# Tip 1: How to find the equilibrium constant

Chemical equilibrium - a state of the chemical system, when the rate of flow of the forward and reverse chemical reaction are equal.That is a condition in which the concentrations of the starting materials and reaction products (or their partial pressure) does not change.A constant balance Cr - is the value determined by the ratio between the concentrations or pressures.
Guide
1
Let's say you need to calculate the constant balance .If it is a reaction between the gases, which is also a product of the gas, the equilibrium constant calculated by the partial pressures of the components.For example, consider the reaction of catalytic oxidation of sulfur dioxide to sulfur trioxide (a raw material for the production of sulfuric acid).It proceeds as follows: 2SO2 + O2 = 2SO ^ 3.
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Given the factors facing the molecules of sulfur dioxide and sulfur trioxide, formula constants balance will be as follows: P ^ 2 SO3 / p ^ 2 SO2 x pO2
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If the reaction occurs in any solution and you
are aware of molar concentrations of the starting materials and products, the formula used to calculate the constant balance reversible chemical reaction A + B = B + D, is as follows: Kp = [A][B] / [B] [D].
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Calculate the equilibrium constant chemical reaction using a known amount of change in the Gibbs energy (these data can be found in chemical handbooks).The calculation is as follows: ΔG = -RT lnKr ie lnKr = -ΔG / RT.Calculating the value of the natural logarithm of Cr, you can easily define itself and the value of the equilibrium constant .
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In calculating the equilibrium constants , remember that the amount of change in the Gibbs energy depends only on the initial and final state of the system and not on the intermediate stages.In other words, you could care less what ways has been obtained from the source material end, Gibbs energy change will still be the same.Therefore, if you for some reason can not determine ΔG for a particular reaction, it is possible to settle on the intermediate reactions (it is only important that they eventually led to the formation of the desired substance, we end).

# Tip 2: How to determine the equilibrium constant

equilibrium constant characterizes displacement reversible chemical reaction toward the formation of reaction products or the starting materials.Calculate constant balance in various ways, depending on the task.
you need
• - handle;
• - paper records;
• - Calculator.
Guide
1
constant equilibrium can be expressed through the equilibrium concentrations of the reactants - that is, the concentration of substances at a time when the rate of the forward reaction is equal to the speed feedback.Suppose that a reversible reaction of compounds A and B at defined conditions, to yield a compound C: nA mB + ↔ ZC, wherein n, m, z - the coefficients in equation reactions. constant equilibrium can be expressed: Kc = [C] ^ z / ([A] ^ n * [B] ^ m), where [C], [A], [B] - equilibrium concentrations of substances ..
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In the first type of tasks you want to define a constant balance of equilibrium concentrations of substances.Equilibrium concentration may not be specified directly.With their decision to first write the reaction equation, set up the coefficients.
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Example: nitrogen monoxide under certain conditions reacts with oxygen c form NO2.Given the initial concentrations of the substances of NO and O2 - 18 mol / L and 10 mol / l.It is known that 60% reacted O2.Required to find constant equilibrium reaction.
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Write down the reaction equation, set up the coefficients.Note, in which the ratio of the reactants.Calculate the concentration of O2, reacted: 10mol * 0.6 = 6 mol / l.From the reaction equation get the concentration of reacted NO - 12 mol / L.And the concentration of NO2 - 12 mol / L.
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Determine the amount of unreacted NO: 18-12 = 6 mol.And unreacted oxygen: 10-6 = 4 mol.Calculate constant balance : Kc = 12 ^ 2 / (2 ^ 6 * 4) = 1.
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If in the problem are given constant speed forward and reverse reactions, get constant equilibrium from the relation: K = k1 / k2, where k1, k2 - constant speed forward and reverse chemical reactions.
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an isothermal process and the process of isobaric constant balance can be found from the equation of the standard Gibbs energy change: ΔGr-and = -RT * lnKc = -8,31T * 2,3lgKc, wherein R- the universal gas constant, equal to 8.31;T - reaction temperature, K;lnKc - the natural logarithm of the equilibrium constant .For convenience, it is converted to decimal lgKc multiplying by a factor of 2.3.
8
Calculate the standard Gibbs energy of the reaction, you can out of the equation for the isothermal isobaric process: ΔG = ΔH - T ΔS, where T - the reaction temperature, K;ΔH - enthalpy kJ / mol;ΔS - entropy J / (mole-degree).The value of the enthalpy and entropy of 1 mole of basic chemicals at 25 ° C are given in reference literature.If the reaction temperature is different from 25 °, the enthalpy and entropy should be given in the problem.
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also value ΔG of the reaction at 25 ° C, you can find folding potentials education ΔGobr each of the reaction products and ΔGobr deducted from the initial substances.The values ​​of the potential formation of 25 ° C for 1 mole of different substances are given in the reference tables.
Note
If the reaction partners are in different states of aggregation, into the formula for the determination of the equilibrium constants include the concentration of substances in a mobile (gas or liquid)state.
Sources:
• how to determine the equilibrium constant

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