equilibrium
(noun)
The state of a reaction in which the rates of the forward and reverse reactions are the same.
(noun)
The state of a reaction in which the rates of the forward and reverse reactions are equal.
Examples of equilibrium in the following topics:
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Homogeneous versus Heterogeneous Solution Equilibria
- The equilibrium constants for homogeneous and heterogeneous solutions need to be calculated differently.
- The equilibrium constant K for a given reaction is defined as the ratio of the products of a reaction to the reactants, measured at equilibrium.
- The reaction quotient measured at equilibrium is the equilibrium constant K.
- A heterogeneous equilibrium is a system in which reactants and products are found in two or more phases.
- The equilibrium constant K is simply [Br2], with the concentration of the pure liquid Br2 excluded.
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Equilibrium Constant Expression
- The equilibrium constant is an expression that gives the ratio of reactants and products at equilibrium.
- This constant is known as the equilibrium constant.
- Their activity is 1, so they do not need to be written in the equilibrium constant.
- The progress of an equilibrium reaction can be visualized.
- The equilibrium constant,denoted by K, is the ratio of products to reactants at equilibrium.
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Reaction Quotients
- If Q < Keq, the reaction will move to the right (in the forward direction) in order to reach equilibrium.
- If Q > Keq, the reaction will move to the left (in the reverse direction) in order to reach equilibrium.
- Eventually, the concentrations become constant; at this point, the reaction is at equilibrium.
- The equilibrium constant, Keq, can be expressed as follows:
- However, most reactions will generally reach equilibrium in a finite period of time.
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Equilibrium
- The concentrations of A and B will change over time, until they reach equilibrium.
- This is the point at which the system has reached chemical equilibrium.
- While there are various factors that can increase or decrease the amount of time it takes for a given system to reach equilibrium, the equilibrium position itself is unaffected by these factors.
- We also explain how the concentrations remain constant at equilibrium.
- Chemical equilibrium is akin to two cities connected a bridge.
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Heterogeneous and Multiple Equilibria
- Therefore, the equilibrium expression for this reaction will be written as:
- Both steps must be included in the equilibrium constant equation.
- K1 and K2 are examples the equilibrium constants for each step.
- Notice that the equilibrium expression for the overall reaction, Keq, is equal to the product of the equilibrium expressions for the two reaction steps.
- Calculate the equilibrium constant of a multiple-step reaction, given the equilibrium constant for each step
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Expressing the Equilibrium Constant of a Gas in Terms of Pressure
- Up to this point, we have been discussing equilibrium constants in terms of concentration.
- Our equilibrium constant in terms of partial pressures, designated KP, is given as:
- Note that this expression is extremely similar to KC, the equilibrium expression written in terms of concentrations.
- Inside this tank, propane is compressed into a liquid, which is in equilibrium with its gaseous headspace.
- Write the equilibrium expression, KP, in terms of the partial pressures of a gas-phase reaction
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Calculating Equilibrium Concentrations
- Equilibrium constants can be used to calculate the concentrations of reactants and products that will be present at equilibrium.
- ICE stands for "initial, change, equilibrium."
- Only one of these values involves equilibrium concentrations that are actually possible.
- Knowing the initial concentration values and equilibrium constant we were able to calculate the equilibrium concentrations for N2, O2 and NO.
- Calculate the concentrations of reaction components at equilibrium given the starting concentrations and the equilibrium constant
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Predicting the Direction of a Reaction
- Equilibrium constants can be used to predict whether a reaction will favor the products or the reactants.
- K is the equilibrium ratio of products to reactants.
- The equilibrium constant is only used when a reaction has reached equilibrium.
- This value is greater than the equilibrium value: there is now too much of the products for equilibrium to exist.
- Evaluate whether a chemical reaction has reached equilibrium from the reaction coefficient (Q) and the equilibrium constant (K), and use the latter to predict whether the reaction will favor the reactants or products
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Calculating the pH of a Buffer Solution
- The pH of a buffer solution can be calculated from the equilibrium constant and the initial concentration of the acid.
- The strength of a weak acid is usually represented as an equilibrium constant.
- The acid-dissociation equilibrium constant (Ka), which measures the propensity of an acid to dissociate, for the reaction is:
- ICE (Initial, Change, Equilibrium) tables are very helpful tools for understanding equilibrium and for calculating the pH of a buffer solution.
- Describes the final concentration of the reactants and products at equilibrium.
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The Effect of a Catalyst
- Catalysts speed up the rate of a reaction, but do not have an affect on the equilibrium position.
- Reactions can be sped up by the addition of a catalyst, including reversible reactions involving a final equilibrium state.
- Recall that for a reversible reaction, the equilibrium state is one in which the forward and reverse reaction rates are equal.
- In the presence of a catalyst, both the forward and reverse reaction rates will speed up equally, thereby allowing the system to reach equilibrium faster.
- To reiterate, catalysts do not affect the equilibrium state of a reaction.