Worked example: Using the reaction quotient to predict a pressure change | Khan Academy

Khan Academy
16 May 201605:53
EducationalLearning
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TLDRThe video explains how to determine the change in total pressure in a reaction vessel as it approaches equilibrium. Using a reaction quotient (Q), the video demonstrates that Q is less than the equilibrium constant (K), indicating the system is not at equilibrium. As the reaction shifts to produce more products, the number of gas molecules decreases, leading to a reduction in total pressure. The explanation is broken down into calculating Q, comparing it to K, and understanding the resulting shift in reaction dynamics.

Takeaways
  • 🔍 The reaction vessel contains 1.2 moles of CO, 1.5 moles of H2, and 2.0 moles of CH3OH.
  • 🔄 CO reacts with H2 in a 1:2 ratio to produce CH3OH, and the reaction is reversible.
  • 🧪 The equilibrium constant (K) for the reaction is 14.5 at a constant temperature.
  • 📊 To determine if the system is at equilibrium, the reaction quotient (Q) is calculated.
  • 📉 Q is calculated using the concentrations of CO, H2, and CH3OH, which are 1.2 M, 1.5 M, and 2.0 M respectively.
  • 🧮 Q is found to be 0.74, which is less than K (14.5), indicating the system is not at equilibrium.
  • ➡️ Since Q < K, the reaction will shift towards the products to reach equilibrium.
  • ⚖️ As the reaction shifts towards the products, the total pressure in the system will decrease.
  • 🔬 The decrease in total pressure is due to the reduction in the number of gas molecules as the reaction progresses towards equilibrium.
  • 📈 The system initially has 3 moles of reactant gas and 1 mole of product gas, favoring the side with fewer gas molecules as it shifts to equilibrium.
Q & A

  • What is the initial concentration of carbon monoxide in the reaction vessel?

    -The initial concentration of carbon monoxide is 1.2 M (molar).

  • What does the equilibrium constant (Kc) of 14.5 indicate about the reaction?

    -The equilibrium constant (Kc) of 14.5 indicates the ratio of product to reactant concentrations at equilibrium, with a higher value suggesting the reaction favors the formation of products.

  • How is the reaction quotient (Q) calculated for this reaction?

    -The reaction quotient (Q) is calculated using the formula Q = [CH3OH] / ([H2]^2 * [CO]), where the concentrations of methanol, hydrogen, and carbon monoxide are used.

  • What is the significance of Q being less than K in this reaction?

    -Q being less than K indicates that the reaction is not at equilibrium and will shift to produce more products to reach equilibrium.

  • How do you determine if the system is at equilibrium using Q and K?

    -By comparing Q to K: if Q = K, the system is at equilibrium; if Q < K, the system will shift to produce more products; if Q > K, the system will shift to produce more reactants.

  • Why will the total pressure decrease as the system approaches equilibrium?

    -The total pressure will decrease because the reaction shifts to favor the side with fewer gas molecules (products), reducing the total number of gas molecules and thus the pressure.

  • How do you calculate the initial concentrations of the gases in the reaction vessel?

    -The initial concentrations are calculated by dividing the number of moles of each gas by the volume of the vessel (1 liter), making the concentration equal to the number of moles.

  • What does the number line representation of Q and K help visualize?

    -The number line helps visualize the relative positions of Q and K, showing how the reaction will shift (towards products or reactants) to reach equilibrium.

  • What is the relationship between the number of gas molecules and total pressure in the system?

    -The total pressure is directly related to the number of gas molecules in the system; more gas molecules result in higher pressure, and fewer gas molecules result in lower pressure.

  • What initial information is given about the quantities of the gases in the reaction vessel?

    -The reaction vessel contains 1.2 moles of carbon monoxide, 1.5 moles of hydrogen gas, and 2.0 moles of methanol gas.

Outlines
00:00
🔍 Initial Reaction Analysis and Equilibrium Determination

This paragraph introduces a chemical reaction in a one-liter vessel with given moles of carbon monoxide, hydrogen, and methanol gases. It explains the reversible nature of the reaction and the use of the reaction quotient 'Q' to determine if the system is at equilibrium. The equilibrium constant 'K' is provided, and the initial concentrations are calculated based on the moles and volume. The calculation of 'Q' reveals that the system is not at equilibrium since 'Q' is less than 'K', indicating that the reaction will proceed to produce more products, thus changing the pressures within the system.

05:03
📉 Predicting Pressure Change Towards Equilibrium

The second paragraph delves into the implications of the reaction quotient 'Q' being less than the equilibrium constant 'K'. It discusses how the system will shift towards the production of more methanol to reach equilibrium. This shift is visualized on a number line to show the relative values of 'Q' and 'K'. The paragraph explains that the reaction will favor the side with fewer gas molecules, which in this case, are the products. Consequently, as the reaction progresses towards equilibrium, the total pressure will decrease due to the reduction in the number of gas molecules in the system.

Mindmap
Keywords
💡Reaction Vessel
A reaction vessel is a container where chemical reactions occur. It is crucial in this video as it houses the gases involved in the reaction. The script mentions a one-liter reaction vessel containing specific amounts of carbon monoxide, hydrogen, and methanol, which sets the stage for the chemical equilibrium discussion.
💡Moles
Moles are a measure of the amount of a substance, typically used in chemistry to quantify the number of atoms, molecules, or ions. In the script, the initial amounts of carbon monoxide, hydrogen, and methanol are given in moles, which is essential for calculating concentrations and understanding the reaction's stoichiometry.
💡Chemical Equilibrium
Chemical equilibrium refers to a state in a reversible chemical reaction where the rate of the forward reaction equals the rate of the reverse reaction, resulting in no net change in the concentrations of reactants and products. The video discusses how the system will change to reach this state, given the initial conditions and the equilibrium constant.
💡Equilibrium Constant (K)
The equilibrium constant (K) is a measure of the extent to which a reversible reaction proceeds to completion. It is given as 14.5 in the script for the reaction between carbon monoxide and hydrogen to form methanol, and it is used to determine whether the reaction is at equilibrium or will proceed in a certain direction.
💡Reaction Quotient (Q)
The reaction quotient (Q) is a preliminary calculation used to determine the direction in which a reaction will proceed to reach equilibrium. It is calculated using the initial concentrations of reactants and products. In the script, Q is calculated to be 0.74, indicating that the system is not at equilibrium.
💡Concentration
Concentration in chemistry is the amount of a substance per unit volume, typically expressed in molarity (moles per liter). The script uses the concentrations of the gases to calculate Q and to understand how the reaction will proceed towards equilibrium.
💡Stoichiometric Ratio
The stoichiometric ratio defines the proportion in which reactants and products are involved in a chemical reaction. In the script, the reaction between carbon monoxide and hydrogen to form methanol has a one-to-two ratio, which is important for calculating the changes in concentrations as the reaction proceeds.
💡Total Pressure
Total pressure in a gaseous system is the sum of the partial pressures of the individual gases. The script discusses how the total pressure will change as the reaction approaches equilibrium, with a focus on the relationship between the number of gas molecules and pressure.
💡Le Chatelier's Principle
Le Chatelier's Principle predicts how a system at equilibrium will respond to changes in conditions such as pressure, temperature, or concentration. Although not explicitly mentioned in the script, the principle underlies the discussion of how the system will adjust to reach equilibrium when Q is less than K.
💡Mole Ratio
The mole ratio is the ratio of the amounts of substances involved in a chemical reaction, which is essential for understanding the stoichiometry of the reaction. In the script, the mole ratio is used to calculate the initial concentrations and to predict the shift towards products to reach equilibrium.
💡Partial Pressure
Partial pressure is the pressure exerted by an individual gas in a mixture of gases. While not directly mentioned in the script, the concept is implied when discussing how the shift in reaction will affect the total pressure, as each gas contributes to the total pressure based on its mole fraction.
Highlights

A one litre reaction vessel contains 1.2 moles of carbon monoxide, 1.5 moles of hydrogen gas, and 2.0 moles of methanol gas.

The reaction between carbon monoxide and hydrogen to produce methanol is reversible.

The equilibrium constant for the reaction is 14.5 at a certain temperature.

The system's temperature remains constant throughout the process.

The reaction quotient 'Q' is calculated using the product concentration of methanol divided by the concentration of hydrogen squared and carbon monoxide.

Initial concentrations are equal to the number of moles due to the one-liter volume.

Initial concentrations are 1.2 molar for carbon monoxide, 1.5 molar for hydrogen, and 2.0 molar for methanol.

The calculated reaction quotient 'Q' at the initial state is 0.74.

'Q' is not equal to 'KC', indicating the system is not at equilibrium.

The system will change pressures as it attempts to reach equilibrium.

The reaction quotient 'Q' is less than the equilibrium constant 'K', suggesting a shift towards products.

The reaction will shift to favor the production of more products to reach equilibrium.

The total pressure is related to the moles of gas in the system.

Shifting to favor the reactants, which have fewer gas molecules, will reduce the total pressure.

As the reaction approaches equilibrium, the total pressure will decrease due to the reduction in gas molecules.

The reaction quotient 'Q' being less than 'K' is the reason for the decrease in total pressure.

Transcripts

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Related Tags
Chemical ReactionPressure ChangeEquilibriumReversible ReactionMethanol ProductionCarbon MonoxideHydrogen GasReaction QuotientMolar ConcentrationGas Molecules