Q.

So if Q was in the forward direction to get to K, your chemical reaction moves in the forward direction. If Q moves in the reverse direction to get to K, then your chemical reaction also moves in the reverse direction. Here we're going to say shifting towards a side causes all molecules on that side to increase in amount. So here let's take a look at this graph where we're doing a Q to K comparison.

In the first one, Q will be less than K, so here the reaction will shift. So Q is less than K. Let's say that Q is equal to 10 and K is equal to 30. Q will always shift to K to get to equilibrium or maintain equilibrium. So Q would move this way. So the reaction will follow suit. The reaction will shift to the right to maintain equilibrium. Here our chemical equation is 2 moles of NO gas plus BR₂ liquid gives us two moles of NOBr gas. It would also move in the same direction.

Now remember, wherever we're heading to the site we're going to will increase in amount. So our product amount would increase. But there's a balance in terms of this. If my product side is increasing, then my reactant side has to be decreasing. Now let's flip everything here. Now let's say that Q is greater than K. So Q is 75. K is still 30. So Q will always shift to K. So Q will go this way. So the reaction will shift in the reverse direction or to the left to maintain equilibrium.

So in the same way it goes this way, we're heading towards the reactant side. Since we're going towards the reactant side, the reactants would be increasing in amount. Things have to balance out. If the reactant side is increasing, then my product side has to be decreasing. Now finally, let's look at if Q is equal to K. So here, if Q is equal to K, then the reaction is said to be at equilibrium. Since they're both equal to 30, Q can't shift anywhere, it's already at K.

And if we're not shifting anywhere, then my chemical reaction wouldn't be shifting anywhere. So you would say that the reactants and product amounts are unchanged or you can say they're constant, they're not increasing or decreasing, they are where they are. OK. So they're remaining unchanged or constant. OK, so that's the way you want to take a look when it comes to your reaction quotient Q. If you know what it is, you can compare it to K and that can help you determine which direction you're going to shift to maintain equilibrium. This can be directly applied to your chemical reaction.

Reaction Quotient Example

Hey everyone, so for this example question, it says for the reaction we have 4 moles of hydrogen bromide gas reacting with one mole of oxygen gas to produce 2 moles of bromine gas +2 water liquids. We're told that the equilibrium constant is 0.063 at 400 Kelvin. Now if the reaction quotient is 0.100, which of the following statements is true?

All right, so remember our equilibrium constant is K and our reaction quotient is Q. When we have both Q&K, we can compare them to one another to tell which direction our chemical reaction will shift. So do a number line. Here K represents our equilibrium amount, so put it in the middle. Now, is Q bigger than, less than or equal to K? Well, Q is 0.100, so Q is larger. But remember, Q will always shift towards K. To maintain equilibrium. The direction queue shifts in the same direction that our chemical reaction will shift, So our chemical reaction also shifts in the reverse direction.

Now remember, the direction we're moving towards will be increasing an amount. Since we're moving towards the reactants, our reactants would be increasing in amount and things must be balanced. If our reactant site is increasing, then our product site has to be decreasing. Based on this information, we have to say which of the following statements is true.

So if we take a look here, it says HBR will decrease. Well HBR is a reactant. We're moving towards a reactant site so it should be increasing not decreasing. So this is false, O₂ will increase. This is true. It's a reactant, so it should be going up. Next BR₂ will increase, H₂O will increase. Both of these are products we're moving away from the product side, so they should be decreasing an amount, not increasing amount. So out of all the statements, only option B is true.

For the reaction: 2 CO₂ (g)  ⇌ 2 CO (g) + 2 O₂  (g), the equilibrium constant is 3.12 × 10⁻⁴ at 400 K, while the reaction quotient is 4.18 × 10⁻⁴. If initially we have 0.20 atm CO₂, 0.30 atm CO and 0.15 atm O₂, which of the following statements is not true? 

a) The pressure of CO₂ will be greater than 0.20 atm. 

b) The pressure of CO will be less than 0.30 atm. 

c) The pressure of O₂ will be greater than 0.15 atm. 

d) The pressure of O₂ will be less than 0.15 atm. 

e) The reaction will favor reactants. 

• A. The pressure of CO₂ will be greater than 0.20 atm.
• B. The pressure of CO will be less than 0.30 atm.
• C. The pressure of O₂ will be greater than 0.15 atm.
• D. The pressure of O₂ will be less than 0.15 atm.
• E. The reaction will favor reactants.

The equilibrium constant for the following gas phase reaction is 0.75 at 750 K. After a short time, analysis of a small amount of the reaction mixture shows the concentrations to be [NOBr] = 1.25 M, [NO] = 0.80 M and [Br₂] = 0.50 M. Which of the following statements is/are true? 

2 NOBr (g)  ⇌ NO (g) + Br₂ (g)

a) The reaction mixture is at equilibrium. 

b) No further reaction will occur. 

c) The partial pressure of NOBr will increase. 

d) The partial pressure of NO will increase. 

e) The reaction will shift to the left, the reactant side. 

• A. The reaction mixture is at equilibrium.
• B. No further reaction will occur.
• C. The partial pressure of NOBr will increase.
• D. The partial pressure of NO will increase.
• E. The reaction will shift to the left, the reactant side.

Here’s what students ask on this topic:

How can one find the reaction quotient?

To find the reaction quotient (Q), you'll need to know the balanced chemical equation for the reaction and the concentrations or partial pressures of the reactants and products at a given moment, not necessarily at equilibrium.

The reaction quotient is calculated similarly to the equilibrium constant (K), but with the current concentrations or pressures. Here's how you do it:

1. Write down the balanced chemical equation for the reaction.
2. For each reactant and product, write down its molarity (for solutions) or partial pressure (for gases) at the state you're interested in.
3. Use the expression for the equilibrium constant, but plug in the current values. The general form of the expression is:

Q = ^{[C]c [D]d} / _{[A]a [B]b}

where [A], [B], [C], and [D] are the molar concentrations (or partial pressures) of the reactants and products, and a, b, c, and d are the stoichiometric coefficients from the balanced equation.

4. Calculate Q using the values you've gathered. Remember that solids and pure liquids do not appear in the Q expression, as their concentrations are constants.

The reaction quotient helps you determine the direction in which the reaction will proceed to reach equilibrium: if Q < K, the reaction goes

How do you calculate the reaction quotient?

To calculate the reaction quotient (Q), you need to write down the balanced chemical equation for the reaction and identify the reactants and products. The reaction quotient is a ratio of the concentrations of the products raised to the power of their coefficients in the balanced equation to the concentrations of the reactants raised to the power of their coefficients.

Here's the general formula for a reaction quotient:

$Q=\frac{{\left[}^C{\left[}^D}{{\left[}^A{\left[}^B}$

In this formula:

• [A], [B], [C], and [D] represent the molar concentrations of the reactants and products.
• a, b, c, and d are the stoichiometric coefficients from the balanced chemical equation.

For gases, you can use partial pressures instead of concentrations. It's important to note that solids and pure liquids are omitted from the reaction quotient because their concentrations do not change during the reaction.

To calculate Q, plug in the current concentrations or partial pressures of the reactants and products into the formula and perform the arithmetic. Remember that Q can help you predict which way a reaction will proceed to reach equilibrium: if Q < K (the equilibrium constant), the reaction goes forward; if Q > K, the reaction goes in reverse.

What is the reaction quotient?

The reaction quotient, denoted as Q, is a measure that compares the relative amounts of products and reactants present during a reaction at a given point in time. It is calculated using the same expression as the equilibrium constant (K), but unlike K, which is only calculated at equilibrium, Q can be determined at any stage of the reaction.

To calculate Q, you take the concentrations (or pressures for gases) of the products raised to the power of their coefficients in the balanced chemical equation and divide by the concentrations of the reactants raised to the power of their coefficients. If Q is less than K, the reaction will proceed in the forward direction to reach equilibrium. If Q is greater than K, the reaction will shift in the reverse direction to reach equilibrium. When Q equals K, the system is at equilibrium, and no net change occurs in the concentrations of reactants and products.

When the reaction quotient (Q) is greater than the equilibrium constant (K), what happens to the reaction?

When the reaction quotient (Q) is greater than the equilibrium constant (K), it indicates that the concentrations of the products are higher relative to the reactants than they would be at equilibrium. In order to reach equilibrium, the reaction will shift to favor the formation of reactants. This shift is described by Le Chatelier's Principle, which states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.

In practical terms, if Q > K, the reaction will proceed in the reverse direction to reduce the amount of products and increase the amount of reactants until Q equals K. This process will continue until the system has reached its equilibrium state once again, where the forward and reverse reactions occur at the same rate and the concentrations of reactants and products remain constant over time.

Which description best captures the relationship between the equilibrium constant and the reaction quotient?

The equilibrium constant (K) and the reaction quotient (Q) are closely related concepts that describe the state of a chemical reaction. The equilibrium constant is a fixed value that represents the ratio of the concentrations of the products to the reactants at equilibrium for a given temperature. It is determined by the particular reaction and does not change unless the temperature changes.

On the other hand, the reaction quotient (Q) is the same ratio of the concentrations of the products to the reactants, but it can be calculated at any point during the reaction, not just at equilibrium.

The relationship between K and Q can indicate the direction in which a reaction will proceed to reach equilibrium:

• If Q < K, the reaction will proceed in the forward direction, producing more products.
• If Q > K, the reaction will proceed in the reverse direction, producing more reactants.
• If Q = K, the reaction is at equilibrium, and no net change in the concentrations of reactants and products occurs.