# Chemical reaction and equilibrium constant expression

We can express Kc values in terms of moles per liter for both, but when dealing with gases it is often more convenient to use partial pressures. The procedure used in this section to derive the equilibrium constant expression only works with reactions that occur in a single step, such as the transfer of a chlorine atom from ClNO2 to NO.

Note that when V is expressed in liters and P in atmospheres, R must have the value 0. The ideal gas law relates the partial pressure of a gas to the number of moles and its volume: Most of the equilibria we deal with in this course occur in liquid solutions and gaseous mixtures.

The numerator of the equilibrium constant expression is the product of the concentrations of the "products" of the reaction raised to a power equal to the coefficient for this component in the balanced equation for the reaction.

The justification for this omission is that water is both the solvent and reactant, but only the tiny portion that acts as a reactant would ordinarly go in the equilibrium expression.

Throwing out the constant-concentration terms can lead to some rather sparse-looking equilibrium expressions. Rules for Writing Equilibrium Constant Expressions Even Chemical reaction and equilibrium constant expression chemical reactions that reach equilibrium occur in both directions, the reagents on the right side of the equation are assumed to be the "products" of the reaction and the reagents on the left side of the equation are assumed to be the "reactants.

Some might question, however, why the equilibrium constant expressions in the preceding exercise are expressed in terms of the concentrations of the gases in units of moles per liter. The amount of water consumed in the reaction is so minute because K is very small that any change in the concentration of H2O from that of pure water The last two processes 3 and 4 represent changes of state phase changes which can be treated exactly the same as chemical reactions.

This choice of units is indicated by adding a subscript "c" to the symbols for the equilibrium constants, to show that they were calculated from the concentrations of the components of the reaction.

Adding a catalyst will affect both the forward reaction and the reverse reaction in the same way and will not have an effect on the equilibrium constant. If mineral acid is added to the acetic acid mixture, increasing the concentration of hydronium ion, the amount of dissociation must decrease as the reaction is driven to the left in accordance with this principle.

For most practical purposes, the differences between these values are so small that they can be neglected. However, the law of mass action is valid only for concerted one-step reactions that proceed through a single transition state and is not valid in general because rate equations do not, in general, follow the stoichiometry of the reaction as Guldberg and Waage had proposed see, for example, nucleophilic aliphatic substitution by SN1 or reaction of hydrogen and bromine to form hydrogen bromide.

No matter what combination of concentrations of reactants and products we start with, the reaction will reach equilibrium when the ratio of the concentrations defined by the equilibrium constant expression is equal to the equilibrium constant for the reaction.

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.

For example, adding more S from the outside will cause an excess of products, and the system will try to counteract this by increasing the reverse reaction and pushing the equilibrium point backward though the equilibrium constant will stay the same.

This is most frequently seen in solubility equilibria, but there are many other reactions in which solids are directly involved: The concentrations of the two ions will be independent of the quantity of solid CaF2 in contact with the water; in other words, the system can be in equilibrium as long as any CaF2 at all is present.

Converting between Kp and Kc It is sometimes necessary to convert between equilibrium constants expressed in different units. Sometimes the symbols KcKxand Km are used to denote these forms of the equilibrium constant. For example, the equilibrium expression for each of the processes shown in the following table consists solely of a single term involving the partial pressure of a gas: This temperature corresponds, of course, to the boiling point of water.

This is an example of dynamic equilibrium. But how do you translate this into a format that relates to the actual chemical system you are interested in?In a chemical reaction, chemical equilibrium is the state in which both reactants and products are present in concentrations which have no further tendency to change with time, The equilibrium constant expression is therefore usually written as.

Reversible reactions, equilibrium, and the equilibrium constant K. How to calculate K, and how to use K to determine if a reaction strongly favors products or reactants at equilibrium.

Factors that affect chemical equilibrium. For reactions that are not at equilibrium, we can write a similar expression called the reaction quotient Q Q Q.

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.

The numerator of the equilibrium constant expression is the product of the concentrations of the "products" of the reaction raised to a power equal to the coefficient for this component in the balanced equation for the reaction.

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Product concentrations appear in the numerator of an equilibrium constant expression 2. A reaction favors the formation of products d.

always constant during a chemical reaction.

Chemical reaction and equilibrium constant expression
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