Question

Consider an equilibrium mixture of four chemicals (A, B, C, and D, all gases) reacting in a closed flask according to the following equation: $$A+B\rightleftharpoons C+D$$ a. You add more $A$ to the flask. How does the concentration of each chemical compare to its original concentration after equilibrium is reestablished? Justify your answer. b. You have the original set-up at equilibrium, and add more D to the flask. How does the concentration of each chemical compare to its original concentration after equilibrium is reestablished? Justify your answer.

Short answer

a. After adding more A and re-establishing equilibrium: - A's concentration is higher than originally. - B's concentration is lower than originally. - C's concentration is higher than originally. - D's concentration is higher than originally. b. After adding more D and re-establishing equilibrium: - A's concentration is higher than originally. - B's concentration is higher than originally. - C's concentration is lower than originally. - D's concentration is higher than originally.

Step-by-step solution

Part a: Adding more A to the flask

According to Le Châtelier's principle, when a change is made to a system in equilibrium, the system will adjust to counteract that change and re-establish equilibrium. In this case, more A is added to the flask. 1. When more A is added to the system, the concentration of A increases. 2. This increase in the concentration of A causes the forward reaction to proceed at a faster rate compared to the reverse reaction. 3. As the forward reaction proceeds, the concentrations of B decrease, and the concentrations of C and D increase. 4. The equilibrium will shift to the right, consuming A and B, and producing more C and D, until a new equilibrium is established. So, after the equilibrium is re-established: - The concentration of A is higher than its original concentration. - The concentration of B is lower than its original concentration. - The concentration of C is higher than its original concentration. - The concentration of D is higher than its original concentration.

Part b: Adding more D to the flask

Following Le Châtelier's principle, we will analyze the effect on the equilibrium when additional D is added to the system. 1. When more D is added to the system, the concentration of D increases. 2. This increase in the concentration of D causes the reverse reaction to proceed at a faster rate compared to the forward reaction. 3. As the reverse reaction proceeds, the concentrations of A and B increase, and the concentration of C decreases. 4. The equilibrium will shift to the left, consuming C and D, and producing more A and B, until a new equilibrium is established. So, after the equilibrium is re-established: - The concentration of A is higher than its original concentration. - The concentration of B is higher than its original concentration. - The concentration of C is lower than its original concentration. - The concentration of D is higher than its original concentration.

Key concepts

Le Châtelier's Principle

Imagine sitting comfortably in your room, and suddenly someone opens the window. It gets chilly, so what do you do? You probably reach for a sweater to counter the change in temperature and maintain comfort. This ability to adjust to new circumstances is not unlike a scientific concept known as Le Châtelier's principle. This principle states that if an external change is applied to a system in equilibrium, the system will adjust itself in a way to counteract the change and achieve a new balance.

Applying this to a chemical reaction, suppose we have a reaction of gases A and B forming C and D. If gas A is suddenly added to the mix, there's a disturbance, just like the cold air rushing into your room. The reaction responds by 'putting on a sweater,' meaning it will produce more of C and D to absorb the excess A, shifting the equilibrium to the right. Understanding Le Châtelier's principle is like having a cheat sheet for predicting how a reaction will respond to changes.

Equilibrium Re-establishment

Equilibrium re-establishment is the chemical system's 'bounce back' process after being disturbed. A chemical equilibrium is like a set of scales in perfect balance, but when one side becomes heavier, the scales tilt. That's when the system starts working out how to balance the scales again. In the case of adding more A to our reaction in the closed flask, increasing A is like adding more weight to one side. The system will do its gymnastics, making more C and D, until that set of scales is balanced once again. It is a dynamic dance of breaking and forming bonds until every participant—the reactants and products—finds their new place in the mix. Properly grasping the concept of equilibrium re-establishment can transform a student's understanding of chemical reactions from static pictures to vivid, moving scenes.

Reaction Rate

The reaction rate tells us how quickly the reactants turn into products. Like sprinters on a track, the speed at which they zip to the finish line varies, and every race can have different things affecting their speed—shoes, wind, even how much lunch they had! In chemistry, factors like concentration, temperature, and catalysts play similar roles, altering the speed of a reaction.

When more reactant, like A, is added to our system, it's similar to giving the runners a tailwind; it speeds them up. But in this case, it specifically speeds up the forward reaction. More A means more collisions between A and B, and with more successful collisions, we get more C and D being formed, faster. Learning about reaction rates is not just about speed but understanding the very essence of how reactions proceed over time.

Concentration Changes

The concept of concentration changes in a chemical reaction is much like adding ingredients to a recipe. If the recipe calls for two carrots and you throw in an extra one, the carrot flavor becomes more pronounced. In our chemical system, when we introduce more of a reactant or product, like A or D, we alter the 'flavor' of the equilibrium mixture. This change in concentration sets off a chain reaction (pun intended!) where products and reactants adjust their amounts to get that perfect taste—or equilibrium—back.

Understanding concentration changes in the context of equilibrium isn’t just about having more or fewer molecules in a flask; it's about envisioning a microscopic tug of war, where each side pulls with a strength proportionate to their numbers. Students who visualize and relate to these changes are better equipped to handle complex reaction scenarios with confidence.