What is Equilibrium constant: Definition and 97 Discussions

The equilibrium constant of a chemical reaction is the value of its reaction quotient at chemical equilibrium, a state approached by a dynamic chemical system after sufficient time has elapsed at which its composition has no measurable tendency towards further change. For a given set of reaction conditions, the equilibrium constant is independent of the initial analytical concentrations of the reactant and product species in the mixture. Thus, given the initial composition of a system, known equilibrium constant values can be used to determine the composition of the system at equilibrium. However, reaction parameters like temperature, solvent, and ionic strength may all influence the value of the equilibrium constant.
A knowledge of equilibrium constants is essential for the understanding of many chemical systems, as well as biochemical processes such as oxygen transport by hemoglobin in blood and acid–base homeostasis in the human body.
Stability constants, formation constants, binding constants, association constants and dissociation constants are all types of equilibrium constants.

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  1. E

    I Equilibrium constant change with stoichiometric doubling (Callen)?

    Callen asks us (with respect to an ideal gas) I had thought to proceed as follow. We have the definition for the singular reaction: $$\ln K_s(T) = - \sum_j \nu_j \phi_j(T).$$ Now a reaction which is the sum of this reaction with itself (doubled reaction) has ##\nu_j \to 2\nu_j## so that its...
  2. tbn032

    Chemistry Confusion in relation of Gibbs free energy and equilibrium constant

    SO2(g)+1/2O2(g)⇌SO3(g);ΔHo=-98.32KJ/mole,ΔSo=-95J/(mole-K). find Kp at 298 Kelvin? In given question at first Δ G will be calculated using formula ΔG = Δ H – T x ΔS, by putting the given values in formula we get ΔG = -70.01 kJ/mol. Then Keq will be calculated using equation = Δ G = -RT ln Keq...
  3. Huzaifa

    Chemistry Effect of temperature change in Le Chatelier's Principle

    The effect of temperature change in Le Chatelier's Principle is given by the equation $$ \log \left(\frac{\mathrm{K}_{2}}{\mathrm{~K}_{1}}\right)=\frac{\Delta \mathrm{H}}{2.303 \mathrm{R}}\left[\frac{1}{\mathrm{~T}_{1}}-\frac{1}{\mathrm{~T}_{2}}\right] $$. How to derive $$ \log...
  4. SilverSoldier

    Approximations in Chemical Equilibrium (add a weak acid HA into pure water)

    Suppose we add a weak acid HA into pure water, so that upon addition its initial concentration is c. The following equilibria should establish in the system. $$\text{HA}+\text{H}_2\text{O}\rightleftharpoons\text{H}_3\text{O}^++\text{A}^-$$...
  5. I

    Calculate Equilibrium Constant for Polyprotic Acid Reaction

    The principal equilibrium in a solution of NaHCO3 is HCO3-(aq) + HCO3-(aq) <-> H2CO3(aq) + CO32-(aq) Calculate the value of the equilibrium constant for this reaction. My solution: This overall reaction is the same as the sum of the following reactions: HCO3-(aq) <-> H2CO3(aq) + OH-(aq)...
  6. W

    Thermal Physics: Interpretation of equilibrium constant

    For a reaction defined as such, $$A\rightleftharpoons B$$ the equilibrium constant ##K## is defined by ##K = p_B / p_A##, with ##p## denoting the partial pressure (edit: at equilibrium). However, if ##K<<1##, which implies ##p_A >> p_B##, it is said that the backwards reaction dominates and that...
  7. M

    Kc = 1.88 for 2HI(g) ↔ H2(g) + I2(g)

    Homework Statement Kc = 1.37 for HI(g) ↔ 1/2 H2(g) + 1/2 I2(g). What is Kc (to 3 decimal places) for H2(g) + I2(g) ↔ 2HI(g)? Homework Equations Kc= Products/Reactants, Ice Tables The Attempt at a Solution I tried an ice table, but I’m not sure what to do with the Kc value I’m given when I...
  8. harambe

    Equilibrium Constant for Alcohol and Acetic Acid Reaction at 25°C | Homework

    Homework Statement One mole of pure ethyl alcohol was treated with one mol of pure acetic acid at 25 °C.One third of acid changes into ester at equilibrium The equilibrium constant for the reaction will be Homework Equations C2H5OH+CH3COOH------>C2H5COOCH3[/B] K={A}^m{B}n/{C}^x{D}^z where...
  9. S

    Thermodynamics equilibrium constant problem

    Homework Statement N2 + 3H2 --> 2NH3 (all gases) Why is such a high pressure needed? Calculate the equilibrium constant at 500k then estimate the percentage conversion at equilibrium at 1 bar total pressure, assuming the stoichiometric ratio of N2:H2 is 1:3. Repeat the process at 50 bar...
  10. C

    Equilibrium constant expression; why exponents?

    Hello all. I am wondering why we raise the concentration to the exponent of the stochiometric coefficient. I understand why we take the products of reactants or products. For instance, let me give an example to show where I am getting mixed up. Let's say 2A +3B = 8C as a reversible...
  11. M

    MHB Calculate Equilibrium Constant of 2BrCl <=> Br2 + Cl2

    bromine monochloride dissociated on heating the following equation 2BrCl <=> Br2 + Cl2 .9 moles of BrCl were heated in a 5 liter vessel until equilibrium was established . the amount of free chlorine in the mixture was found to be .4 moles calculate the equilibrium constant for the reaction...
  12. U

    Pressure dependence of the equilibrium constant for an ideal gas

    I read in some scripts that equilibrium constant for an ideal gas is not a function of pressure: But that is not generally true! Since: $$\left (\frac{\partial \Delta_{R} G}{\partial p} \right )_{T,\vec{n}}=\Delta_{R} V$$ and $$\Delta_{R} G^{0}=-RT\cdot \ln K$$ it should be: $$\left...
  13. W

    Equilibrium constant Kp,eq issue

    Homework Statement question1-sentence https://www.physicsforums.com/attachments/105257 question1-answer https://www.physicsforums.com/attachments/105258 question2-sentence https://www.physicsforums.com/attachments/105259 question2-answer https://www.physicsforums.com/attachments/105260...
  14. Aafia

    Calculate equilibrium constant?

    Homework Statement [/B] The following reaction was allowed to reach the state of equilibrium 2A+B <====> C The initial amounts of reactant present in one litre of solution were 0.5 mole of A and 0.6 mole of B. At equilibrium the amounts were 0.2 mole of A and 0.45 mole of B and 0.15 mole of...
  15. Saracen Rue

    Equilibrium Constant of a Heterogeneous Reaction

    Homework Statement Write an expression for Kc for the following at equilibrium; CaCO3(s) + CO2(g) + H2O(l) ⇔ Ca2+(aq) + 2HCO3-(aq) Homework Equations K = [products]^(coefficients)/[products]^(coefficients) The Attempt at a Solution I know that pure solid and liquid substances should be set...
  16. E

    Trouble with Gibbs Free Energy & Equilibrium Constant Calc.

    Hello. A known equation that is useful for calculating equilbrium constants is: ΔG° = -RT * ln(K) This is all well and good. Given a standard gibbs free energy of reaction for some given reaction, the equilibrium constant for the reaction can be found. My trouble is in which ΔG° to use. For...
  17. TheSodesa

    Precipitation of lead hydroxide in the presence of EDTA

    Homework Statement 1. The question EthyleneDiamineTetra-acetate(##EDTA^{4-}##) is used in chemical analysis as a complex-former. ##EDTA^{4-}## is also used to treat heavy metal poisoning, as it forms water soluble complexes with them which can then be easily removed by the body. The reaction...
  18. K

    Solving Dissociation Reaction Homework

    Homework Statement Hi, I tried the following task: I tried to solve it, but apparently it's not correct: Can someone show me the right way to do this?Homework Equations See above The Attempt at a Solution see above[/B]
  19. G

    Continuous process and equilibrium constant.

    Assuming that we have a chemical plant which produces methanol through the following equations: CH4 + H2O <--> CO + 3H2 CO + 2H2 <--> CH3OH CO2 + 3H2 <--> CH3OH + H2O I know that with specific pressure, temperature and flow rates, I can produce reactions with specific equilibrium constants. Are...
  20. N

    When to Use Kw & Ka for Acid Dissociation Calculations

    So I've learned that in strong acids that dissociate completely, the concentration of H+ is the same as the concentration of the initial solution. So 1M of a strong acid will create 1M of H+, meaning the pH is 0. I've also learned that in weak acids, the whole thing doesn't dissociate. I have...
  21. G

    Gibbs free energy and equilibrium constant at a high T

    I posted this earlier, but I just realized it might have been in the wrong section. Sorry Okay, so consider you have system in which ΔG<0 and ΔS>0. Using Gibbs free energy (ΔG=ΔH-TΔS), you'll know that it will always be negative. As the temperature increases, it will actually become more and...
  22. MexChemE

    Equilibrium of ammonia synthesis

    Homework Statement In order to obtain commercial conversion levels in the production of ammonia, reactors operate between a pressure range from 150 to 300 atm, and a temperature range from 700 to 750 K. Calculate the mole fraction of N2 when the reaction reaches equilibrium at 300 atm and 723...
  23. patrickbotros

    When to use which Equilibrium Constant. (Kp vs. Kc)

    Hello forum! My name's Patrick, and I'm taking Chemistry 212. We're learning about equilibrium and I'm constantly getting K's confused. For example: In the equation ΔG=-RT*ln(K), I have no clue whether to use the Kp value or the Kc value. Could I have used either? I just need a very thorough...
  24. F

    Lennard-Jones Potential: Derive an expression for equilibrium constant

    Edit: well this is frustrating, not sure why the itex things aren't working.. Edit2: I've attached a picture of the question at the bottom as well. Homework Statement The cohesive energy of a solid noble gas may be written as (See picture below) Derive an expression for the...
  25. J

    Relationship between Free energy change and Equilibrium constant (K)

    1. I have a question regarding the equation ΔG° = -RT lnK. When solving for the equilibrium constant K, what is the relationship between the vapor pressure of each molecular compound in a reaction? ΔG° = Free energy change R = Universal gas constant T = Temperature K = equilibrium...
  26. D

    Equilibrium constant expression

    Hello I'm solving previous exams and there is this exercise that appears in almost every exam : Homework Statement Suppose v moles of a gas H2O are enclosed in a constant volume V at a low temperature so the gas is in aN undissociated state. At highest temperatureS, H2O molecules are...
  27. L

    Calculating equilibrium constant

    Homework Statement Estimate the value of the equilibrium constant at 670K for the following reaction: 2NO2(g)⇌N2O4(g)Homework Equations ΔG°rxn = -RT*lnKThe Attempt at a Solution This should be an easy problem. But for some reason, MasteringChem won't accept it, so I'm wondering if I did...
  28. W

    Osmotic Pressure and Equilibrium constant

    Homework Statement An aqueous solution contains 0.84 g/L of an organic detergent. The osmotic pressure of this solution is 54.4 torr, at 25°C. What is the molar mass (in g/mol) of this detergent? Homework Equations ##pi=MRT## ##R=0.08206L*atm/(mol*K)## The Attempt at a Solution...
  29. C

    How Much CO Remains at Equilibrium in a 3L Flask?

    Homework Statement H2+CO2=CO+H2O if 1.50 mol of each chemical species are placed in a 3.00l flask and allowed to achieve the equilibrium above, what mass of Co will be present at equilibrium? Homework Equations Keq= concentration of products each raised to the number of moles in...
  30. U

    Why is equilibrium constant K a product of the concentrations?

    So in a given amount of solution, the equilibrium constant is the product of the concentrations of the ionic species present, raised to the power of the stoichiometric coefficient. Wouldn't it make more sense if it was the sum of the concentrations though,since basically the net number of ions...
  31. B

    Nernst equation and equilibrium constant

    The equation ΔG=-nFEcell, I understand. I also understand that, at unity activities of all species, Q=1 so R*T*ln(Q)=R*T*ln(1)=0. And therefore Ecell=E°cell and ΔG=ΔG°. However, surely we cannot then write that ΔG°=-nFE°cell=-R*T*ln(K), since ΔG° is measured at unity activity when Q=1, rather...
  32. A

    Equilibrium Constant Calculation

    Homework Statement With the 5 equations, the equilibrium contants can be calculated at the bottom. (See image) My complication I am aware that X = 200 thus that value remains at that. Also, From the fourth equation K_{out} = Cl_{out}. But, I cannot seem to work out the values. If not...
  33. H

    Relation between gibbs free energy and equilibrium constant

    I am familiar with the equation ΔG=ΔG°+RT ln(Q).But I can't derive it.We have to use the equation to derive nernst equation. So please help.
  34. T

    Equilibrium Constant Table Method

    Hi, I just want someone to explain this table to me please, I am totally confused especially with the Change(mol). Question below:
  35. M

    Equilibrium constant vs rate constant - where kinetics meets thermodynamics?

    Equilibrium constant vs rate constant - where kinetics meets thermodynamics?? The Equilibrium Law for aA + bB ⇌ cC + dD gives Kc = [C]c [D]d / [A]a x [B]b at a given temperature. Kc* is also equal to the ratio of forward to reverse rate constants kfwd/krev. The rate expression for the forward...
  36. M

    Temperature and the equilibrium constant

    The equilibrium constant changes with a change in temperature, I am confused as to the directions and why Qc is behaving oppositely of what I think it does For example, my textbook says a temperature increase favors an endothermic reaction (left to right) and a temperature decrease favors the...
  37. S

    Finding an equilibrium constant

    Hello, I'm new to the forums and did in fact search for my thread before posting. I couldn't find anything that helped me with what I'm looking for. With that said, the problem I'm having is: 1) The value of Keq for the equilibrium H2+I2 → 2 HI is 794 at 25°C. What is the value of Keq for the...
  38. J

    Does the equilibrium constant have units?

    According to my teacher in my chemistry lesson, the equilibrium constant has different units depending on how many moles of reactant and product there are. But the idea of a constant which changes it's units depending on the situation seems a bit dodgy to me, especially as it seems to be related...
  39. H

    Equilibrium constant contradiction

    I recently studied that equilibrium constant is independent of concentration. But if you multiply both sides of equation by any number, equilibrium constant changes. Isn't this contradictory?
  40. M

    Equilibrium constant and concentration

    I am aware that the equilibrium constant, Kc for the reaction {aA}~~+~~{bB}~~~\mathop{\rightleftharpoons}^{k_f}_{k_b}~~~{cC}~~+~~{dD} is \frac{k_f}{k_b} = \frac{[A]^a [B]^b}{[C]^c [D]^d} Now my question is, are the stoichiometric coefficients a,b,c and d, written in simplest whole number...
  41. M

    Solving for equilibrium constant

    Given the following reaction data at 25oC, PbCl2  Pb+2 + 2Cl- Kc = 1.8*10-10 2Ag+ + 2Cl-  2AgCl Kc = 2.48*105 Calculate Kc for the reaction of PbCl2 and Ag+. a. 7.2*10-16 b. 1.4*1015 c. 4.4*105 d. 4.5*10-5 e. can not solve Is this solveable? I don't know where to start>?
  42. Saladsamurai

    Equilibrium Constant: Valid under what conditions?

    Hello all! :smile: I am wondering: Since we derive the equilibrium constant by minimizing the Gibbs free energy, it means that me are taking advantage of the fact that at the minimum, we have (dG)_{T,P,m}=0. Does this mean that in order to determine the products of a reaction using the...
  43. W

    The equilibrium constant and solids

    Why are the concentrations of solids not incorporated into the equilibrium expression? All the books say it's because the "concentration of a pure solid is constant", but that doesn't make sense to me... If the solid is dissociating to produce a given product, why doesn't the concentration...
  44. M

    Equilibrium constant rules help.

    I was wondering where i could find a source for the rules to find Kc from a chemical formula? My professor tonight was changeing the formula like this: H2+Br2+2HBr then showed that 2H2+2Br2=4HBr...and getting different Kc values for each. I think that this is straight out wrong but she's the...
  45. A

    Calculate the equilibrium constant Kc for the reaction at this temperature

    Homework Statement H2 + I2 -> 2HI A reaction mixture in a 3.67 L flask at a certain temperature initially contains 0.763 g H2 and 96.9 g I2. At equilibrium, the flask contains 90.4 g HI. Calculate the equilibrium constant Kc for the reaction at this temperature. Homework Equations The...
  46. R

    Equilibrium constant question?

    Equilibrium constant question? Homework Statement A mixture of 0.229 mol CO2, 0.229 mol of H2, and 0.328 mol of H2O are placed in a 2.0 L container. The following equilibrium is established: CO2 + H2 <-> CO + H2O. The equilibrium concentration of H2O is 0.210 M. Calculate the equilibrium...
  47. G

    Derivation of the Equilibrium Constant

    I'm trying to understand this: http://en.wikipedia.org/wiki/Equilibrium_constant#Derivation_from_Gibbs_Free_Energy And there are few steps that I don't get. My first question is probably a stupid one, but where does the \Sigmav_{j}A_{j}=0 come from? What exactly is it summing up, the number of...
  48. H

    Chemical Equilibrium Constant Calculation

    Homework Statement If Q = 1.0 1042 for the reaction C(s) + O2(g) → CO2(g) at 25°C, calculate Keq. dGof (Kj/mol) of... C(s)=0 O2(g)=0 CO2(g)=-394.36 Homework Equations dGorxn = -R*T*ln(K)The Attempt at a Solution -394.36*1000 = -8.314472(298)ln(K) K=1.3e69 This is the wrong answer. I'm...
  49. A

    Equilibrium constant expression

    Given 2NO(g) + Br2(g) <--------_____> 2NOBr(g), Concentration (M) [NO] [Br2] [NOBr] Initial 2.5 5.0 1.0 Change P Q...