The result would be the same. Privacy Policy. Skip to main content. Chemical Equilibrium. Search for:. Calculating the Equilibrium Constant Calculating Equilibrium Concentrations Equilibrium constants can be used to calculate the concentrations of reactants and products that will be present at equilibrium. Learning Objectives Calculate the concentrations of reaction components at equilibrium given the starting concentrations and the equilibrium constant.
Key Takeaways Key Points If you know K c and the initial concentrations for a reaction, you can calculate the equilibrium concentrations. Using the ICE chart and equilibrium-constant equation, you can write an expression to describe the concentration changes in the reactants and products.
Key Terms equilibrium : The state of a reaction in which the rates of the forward and reverse reactions are the same. Licenses and Attributions. This is the humidity that will be maintained if the hydrate is placed in a closed container of dry air. Anhydrous CaSO 4 forms compact, powdery crystals, whereas the elongated crystals of the hemihydrate bind themselves into a cement-like mass that makes this material useful for making art objects, casts for immobilizing damaged limbs, and as a construction material fireproofing, drywall.
Chem1 Virtual Textbook. Learning Objectives Make sure you thoroughly understand the following essential ideas: The equilibrium quotient Q is the value of the equilibrium expression of a reaction for any arbitrary set of concentrations or partial pressures of the reaction components. The equilibrium constant K is the value of Q when the reaction is at equilibrium. K has a unique value for a given reaction at a fixed temperature and pressure. Q and K can be expressed in terms of concentrations, partial pressures, or, when appropriate, in some combination of these.
For a reaction in which all the components are gases, Q c and K c will have different values except in the special case in which the total number of moles of gas does not change.
Concentration terms for substances whose concentrations do not change in the reaction do not appear in equilibrium expressions. The most common examples are [H 2 O] when the reaction takes place in aqueous solution so that [H 2 O] is effectively constant at A reaction whose equilibrium constant is in the range of about 0. Q and K are conventionally treated as dimensionless quantities, and need not ordinarily have units associated with them.
Heterogeneous reactions are those in which two or more phases are involved; homogeneous reactions take place in a single phase. The equilibrium expression can be manipulated and combined in the following ways: If the reaction is written in reverse, Q becomes Q —1 ; If the coefficients of an equation are multiplied by n , Q becomes Q n ; Q for the sum of two reactions that is, for two reactions that take place in sequence is the product Q 1 Q 2.
Pressures can Express Concentrations Although we commonly write equilibrium quotients and equilibrium constants in terms of molar concentrations, any concentration-like term can be used, including mole fraction and molality.
A note about pressure and concentration units In this lesson and in most of the others in this set, we express concentrations in mol L —1 and pressures in atmospheres. Do not show unchanging concentrations!
The substance is also the Solvent This happens all the time in acid-base chemistry. This temperature corresponds, of course, to the boiling point of water. The normal boiling point of a liquid is the temperature at which the partial pressure of its vapor is 1 atm. Kinetically Hindered Reactions Although it is by no means a general rule, it frequently happens that reactions having very large equilibrium constants are kinetically hindered , often to the extent that the reaction essentially does not take place.
Do Equilibrium Constants have Units? The rules are very simple: Writing the equation in reverse will invert the equilibrium expression; Multiplying the coefficients by a common factor will raise Q or K to the corresponding power.
Multi-step Equilibria Rule T he equilibrium constant for the sum of two or more reactions is the product of the equilibrium constants for each of the steps. More on heterogeneous reactions Heterogeneous reactions are those involving more than one phase. The vapor pressure of solid hydrates A particularly interesting type of heterogeneous reaction is one in which a solid is in equilibrium with a gas.
Thermal decomposition of limestone, a first step in the manufacture of cement. Vaporization of water. If the cover is removed so that this pressure cannot be maintained, the system will cease to be at equilibrium and the water will evaporate. The only way to heat water above its normal boiling point is to do so in a closed container that can withstand the increased vapor pressure.
These two very different values of K illustrate very nicely why reducing combustion-chamber temperatures in automobile engines is environmentally friendly. See the discussion of this reaction in the section on the Haber process. Dissociation of any stable molecule into its atoms is endothermic. This means that all molecules will decompose at sufficiently high temperatures.
The thermodynamically correct equilibrium constant expression relates the activities of all of the species present in the reaction. Although the concept of activity is too advanced for a typical General Chemistry course, it is essential that the explanation of the derivation of the equilibrium constant expression starts with activities so that no misconceptions occur.
For the hypothetical reaction:. To avoid the use of activities, and to simplify experimental measurements, the equilibrium constant of concentration approximates the activities of solutes and gases in dilute solutions with their respective molarities.
However, the activities of solids, pure liquids, and solvents are not approximated with their molarities. Instead these activities are defined to have a value equal to 1 one. Here, the letters inside the brackets represent the concentration in molarity of each substance.
Notice the mathematical product of the chemical products raised to the powers of their respective coefficients is the numerator of the ratio and the mathematical product of the reactants raised to the powers of their respective coefficients is the denominator. This is the case for every equilibrium constant. A ratio of molarities of products over reactants is usually used when most of the species involved are dissolved in water.
A ratio of concentrations can also be used for reactions involving gases if the volume of the container is known. Gaseous reaction equilibria are often expressed in terms of partial pressures. The equilibrium constant of pressure gives the ratio of pressure of products over reactants for a reaction that is at equilibrium again, the pressures of all species are raised to the powers of their respective coefficients.
It is used to determine which way the reaction will proceed at any given point in time. The most important consideration for a heterogeneous mixture is that solids and pure liquids and solvents have an activity that has a fixed value of 1. From a mathematical perspective, with the activities of solids and liquids and solvents equal one, these substances do not affect the overall K or Q value. This convention is extremely important to remember, especially in dealing with heterogeneous solutions.
In this case, since solids and liquids have a fixed value of 1, the numerical value of the expression is independent of the amounts of A and B.
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