If you're seeing this message, it means we're having trouble loading external resources on our website. If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked. The reaction quotient (\(Q\)) measures the relative amounts of products and reactants present during a reaction at a particular point in time. The reaction quotient aids in figuring out which direction a reaction is likely to proceed, given either the pressures or the concentrations of the reactants and the products. The \(Q\) value can be compared to the Equilibrium Constant, \(K\), to determine the direction of the reaction that is taking place. The main difference between \(K\) and \(Q\) is that \(K\) describes a reaction that is at equilibrium, whereas \(Q\) describes a reaction that is not at equilibrium. To determine \(Q\), the concentrations of the reactants and products must be known. For a given general chemical equation: \[aA + bB \rightleftharpoons cC + dD \tag{1}\nonumber \] the Q equation is written by multiplying the activities (which are approximated by concentrations) for the species of the products and dividing by the activities of the reactants. If any component in the reaction has a coefficient, indicated above with lower case letters, the concentration is raised to the power of the coefficient. \(Q\) for the above equation is therefore: \[Q_c = \dfrac{[C]^c[D]^d}{[A]^a[B]^b} \tag{2}\nonumber \]
This equation only shows components in the gaseous or aqueous states. Each pure liquid or solid has an activity of one and can be functionally omitted. Equilibrium constants really contain a ratio of concentrations (actual concentration divided by the reference concentration that defines the standard state). Because the standard state for concentrations is usually chosen to be 1 mol/L, it is not written out in practical applications. Hence, the ratio does not contain units. A comparison of \(Q\) with \(K\) indicates which way the reaction shifts and which side of the reaction is favored:
Another important concept that is used in the calculation of the reaction quotient is called an activity. For example, consider the \(Q\) equation for this acid/base reaction: \[\ce{CH_3CH_2CO_2H(aq) + H_2O(l) <=> H_3O^{+}(aq) + CH_3CH_2CO_2^{-}(aq)} \nonumber \] The \(Q\) equation is written as the concentrations of the products divided by the concentrations of the reactants, but only including components in the gaseous or aqueous states and omitting pure liquid or solid states. The \(Q\) equation for this example is the following: \[Q = \dfrac{[\ce{H3O^{+}(aq)}][\ce{CH3CH2CO2^{-}(aq)}]}{[\ce{CH3CH2CO2H(aq)}]} \nonumber \]
What is the \(Q\) value for this equation? Which direction will the reaction shift if \(K_c\) = 1.0? \[\ce{CO(g) + H_2O(g) \rightleftharpoons CO_2(g) + H_2(g)} \nonumber\]
Solution Step 1: Write the \(Q\) formula: \[Q_c = \dfrac{[CO_2][H_2]}{[CO][H_2O]} \nonumber\] Step 2: Plug in given concentration values: \(\begin{align*} Q_c &= \dfrac{(2.0)(2.0)}{(1.0)(1.0)} \\[4pt] &= 4.0 \end{align*}\] Step 3: Compare \(Q\) to K: Because \(4.0 > 1.0\), then \(Q > K\) and the reaction shifts left toward the reactants. Answer Q= 4.0 and the reaction shifts left.
Find the value of \(Q\) and determine which side of the reaction is favored with \(K=0.5\). \[\ce{HCl(g) + NaOH(aq) \rightleftharpoons NaCl(aq) + H_2O(l)} \nonumber\] with
Solution Step 1: Write the \(Q\) formula. Because the activity of a liquid is 1, we can omit the water component in the equation. \(Q_c = \dfrac{[NaCl{(aq)}]}{[HCl{(g)}][NaOH{(aq)}]}\) Step 2: Plug in given concentrations into the \(Q\) formula: \(Q_c = \dfrac{[6]}{[3.2][4.3]}\) Step 3: Calculate using the given concentrations: \(Q = 0.436\) Step 4: Compare Q to K. The \(Q\) value, 0.436, is less than the given \(K\) value of 0.5, so \(Q < K\). Because \(Q\) < K, the reaction is not at equilibrium and proceeds to the products side to reach dynamic equilibrium once again. Answer: Q= 0.436 and the reaction favors the products.
Given the equation with \(K= 0.040\). Find \(Q\) and determine which direction the reaction will shift to reach equilibrium. \[\ce{N_2(g) + 3H_2(aq) \rightleftharpoons 2NH_3(g)} \nonumber \] with
Solution Step 1: Write the \(Q\) formula: \[Q_c = \dfrac{[NH_3{(g)}]^2}{[N_2{(g)}][H_2{(g)}]^3}\nonumber \] Step 2: Plug in values. Because the concentrations for \(N_2\) and \(H_2\) were given, they can be inserted directly into the equation. However, no concentration value was given for NH3 so the concentration is assumed to be 0. \[Q_c = \dfrac{(0)^2}{(0.04)(0.09)^3}\nonumber \] Step 3: Solve for Q: \[Q=0\nonumber \] Step 4: Compare \(Q\) to K. Because \(K=0.04\) and \(Q=0\), \(K > Q\) and the reaction will shift right to regain equilibrium. Answer: \(Q=0\), the reaction shifts right. The Reaction Quotient is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Kellie Berman, Rebecca Backer, Deepak Nallur, & Deepak Nallur.
We previously saw that knowing the magnitude of the equilibrium constant under a given set of conditions allows chemists to predict the extent of a reaction. Often, however, chemists must decide whether a system has reached equilibrium or if the composition of the mixture will continue to change with time. In this section, we describe how to quantitatively analyze the composition of a reaction mixture to make this determination.
To determine whether a system has reached equilibrium, chemists use a Quantity called the reaction Quotient (\(Q\)). The expression for the reaction Quotient has precisely the same form as the equilibrium constant expression, except that \(Q\) may be derived from a set of values measured at any time during the reaction of any mixture of the reactants and the products, regardless of whether the system is at equilibrium. Therefore, for the following general reaction: \[aA+bB \rightleftharpoons cC+dD \nonumber \] the reaction quotient is defined as follows: \[Q=\dfrac{[C]^c[D]^d}{[A]^a[B]^b} \label{15.6.1} \] To understand how information is obtained using a reaction Quotient, consider the dissociation of dinitrogen tetroxide to nitrogen dioxide, \[\ce{N2O4(g) <=> 2NO2(g)} \nonumber \] for which \(K = 4.65 \times 10^{−3}\) at 298 K. We can write \(Q\) for this reaction as follows: \[Q=\dfrac{[\ce{NO2}]^2}{[\ce{N2O4}]} \label{15.6.2} \] The following table lists data from three experiments in which samples of the reaction mixture were obtained and analyzed at equivalent time intervals, and the corresponding values of \(Q\) were calculated for each. Each experiment begins with different proportions of product and reactant:
As these calculations demonstrate, \(Q\) can have any numerical value between 0 and infinity (undefined); that is, \(Q\) can be greater than, less than, or equal to \(K\). Comparing the magnitudes of \(Q\) and \(K\) enables us to determine whether a reaction mixture is already at equilibrium and, if it is not, predict how its composition will change with time to reach equilibrium (i.e., whether the reaction will proceed to the right or to the left as written). All you need to remember is that the composition of a system not at equilibrium will change in a way that makes \(Q\) approach \(K\):
These points are illustrated graphically in Figure \(\PageIndex{1}\). 
A Video Discussing Using the Reaction Quotient (Q): Using the Reaction Quotient (Q) (opens in new window) [youtu.be]
At elevated temperatures, methane (\(CH_4\)) reacts with water to produce hydrogen and carbon monoxide in what is known as a steam-reforming reaction: \[\ce{CH4(g) + H2O(g) <=> CO(g) + 3H2(g)} \nonumber \] \(K = 2.4 \times 10^{−4}\) at 900 K. Huge amounts of hydrogen are produced from natural gas in this way and are then used for the industrial synthesis of ammonia. If \(1.2 \times 10^{−2}\) mol of \(CH_4\), 8.0 × 10−3 mol of \(H_2O\), \(1.6 \times 10^{−2}\) mol of \(CO\), and \(6.0 \times 10^{−3}\) mol of \(H_2\) are placed in a 2.0 L steel reactor and heated to 900 K, will the reaction be at equilibrium or will it proceed to the right to produce \(\ce{CO}\) and \(\ce{H_2}\) or to the left to form \(\ce{CH_4}\) and \(\ce{H_2O}\)? Given: balanced chemical equation, \(K\), amounts of reactants and products, and volume Asked for: direction of reaction Strategy:
Solution: A We must first find the initial concentrations of the substances present. For example, we have \(1.2 \times 10^{−2} mol\) of \(\ce{CH_4}\) in a 2.0 L container, so \[[\ce{CH4}]=\dfrac{1.2\times 10^{−2} \, \text{mol}}{2.0\; \text{L}}=6.0 \times 10^{−3} M \nonumber \] We can calculate the other concentrations in a similar way:
B We now compute \(Q\) and compare it with \(K\): \[\begin{align*} Q&=\dfrac{[\ce{CO}][\ce{H_2}]^3}{[\ce{CH_4}][\ce{H_2O}]} \\[4pt] &=\dfrac{(8.0 \times 10^{−3})(3.0 \times 10^{−3})^3}{(6.0\times 10^{−3})(4.0 \times 10^{−3})} \\[4pt] &=9.0 \times 10^{−6} \end{align*} \nonumber \] Because \(K = 2.4 \times 10^{−4}\), we see that \(Q < K\). Thus the ratio of the concentrations of products to the concentrations of reactants is less than the ratio for an equilibrium mixture. The reaction will therefore proceed to the right as written, forming \(\ce{H2}\) and \(\ce{CO}\) at the expense of \(\ce{H_2O}\) and \(\ce{CH4}\).
In the water–gas shift reaction introduced in Example \(\PageIndex{1}\), carbon monoxide produced by steam-reforming reaction of methane reacts with steam at elevated temperatures to produce more hydrogen: \[\ce{CO(g) + H_2O(g) <=> CO2(g) + H2(g)} \nonumber \] \(K = 0.64\) at 900 K. If 0.010 mol of both \(\ce{CO}\) and \(\ce{H_2O}\), 0.0080 mol of \(\ce{CO_2}\), and 0.012 mol of \(\ce{H_2}\) are injected into a 4.0 L reactor and heated to 900 K, will the reaction proceed to the left or to the right as written? Answer\(Q = 0.96\). Since (Q > K), so the reaction will proceed to the left, and \(CO\) and \(H_2O\) will form.
By graphing a few equilibrium concentrations for a system at a given temperature and pressure, we can readily see the range of reactant and product concentrations that correspond to equilibrium conditions, for which \(Q = K\). Such a graph allows us to predict what will happen to a reaction when conditions change so that \(Q\) no longer equals \(K\), such as when a reactant concentration or a product concentration is increased or decreased.
Lead carbonate decomposes to lead oxide and carbon dioxide according to the following equation: \[\ce{PbCO3(s) <=> PbO(s) + CO2(g)} \label{15.6.3} \] Because \(\ce{PbCO_3}\) and \(\ce{PbO}\) are solids, the equilibrium constant is simply \[K = [\ce{CO_2}]. \nonumber \] At a given temperature, therefore, any system that contains solid \(\ce{PbCO_3}\) and solid \(\ce{PbO}\) will have exactly the same concentration of \(\ce{CO_2}\) at equilibrium, regardless of the ratio or the amounts of the solids present. This situation is represented in Figure \(\PageIndex{3}\), which shows a plot of \([\ce{CO_2}]\) versus the amount of \(\ce{PbCO_3}\) added. Initially, the added \(\ce{PbCO_3}\) decomposes completely to \(\ce{CO_2}\) because the amount of \(\ce{PbCO_3}\) is not sufficient to give a \(\ce{CO_2}\) concentration equal to \(K\). Thus the left portion of the graph represents a system that is not at equilibrium because it contains only \(\ce{CO2(g)}\) and \(\ce{PbO(s)}\). In contrast, when just enough \(\ce{PbCO_3}\) has been added to give \([CO_2] = K\), the system has reached equilibrium, and adding more \(\ce{PbCO_3}\) has no effect on the \(\ce{CO_2}\) concentration: the graph is a horizontal line. Thus any \(\ce{CO_2}\) concentration that is not on the horizontal line represents a nonequilibrium state, and the system will adjust its composition to achieve equilibrium, provided enough \(\ce{PbCO_3}\) and \(\ce{PbO}\) are present. For example, the point labeled A in Figure \(\PageIndex{2}\) lies above the horizontal line, so it corresponds to a \([\ce{CO_2}]\) that is greater than the equilibrium concentration of \(\ce{CO_2}\) (i.e., \(Q > K\)). To reach equilibrium, the system must decrease \([\ce{CO_2}]\), which it can do only by reacting \(\ce{CO_2}\) with solid \(\ce{PbO}\) to form solid \(\ce{PbCO_3}\). Thus the reaction in Equation \(\ref{15.6.3}\) will proceed to the left as written, until \([\ce{CO_2}] = K\). Conversely, the point labeled B in Figure \(\PageIndex{2}\) lies below the horizontal line, so it corresponds to a \([\ce{CO_2}]\) that is less than the equilibrium concentration of \(\ce{CO_2}\) (i.e., \(Q < K\)). To reach equilibrium, the system must increase \([\ce{CO_2}]\), which it can do only by decomposing solid \(\ce{PbCO_3}\) to form \(\ce{CO_2}\) and solid \(\ce{PbO}\). The reaction in Equation \ref{15.6.3} will therefore proceed to the right as written, until \([\ce{CO_2}] = K\).
In contrast, the reduction of cadmium oxide by hydrogen gives metallic cadmium and water vapor: \[\ce{CdO(s) + H2(g) <=> Cd(s) + H_2O(g)} \label{15.6.4} \] and the equilibrium constant is \[K = \dfrac{[\ce{H_2O}]}{[\ce{H_2}]}. \nonumber \] If \([\ce{H_2O}]\) is doubled at equilibrium, then \([\ce{H2}]\) must also be doubled for the system to remain at equilibrium. A plot of \([\ce{H_2O}]\) versus \([\ce{H_2}]\) at equilibrium is a straight line with a slope of \(K\) (Figure \(\PageIndex{3}\)). Again, only those pairs of concentrations of \(\ce{H_2O}\) and \(\ce{H_2}\) that lie on the line correspond to equilibrium states. Any point representing a pair of concentrations that does not lie on the line corresponds to a nonequilibrium state. In such cases, the reaction in Equation \(\ref{15.6.4}\) will proceed in whichever direction causes the composition of the system to move toward the equilibrium line. For example, point A in Figure \(\PageIndex{3}\) lies below the line, indicating that the \([\ce{H_2O}]/[\ce{H_2}]\) ratio is less than the ratio of an equilibrium mixture (i.e., \(Q < K\)). Thus the reaction in Equation \ref{15.6.4} will proceed to the right as written, consuming \(\ce{H_2}\) and producing \(\ce{H_2O}\), which causes the concentration ratio to move up and to the left toward the equilibrium line. Conversely, point B in Figure \(\PageIndex{3}\) lies above the line, indicating that the \([\ce{H_2O}]/[\ce{H_2}]\) ratio is greater than the ratio of an equilibrium mixture (\(Q > K\)). Thus the reaction in Equation \(\ref{15.6.4}\) will proceed to the left as written, consuming \(\ce{H_2O}\) and producing \(\ce{H_2}\), which causes the concentration ratio to move down and to the right toward the equilibrium line.
In another example, solid ammonium iodide dissociates to gaseous ammonia and hydrogen iodide at elevated temperatures: \[\ce{ NH4I(s) <=> NH3(g) + HI(g)} \label{15.6.5} \] For this system, \(K\) is equal to the product of the concentrations of the two products: \[K = [\ce{NH_3}][\ce{HI}]. \nonumber \] If we double the concentration of \(\ce{NH3}\), the concentration of \(\ce{HI}\) must decrease by approximately a factor of 2 to maintain equilibrium, as shown in Figure \(\PageIndex{4}\). As a result, for a given concentration of either \(\ce{HI}\) or \(\ce{NH_3}\), only a single equilibrium composition that contains equal concentrations of both \(\ce{NH_3}\) and \(\ce{HI}\) is possible, for which \[[\ce{NH_3}] = [\ce{HI}] = \sqrt{K}. \nonumber \] Any point that lies below and to the left of the equilibrium curve (such as point A in Figure \(\PageIndex{4}\)) corresponds to \(Q < K\), and the reaction in Equation \(\ref{15.6.5}\) will therefore proceed to the right as written, causing the composition of the system to move toward the equilibrium line. Conversely, any point that lies above and to the right of the equilibrium curve (such as point B in Figure \(\ref{15.6.5}\)) corresponds to \(Q > K\), and the reaction in Equation \(\ref{15.6.5}\) will therefore proceed to the left as written, again causing the composition of the system to move toward the equilibrium line. By graphing equilibrium concentrations for a given system at a given temperature and pressure, we can predict the direction of reaction of that mixture when the system is not at equilibrium.
The reaction Quotient (\(Q\)) is used to determine whether a system is at equilibrium and if it is not, to predict the direction of reaction. The reaction Quotient (\(Q\) or \(Q_p\)) has the same form as the equilibrium constant expression, but it is derived from concentrations obtained at any time. When a reaction system is at equilibrium, \(Q = K\). Graphs derived by plotting a few equilibrium concentrations for a system at a given temperature and pressure can be used to predict the direction in which a reaction will proceed. Points that do not lie on the line or curve represent nonequilibrium states, and the system will adjust, if it can, to achieve equilibrium. |
When the numerical value of the reaction quotient Q is greater than the equilibrium constant K in which direction does the reaction proceed to reach equilibrium?
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