We use molar concentrations in the following examples, but we will see shortly that partial pressures of the gases may be used as well: \[\ce{C2H6}(g) \rightleftharpoons \ce{C2H4}(g)+\ce{H2}(g) \label{13.3.12a}\], \[K_{eq}=\ce{\dfrac{[C2H4][H2]}{[C2H6]}} \label{13.3.12b}\], \[\ce{3O2}(g) \rightleftharpoons \ce{2O3}(g) \label{13.3.13a}\], \[K_{eq}=\ce{\dfrac{[O3]^2}{[O2]^3}} \label{13.3.13b}\], \[\ce{N2}(g)+\ce{3H2}(g) \rightleftharpoons \ce{2NH3}(g) \label{13.3.14a}\], \[K_{eq}=\ce{\dfrac{[NH3]^2}{[N2][H2]^3}} \label{13.3.14b}\], \[\ce{C3H8}(g)+\ce{5O2}(g) \rightleftharpoons \ce{3CO2}(g)+\ce{4H2O}(g)\label{13.3.15a} \], \[K_{eq}=\ce{\dfrac{[CO2]^3[H2O]^4}{[C3H8][O2]^5}}\label{13.3.15b}\]. K is the numerical value of Q at the end of the reaction, when equilibrium is reached. 2 Add the number of moles of each gas in the sample to find the total number of moles in the gas mixture. The LibreTexts libraries arePowered by NICE CXone Expertand are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. In each of these examples, the equilibrium system is an aqueous solution, as denoted by the aq annotations on the solute formulas. The first is again fairly obvious. . It is used to express the relationship between product pressures and reactant pressures. Before any reaction occurs, we can calculate the value of Q for this reaction. The reaction quotient, Q, is the same as the equilibrium constant expression, but for partial pressures or concentrations of the reactants and products before the system reaches equilibrium. Subsitute values into the 512 Math Consultants 96% Recurring customers 20168+ Customers Get Homework Help. Subsitute values into the expression and solve. The formal definitions of Q and K are quite simple, but they are of limited usefulness unless you are able to relate them to real chemical situations. Thank you so so much for the app developer. How do you find the Q reaction in thermochemistry? It may also be useful to think about different ways pressure can be changed. The only possible change is the conversion of some of these reactants into products. forward, converting reactants into products. by following the same guidelines for deriving concentration-based expressions: \[Q_P=\dfrac{P_{\ce{C2H4}}P_{\ce{H2}}}{P_{\ce{C2H6}}} \label{13.3.20}\]. These cookies track visitors across websites and collect information to provide customized ads. At constant pressure, the change in the enthalpy of a system is equal to the heat flow: H=qp. 2) D etermine the pre-equilibrium concentrations or partial pressures of the reactants and products that are involved in the equilibrium. Kc = 0.078 at 100oC. In the general case in which the concentrations can have any arbitrary values (including zero), this expression is called the reaction quotient (the term equilibrium quotient is also commonly used.) Q is the energy transfer due to thermal reactions such as heating water, cooking, etc. To solve for the partial pressure, you would set up the problem in the same way: The reaction quotient Q is determined the same way as the equilibrium constant, regardless of whether you are given partial pressures or concentration in mol/L. Q doesnt change because it just represents the relative products to reactants concentrations, which do not change with temperature. Knowing is half the battle. This means that the effect will be larger for the reactants. 15. 5 3 8. Calculate G for this reaction at 298 K under the following conditions: PCH3OH=0.895atm and K is determined from the partial pressures. K vs. Q For now, we use brackets to indicate molar concentrations of reactants and products. There are actually multiple solutions to this. anywhere where there is a heat transfer. The reaction quotient Q (article) Join our MCAT Study Group: Check out more MCAT lectures and prep materials on our website: Determine math questions. Explanation: The relationship between G and pressure is: G = G +RT lnQ Where Q is the reaction quotient, that in case of a reaction involving gaseous reactants and products, pressure could be used. The pressure given is the pressure there is and the value you put directly into the products/reactants equation. the shift. Decide mathematic equation. Therefore, for this course we will use partial pressures for gases and molar concentrations for aqueous solutes, all in the same expressions as shown below. If the system is initially in a non-equilibrium state, its composition will tend to change in a direction that moves it to one that is on the line. You're right! Add up the number of moles of the component gases to find n Total. How to find reaction quotient with partial pressure Before any reaction occurs, we can calculate the value of Q for this reaction. Calculating the Equilibrium Constant The concentration of component D is zero, and the partial pressure (or Solve Now. Therefore, Q = (0.5)^2/0.5 = 0.5 for this reaction. Reaction Quotient Chemical Analysis Formulations Instrumental Analysis Pure Substances Sodium Hydroxide Test Test for Anions Test for Metal Ions Testing for Gases Testing for Ions Chemical Reactions Acid-Base Reactions Acid-Base Titration Bond Energy Calculations Decomposition Reaction Electrolysis of Aqueous Solutions The concentration of component D is zero, and the partial pressure (or. The subscript \(P\) in the symbol \(K_P\) designates an equilibrium constant derived using partial pressures instead of concentrations. BUT THIS APP IS AMAZING. The winners are: Princetons Nima Arkani-Hamed, Juan Maldacena, Nathan Seiberg and Edward Witten. Find the molar concentrations or partial pressures of each species involved. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. The LibreTexts libraries arePowered by NICE CXone Expertand are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. This cookie is set by GDPR Cookie Consent plugin. An equilibrium is established for the reaction 2 CO(g) + MoO(s) 2 CO(g) + Mo(s). However, K does change because, with endothermic and exothermic reactions, an increase in temperature leads to an increase in either products or reactants, thus changing the K value. The Nernst equation accurately predicts cell potentials only when the equilibrium quotient term Q is expressed in activities. Im using this for life, really helps with homework,and I love that it explains the steps to you. The volume of the reaction can be changed. the concentrations at equilibrium are [SO2] = 0.90 M, [O2] = 0.35 M, and [SO3] = 1.1 M. What is the value of the equilibrium constant, Keq? Before any product is formed, \(\mathrm{[NO_2]=\dfrac{0.10\:mol}{1.0\:L}}=0.10\:M\), and [N, At equilibrium, the value of the equilibrium constant is equal to the value of the reaction quotient. Subsitute values into the More ways to get app. The equilibrium constant for the oxidation of sulfur dioxide is Kp = 0.14 at 900 K. \[\ce{2 SO_2(g) + O_2(g) \rightleftharpoons 2 SO_3(g)} \nonumber\]. Q can be used to determine which direction a reaction Khan Academy has been translated into dozens of languages, and 15 million people around the globe learn on Khan Academy every month. Since Q > K, the reaction is not at equilibrium, so a net change will occur in a direction that decreases Q. calculate an equilibrium constant but Q can be calculated for any set of You need to ask yourself questions and then do problems to answer those questions. Just make sure your values are all in the same units of atm or bar. the quantities of each species (molarities and/or pressures), all measured Determine in which direction the reaction proceeds as it goes to equilibrium in each of the three experiments shown. Example \(\PageIndex{3}\): Predicting the Direction of Reaction. For example, if we combine the two reactants A and B at concentrations of 1 mol L1 each, the value of Q will be 01=0. To find the reaction quotient Q, multiply the activities for the species of the products and divide by the activities of the reagents, raising each one of . If G Q, and the reaction must proceed to the right to reach equilibrium. As for the reaction quotient, when evaluated in terms of concentrations, it could be noted as \(K_c\). arrow_forward Consider the reaction below: 2 SO(g) 2 SO(g) + O(g) A sealed reactor contains a mixture of SO(g), SO(g), and O(g) with partial pressures: 0.200 bar, 0.250 bar and 0.300 bar, respectively. To find the reaction quotient Q, multiply the activities for the species of the products and divide by the activities of the reagents, raising each one of these values to the power of the corresponding stoichiometric coefficient.7 days ago To calculate Q: Write the expression for the reaction quotient. For astonishing organic chemistry help: https://www.bootcamp.com/chemistryTo see my new Organic Chemistry textbook: https://tophat.com/marketplace/science-&-. You need to solve physics problems. For example, equilibria involving aqueous ions often exhibit equilibrium constants that vary quite significantly (are not constant) at high solution concentrations. So in this case it would be set up as (0.5)^2/(0.5) which equals 0.5. When evaluated using concentrations, it is called Q c or just Q. for Q. Do NOT follow this link or you will be banned from the site! The formula is: PT = P1 + P2 + P3 + PN Where PT is the. Find the molar concentrations or partial pressures of each species involved. Kp is pressure and you just put the pressure values in the equation "Kp=products/reactants". After many, many years, you will have some intuition for the physics you studied. 24/7 help If you need help, we're here for you 24/7. Product concentration too low for equilibrium; net reaction proceeds to, When arbitrary quantities of the different, The status of the reaction system in regard to its equilibrium state is characterized by the value of the, The various terms in the equilibrium expression can have any arbitrary value (including zero); the value of the equilibrium expression itself is called the, If the concentration or pressure terms in the equilibrium expression correspond to the equilibrium state of the system, then. Several examples are provided here: \[\ce{C2H2}(aq)+\ce{2Br2}(aq) \rightleftharpoons \ce{C2H2Br4}(aq)\hspace{20px} \label{13.3.7a}\], \[K_{eq}=\ce{\dfrac{[C2H2Br4]}{[C2H2][Br2]^2}} \label{13.3.7b}\], \[\ce{I2}(aq)+\ce{I-}(aq) \rightleftharpoons \ce{I3-}(aq) \label{13.3.8b}\], \[K_{eq}=\ce{\dfrac{[I3- ]}{[I2][I- ]}} \label{13.3.8c}\], \[\ce{Hg2^2+}(aq)+\ce{NO3-}(aq)+\ce{3H3O+}(aq) \rightleftharpoons \ce{2Hg^2+}(aq)+\ce{HNO2}(aq)+\ce{4H2O}(l) \label{13.3.9a}\], \[K_{eq}=\ce{\dfrac{[Hg^2+]^2[HNO2]}{[Hg2^2+][NO3- ][H3O+]^3}} \label{13.3.9b}\], \[\ce{HF}(aq)+\ce{H2O}(l) \rightleftharpoons \ce{H3O+}(aq)+\ce{F-}(aq) \label{13.3.10a}\], \[K_{eq}=\ce{\dfrac{[H3O+][F- ]}{[HF]}} \label{13.3.10b}\], \[\ce{NH3}(aq)+\ce{H2O}(l) \rightleftharpoons \ce{NH4+}(aq)+\ce{OH-}(aq) \label{13.3.11a}\], \[K_{eq}=\ce{\dfrac{[NH4+][OH- ]}{[NH3]}} \label{13.3.11b}\]. As described in the previous paragraph, the disturbance causes a change in Q; the reaction will shift to re-establish Q = K. The equilibrium constant, Kc is the ratio of the rate constants, so only variables that affect the rate constants can affect Kc. If the initial partial pressures are 0.80 atmospheres for carbon monoxide and 0.40 atmospheres for carbon dioxide, we can use the reaction quotient Q, to predict which direction that reaction will go to reach equilibrium. Thus, the reaction quotient of the reaction is 0.800. b. Write the reaction quotient expression for the ionization of NH 3 in water. The equilibrium constant is related to the concentration (partial pressures) of the products divided by the reactants. Arrow represents the addition of ammonia to the equilibrium mixture; the system responds by following the path back to a new equilibrium state which, as the Le Chatelier principle predicts, contains a smaller quantity of ammonia than was added. This website uses cookies to improve your experience while you navigate through the website. As , EL NORTE is a melodrama divided into three acts. For example, if we combine the two reactants A and B at concentrations of 1 mol L1 each, the value of Q will be 01=0. In this blog post, we will be discussing How to find reaction quotient with partial pressure. The partial pressure of one of the gases in a mixture is the pressure which it would exert if it alone occupied the whole container. Our goal is to find the equilibrium partial pressures of our two gasses, carbon monoxide and carbon dioxide. If the same value of the reaction quotient is observed when the concentrations stop changing in both experiments, then we may be certain that the system has reached equilibrium. To find the reaction quotient Q Q, multiply the activities for the species of the products and divide by the activities of the reagents, raising each one of these values to the power of the corresponding stoichiometric coefficient. Find the reaction quotient. To find the reaction quotient Q, multiply the activities for the species of the products and divide by the activities of the reagents, raising each one of. \[\begin{align} PV&=nRT \label{13.3.16} \\[4pt] P &=\left(\dfrac{n}{V}\right)RT \label{13.3.17} \\[4pt] &=MRT \label{13.3.18} \end{align}\], Thus, at constant temperature, the pressure of a gas is directly proportional to its concentration. . Whenever gases are involved in a reaction, the partial pressure of each gas can be used instead of its concentration in the equation for the reaction quotient because the partial pressure of a gas is directly proportional to its concentration at constant temperature. After completing his doctoral studies, he decided to start "ScienceOxygen" as a way to share his passion for science with others and to provide an accessible and engaging resource for those interested in learning about the latest scientific discoveries. Since H2O(l) is the solvent for these solutions, its concentration does not appear as a term in the \(K_{eq}\) expression, as discussed earlier, even though it may also appear as a reactant or product in the chemical equation. A small value of \(K_{eq}\)much less than 1indicates that equilibrium is attained when only a small proportion of the reactants have been converted into products. W is the net work done on the system. The value of Q in relation to K serves as an index how the composition of the reaction system compares to that of the equilibrium state, and thus it indicates the direction in which any net reaction must proceed. So adding various amounts of the solid to an empty closed vessel (states and ) causes a gradual buildup of iodine vapor. This cookie is set by GDPR Cookie Consent plugin. This relationship can be derived from the ideal gas equation, where M is the molar concentration of gas, \(\dfrac{n}{V}\). How does changing pressure and volume affect equilibrium systems? Using the ideal gas law we know that P= concentration (RT) and therefore Kp=Kc (RT)^n, when atm and molarity, the units for this problem . How does pressure affect Le Chateliers principle? When heated to a consistent temperature, 800 C, different starting mixtures of \(\ce{CO}\), \(\ce{H_2O}\), \(\ce{CO_2}\), and \(\ce{H_2}\) react to reach compositions adhering to the same equilibrium (the value of \(Q\) changes until it equals the value of Keq). Born and raised in the city of London, Alexander Johnson studied biology and chemistry in college and went on to earn a PhD in biochemistry. This value is called the equilibrium constant (\(K\)) of the reaction at that temperature. In such cases, you can calculate the equilibrium constant by using the molar concentration (Kc) of the chemicals, or by using their partial pressure (Kp). Find P Total. Check out 9 similar chemical reactions calculators , Social Media Time Alternatives Calculator, Relation between the reaction quotient and the equilibrium constant, An example of how to calculate the reaction quotient. How is partial pressure calculated? To calculate Q: Write the expression for the reaction quotient. When pure reactants are mixed, \(Q\) is initially zero because there are no products present at that point. The amounts are in moles so a conversion is required. How to divide using partial quotients - So 6 times 6 is 36. Find the molar concentrations or partial pressures of each species involved. 5 1 0 2 = 1. If the initial partial pressures are those in part a, find the equilibrium values of the partial pressures. As a 501(c)(3) nonprofit organization, we would love your help!Donate or volunteer today! For example, the reaction quotient for the reversible reaction, \[\ce{2NO}_{2(g)} \rightleftharpoons \ce{N_2O}_{4(g)} \label{13.3.3}\], \[Q=\ce{\dfrac{[N_2O_4]}{[NO_2]^2}} \label{13.3.4}\], Example \(\PageIndex{1}\): Writing Reaction Quotient Expressions. ln Q is the natural logarithm of the reaction quotient (Q) The reaction quotient (Q) is given by: Q = P A 3 P B P C 2 Where P C, P A, and P B are the partial pressures of C (0.510 atm), A (11.5 atm), and B (8.60 atm), respectively. The answer to the equation is 4. I can solve the math problem for you. Step 2. Formula to calculate Kp. How to find the reaction quotient using the reaction quotient equation; and. . Calculate the reaction quotient and determine the direction in which each of the following reactions will proceed to reach equilibrium. Several examples of equilibria yielding such expressions will be encountered in this section. 7.6 T OPIC: 7.6 P ROPERTIES OF THE E QUILIBRIUM C ONSTANT E NDURING U NDERSTANDING: TRA-7 A system at equilibrium depends on the relationships between concentrations, partial pressures of chemical species, and equilibrium constant K. L EARNING O BJECTIVE: TRA-7.D Represent a multistep process with an overall equilibrium expression, using the constituent K expressions for each individual reaction. Use the information below to determine whether or not a reaction mixture in which the partial pressures of PCl3,Cl2, and PCl5 are 0.21 atm, 0.41 atm. It is easy to see (by simple application of the Le Chatelier principle) that the ratio of Q/K immediately tells us whether, and in which direction, a net reaction will occur as the system moves toward its equilibrium state. If it is less than 1, there will be more reactants. Examples using this approach will be provided in class, as in-class activities, and in homework. (a) The gases behave independently, so the partial pressure of each gas can be determined from the ideal gas equation, using P = nRT/ V : (b) The total pressure is given by the sum of the partial pressures: Check Your Learning 2.5.1 - The Pressure of a Mixture of Gases A 5.73 L flask at 25 C contains 0.0388 mol of N2, 0.147 mol of CO, and 0.0803 Write the expression to find the reaction quotient, Q. What is the value of the reaction quotient before any reaction occurs? In the general case in which the concentrations can have any arbitrary values (including zero), this expression is called the reaction quotient (the term equilibrium quotient is also commonly used.) The reaction quotient aids in figuring out which direction a reaction is likely to proceed, given either the pressures or the . Math is a way of determining the relationships between numbers, shapes, and other mathematical objects. There are two important relationships involving partial pressures. If you're trying to calculate Qp, you would use the same structure as the equilibrium constant, (products)/(reactants), but instead of using their concentrations, you would use their partial pressures. This example problem demonstrates how to find the equilibrium constant of a reaction from equilibrium concentrations of reactants and products . Partial pressures are: P of N 2 N 2 = 0.903 P of H2 H 2 = 0.888 P of N H3 N H 3 = 0.025 Reaction Quotient: The reaction quotient has the same concept. Since the reactants have two moles of gas, the pressures of the reactants are squared. The only possible change is the conversion of some of these reactants into products. Enthalpy (Delta H), on the other hand, is the state of the system, the total heat content. Reaction Quotient: Meaning, Equation & Units. (b) A 5.0-L flask containing 17 g of NH3, 14 g of N2, and 12 g of H2: \[\ce{N2}(g)+\ce{3H2}(g)\ce{2NH3}(g)\hspace{20px}K_{eq}=0.060 \nonumber\]. n Total = n oxygen + n nitrogen. A general equation for a reversible reaction may be written as follows: (2.3.1) m A + n B + x C + y D We can write the reaction quotient ( Q) for this equation. If G > 0, then K. In chemical thermodynamics, the reaction quotient (Qr or just Q) is a dimensionless quantity that provides a measurement of the relative amounts of products and reactants present in a reaction mixture for a reaction with well-defined overall stoichiometry, at a particular point in time. Subsitute values into the Introduction to reaction quotient Qc (video) The reaction quotient Q Q QQ is a measure of the relative amounts of products and reactants present in a reaction at a given time. Plugging in the values, we get: Q = 1 1. To find the reaction quotient Q, multiply the activities for the species of the products and divide by the activities of the reagents, raising each one of these values to the power of the corresponding stoichiometric coefficient. This cookie is set by GDPR Cookie Consent plugin. Find the molar concentrations or partial pressures of each species involved. Partial pressure is calculated by setting the total pressure equal to the partial pressures. 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Note that the concentration of \(\ce{H_2O}_{(g)}\) has been included in the last example because water is not the solvent in this gas-phase reaction and its concentration (and activity) changes. Standard pressure is 1 atm. Their particular values may vary depending on conditions, but the value of the reaction quotient will always equal K (Kc when using concentrations or KP when using partial pressures). The cookie is used to store the user consent for the cookies in the category "Other. The reaction quotient Q is determined the same way as the equilibrium constant, regardless of whether you are given partial pressures or concentration in mol/L. The following diagrams illustrate the relation between Q and K from various standpoints. Q = heat energy (Joules, J) m = mass of a substance (kg) c = specific heat (units J/kgK) is a symbol meaning the change in T = change in temperature (Kelvins, K). I believe you may be confused about how concentration has "per mole" and pressure does not. This page titled 11.3: Reaction Quotient is shared under a CC BY 3.0 license and was authored, remixed, and/or curated by Stephen Lower via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request. The equation for Q, for a general reaction between chemicals A, B, C and D of the form: Is given by: So essentially it's the products multiplied together divided by the reactants multiplied together, each raised to a power equal to their stoichiometric constants (i.e. If instead our mixture consists only of the two products C and D, Q will be indeterminately large (10) and the only possible change will be in the reverse direction. The phases may be any combination of solid, liquid, or gas phases, and solutions. Some heterogeneous equilibria involve chemical changes: \[\ce{PbCl2}(s) \rightleftharpoons \ce{Pb^2+}(aq)+\ce{2Cl-}(aq) \label{13.3.30a}\], \[K_{eq}=\ce{[Pb^2+][Cl- ]^2} \label{13.3.30b}\], \[\ce{CaO}(s)+\ce{CO2}(g) \rightleftharpoons \ce{CaCO3}(s) \label{13.3.31a}\], \[K_{eq}=\dfrac{1}{P_{\ce{CO2}}} \label{13.3.31b}\], \[\ce{C}(s)+\ce{2S}(g) \rightleftharpoons \ce{CS2}(g) \label{13.3.32a}\], \[K_{eq}=\dfrac{P_{\ce{CS2}}}{(P_{\ce S})^2} \label{13.3.32b}\]. Once we know this, we can build an ICE table, which we can then use to calculate the concentrations or partial pressures of the reaction species at equilibrium. A schematic view of this relationship is shown below: It is very important that you be able to work out these relations for yourself, not by memorizing them, but from the definitions of \(Q\) and \(K\). If K < Q, the reaction Since K >Q, the reaction will proceed in the forward direction in order ), \[ Q=\dfrac{[\ce{C}]^x[\ce{D}]^y}{[\ce{A}]^m[\ce{B}]^n} \label{13.3.2}\], The reaction quotient is equal to the molar concentrations of the products of the chemical equation (multiplied together) over the reactants (also multiplied together), with each concentration raised to the power of the coefficient of that substance in the balanced chemical equation. It should be pointed out that using concentrations in these computations is a convenient but simplified approach that sometimes leads to results that seemingly conflict with the law of mass action. Pressure does not have this. The first, titled Arturo Xuncax, is set in an Indian village in Guatemala. Q = K: The system is at equilibrium resulting in no shift. The chemical species involved can be molecules, ions, or a mixture of both. . Problem: For the reaction H 2 (g) + I 2 (g) 2 HI (g) At equilibrium, the concentrations are found to be [H 2] = 0.106 M [I 2] = 0.035 M [HI] = 1.29 M What is the equilibrium constant of this reaction? Subsitute values into the expression and solve. It does not store any personal data. As will be discussed later in this module, the rigorous approach to computing equilibrium constants uses dimensionless 'activities' instead ofconcentrations, and so \(K_{eq}\) values are truly unitless. , Using Standard Molar Entropies), Gibbs Free Energy Concepts and Calculations, Environment, Fossil Fuels, Alternative Fuels, Biological Examples (*DNA Structural Transitions, etc. Since K c is given, the amounts must be expressed as moles per liter ( molarity ). How do you find internal energy from pressure and volume? The reaction quotient (Q) uses the same expression as K but Q uses the concentration or partial pressure values taken at a given point in time, whereas K uses the concentration or partial pressure . Arrow traces the states the system passes through when solid NH4Cl is placed in a closed container. Calculate the partial pressure of N 2 (g) in the mixture.. At first this looks really intimidating with all of the moles given for each gas but if you read the question carefully you realize that it just wants the pressure for nitrogen and you can calculate that . As the reaction proceeds, the value of \(Q\) increases as the concentrations of the products increase and the concentrations of the reactants simultaneously decrease (Figure \(\PageIndex{1}\)). If one species is present in both phases, the equilibrium constant will involve both. Write the expression of the reaction quotient for the ionization of HOCN in water. 17. What is the value of the equilibrium constant for the reaction? (The proper approach is to use a term called the chemical's 'activity,' or reactivity. For now, we use brackets to indicate molar concentrations of reactants and products. A homogeneous equilibrium is an equilibrium in which all components are in the same phase. Water does not participate in a reaction when it's the solvent, and its quantity is so big that its variations are negligible, thus, it is excluded from the calculations. Thus for the process, \[I_{2(s)} \rightleftharpoons I_{2(g)} \nonumber\], all possible equilibrium states of the system lie on the horizontal red line and is independent of the quantity of solid present (as long as there is at least enough to supply the relative tiny quantity of vapor.). In fact, one technique used to determine whether a reaction is truly at equilibrium is to approach equilibrium starting with reactants in one experiment and starting with products in another. The partial pressure of gas B would be PB - and so on. So, if gases are used to calculate one, gases can be used to calculate the other.
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