Intro to Chemical Equilibrium - Online Tutor, Practice Problems & Exam Prep

Chemical equilibrium is a state in which a reversible reaction proceeds at a rate where the forward and reverse reactions occur at equal rates, resulting in no net change in the concentrations of reactants and products. This dynamic process continues with reactants transforming into products and some products re-forming into reactants. Equilibrium can be established in both homogeneous systems, where all reactants and products are in the same phase, and heterogeneous systems, where they exist in different phases. The concept is akin to a party where guests (representing reactants and products) leave and enter at the same rate, maintaining a constant number of attendees. Understanding equilibrium is crucial as it underpins many chemical processes and the principle that not all reactions proceed to completion.

Intro to Chemical Equilibrium

Hey everyone. So in this set of videos, we're going to take a look at chemical equilibrium. Now, what we're going to say first is that most chemical reactions never go to what we call completion. This means that not all the reactants are converted into products. Instead, we reach what's called a chemical equilibrium. Now we're going to say chemical equilibrium is the dynamic process where the rate of the forward and reverse reactions are equal to one another.

Now, what kind of effect will this have? Well, before we get to that, we're also going to say that a dynamic process means that the reaction does not simply stop. That keeps going in both directions. So when we refer to chemical equilibrium, we're going to say these reactions are reversible and because they're reversible, they're going to use double arrows. We're going to have one arrow that moves forward and we're going to have another arrow that moves in the reverse direction.

Now coming back to this idea of forward and reverse reactions being equal, if we take a look at this chemical reaction, we have two reactants here, one larger purple sphere and one smaller one. They combine together going in the forward direction together. When they combine, they become this more like pinkish type of compound. Now here since we're talking about the four direction, we look at the rate of the four direction. But since we're talking about chemical equilibrium, there's a chance of this react of this product that's been formed that some of it moves backwards and reforms are reactants.

Here we're going to say at equilibrium the rate of the forward reaction equals the rate of the reverse. That means there is no net change in concentrations. So basically we are losing some reactants to make products, but then some of that product goes in the reverse direction to recreate our reactants. So there's no real change in the amounts of products and reactants for the amount that we're losing. We're gaining it right back.

Now here, if we took a look at this, this graph here, we're going to say that this purple line represents our reactants. Remember, we breakdown reactants to help us reassemble them into products. Because of this, we're going to see a gradual decreasing of our reactants up to this point of 15 seconds. And then this pinkish fuchsia color is our product that's being formed. Initially it starts off as zero because at the beginning of a chemical reaction there is no product. We're going to start making product and eventually it's going to hit this point here 15 seconds.

That 15 seconds represents where equilibrium has been established. And if we're at chemical equilibrium, we have a reversible reaction, so they're moving forward and backwards, thereby there's no net change in our concentrations, which is why we see for the most part the level of reactants and products kind of staying constant. Now if we want to think of this in real world aspects, we could think of a party. In the party we have a combination of people represent products and reactants. We can say that we have products and react to one of each, leaving the party to go outside. But as they leave, we have another set of products and reactants entering the party. So nothing is really lost or gained. There's no net change in the number of people. Two left, but two came back in.

Now here we're going to say that chemical equilibrium can be either homogeneous or heterogeneous. Now when we say homogeneous, we're talking about homogeneous equilibrium. This is where reactants and products are present in the same state of matter. So basically all the compounds within the chemical reaction are gases, or all are aqueous, or all our liquids are solid. It's A1 type of phase. They're all the same. In a heterogeneous equilibrium, we have reactants and products that are in different phases. Maybe our balanced chemical reaction has a mixture of solids and gases present that represent a heterogeneous equilibrium. So remember, when we reach chemical equilibrium, there is no net change in the amount of reactants and products. Whatever is lost is being regained because we are reversible reaction.

Intro to Chemical Equilibrium Example

So describe what happens to a reaction when it reaches chemical equilibrium. Now here for A it says the reaction stops. Remember we said that in a dynamic process the reaction doesn't simply stop. So A could not be an answer.

B reactants form products as fast as products form reactants. This is an answer because remember when we get to chemical equilibrium we have a reversible reaction, meaning the rate of the reverse and the rate of the Ford reaction are equal to one another. So the pace at which reactants or products are being formed is equal. This leads to no net change in the concentrations of either your reactants or your products.

Here the collision frequencies of products and reactants are equal or identical. We never went into this detail, so we don't know for certain that this contributes in the same way that our rates would OK, and because of that we can't say it's a vital answer.

4D rate of forward and rate of reverse reactions are equal to 0. This would only happen if the reaction stopped. And again, since we're at equilibrium into dynamic process so the reaction doesn't stop there, would it be equal to 0? They'd be equal to one another. So out of all the choices given, only option B is the correct answer.

Which of the following does not represent a heterogenous equilibrium?  

I) CH₄(g) + 2 O₂(g) ⇌ CO₂(g) + 2 H₂O(g)                  
II) CO₂(g) + C(s) ⇌ 2 CO(g)
III) 2 H₂O(l) ⇌ 2 H₂(g) + O₂(g)
IV) CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO^–(aq) + H₃O⁺(aq)

• A. Only I)
• B. Only II)
• C. Only III)
• D. Only IV)
• E. Both I) & IV)
• F. Both II) & III)

Here’s what students ask on this topic:

What is chemical equilibrium?

Chemical equilibrium is a state in a chemical reaction where the rates of the forward and reverse reactions are equal, resulting in no net change in the concentration of reactants and products over time. This dynamic balance occurs when the process of the reactants converting into products is happening at the same rate as the process of the products converting back into reactants.

At equilibrium, the system isn't static as molecules continue to react with each other, but there's a constant ratio between the concentration of the reactants and the products. This ratio is described by the equilibrium constant ($K_{\mathrm{eq}}$), which is specific to each reaction at a given temperature. The value of $K_{\mathrm{eq}}$ gives an idea of the position of equilibrium; a large $K_{\mathrm{eq}}$ means that the equilibrium lies to the right with more products, while a small $K_{\mathrm{eq}}$ means it lies to the left with more reactants.

Understanding chemical equilibrium is crucial in fields like chemistry and biochemistry, as it applies to many reactions in nature and industrial processes. It helps predict the concentrations of substances in a system at equilibrium, which is essential for controlling chemical reactions in various applications.

Which of the following statements about the equilibrium of chemical reactions is correct?

Equilibrium in chemical reactions refers to the state 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 over time. This dynamic state means that reactions are still occurring, but the overall concentrations remain constant. It's important to note that equilibrium does not imply that the reactants and products are present in equal amounts, but rather that their ratios are constant at a given temperature and pressure. The position of equilibrium is described by the equilibrium constant ($K$), which is a ratio of the concentration of products to the concentration of reactants, each raised to the power of their stoichiometric coefficients. This constant is specific for a particular reaction at a specific temperature. If a system at equilibrium is disturbed (by changing concentration, pressure, or temperature), the position of equilibrium will shift in a direction that tends to counteract the disturbance, as described by Le Chatelier's principle.

When is chemical equilibrium reached?

Chemical equilibrium is reached in a chemical reaction when the rate of the forward reaction equals the rate of the reverse reaction. At this point, the concentrations of reactants and products remain constant over time, although both reactions continue to occur. It's important to understand that equilibrium does not mean the reactants and products are present in equal amounts, but rather that their ratios are stable. This state of balance is dynamic, meaning that individual molecules are still reacting and interconverting, but there's no net change in the overall concentrations of the chemical species involved. Equilibrium can be reached at various points depending on the nature of the reaction and the conditions, such as temperature and pressure, under which the reaction is occurring. The conditions at which equilibrium is established are described by the reaction's equilibrium constant ($K_{\mathrm{eq}}$), which is a specific value at a given temperature.

Which equation represents a chemical equilibrium?

A chemical equilibrium is represented by an equation where the reactants and products are in a dynamic balance, meaning the rate at which the reactants are converting into products is equal to the rate at which the products are converting back into reactants. This is typically denoted by a double-headed arrow (⇌) between the reactants and products.

For example, the chemical equilibrium of hydrogen gas (H₂) reacting with iodine gas (I₂) to form hydrogen iodide (HI) can be represented by the following equation:

$H_2\left(g\right)+I_2\left(g\right)\rightleftharpoons 2\mathrm{HI}\left(g\right)$

In this equation, the reactants H₂ and I₂ are on the left side, and the product HI is on the right side. The double-headed arrow indicates that the reaction can proceed in both the forward direction (towards the products) and the reverse direction (back to the reactants), and at equilibrium, the concentrations of all species remain constant over time.

Which of the following statements best describes a system at chemical equilibrium?

A system at chemical equilibrium is characterized by a state in which the rate of the forward chemical reaction equals the rate of the reverse reaction. In this state, the concentrations of reactants and products remain constant over time, not because the reactions have stopped, but because they are proceeding at equal rates. It's important to note that being at equilibrium does not necessarily mean that the reactants and products are present in equal amounts, but rather that their ratios are stable. This concept is governed by the principle of dynamic equilibrium, indicating that while individual molecules are continuously reacting, there is no net change in the concentrations of the substances involved. This equilibrium can be disturbed by changes in temperature, pressure, or concentration, as described by Le Chatelier's Principle.