Suppose you need to calculate the constant of equilibrium. If we are talking about a reaction between gases, the product of which is also gas, the constant of equilibrium is calculated using the partial pressure of the components. For example, consider the reaction of catalytic oxidation of sulphur dioxide to sulphuric anhydride (raw material for the production of sulfuric acid). It proceeds as follows:2SO2 + O2 = 2SO^3.
Taking into account factors faced by the molecules of sulphur dioxide and sulphuric anhydride, the formula of the constant of equilibrium will look as follows:P^2 SO3/p^2 x pO2 SO2
If the reaction occurs in any solution, and you know the molar concentration of the starting materials and products, the formula is used to calculate the constant of equilibrium of the reversible chemical reaction A + B = B + G, would be as follows:Cu = [A][B]/[B][G].
Calculate the constant of equilibrium of a chemical reaction using a known amount of change of the Gibbs energy (this data you can find in the chemical reference). The calculation is made according to the following formula:∆G = -RT lnKр, that is lnKр = -∆G/RT. Calculating the value of the natural logarithm of the CD, you can easily determine the value of constant equilibrium.
When calculating the constants of equilibrium, remember that the amount of change of the Gibbs energy depends only on final and initial state of the system and not on the intermediate stages. In other words, you are completely indifferent to the ways in which it was received by the destination from the source substance, the Gibbs energy change will still be the same. So, if you for some reason can't determine ∆G for a particular reaction, can be estimated by intermediate reactions (it is important only that they eventually led to the formation of we need ultimate substances).
Advice 2: How to determine the equilibrium constant
Constant equilibrium characterizes the displacement of the reversible chemical reaction toward the formation of the reaction products or starting materials. To calculate the constant of equilibrium in a variety of ways, depending on the conditions of the problem.
You will need
- - handle;
- paper for records;
A constant equilibrium can be expressed using equilibrium concentrations of the reaction participants, i.e. the concentration of substances at a time when the speed of the forward reaction equals the rate of reverse. Given the reversible reaction of substances A and b In certain conditions with the formation of substances: na+mB ↔ zС, where n, m, z – the coefficients in the equation reactions. A constant equilibrium can be expressed by: Kc = [C]^z/ ([A]^n*[B]^m), where [C], [A], [B] is the equilibrium concentration of substances..
In the first type of task you want to define the constant of equilibrium of the equilibrium concentrations. Equilibrium concentrations may not be asked directly. When solution first write down the reaction equation, place coefficients.
Example: nitrogen monoxide under certain conditions, reacts with oxygen c NO2 formation. The initial concentrations of NO and O2 – 18 mol/l and 10 mol/L. it is Known that methane 60% O2. You want to find the constant of equilibrium reaction.
Write down the reaction equation, place coefficients. Please note, the proportions in which substances react. Calculate the concentration of O2, which entered into reaction: 10моль*0,6 = 6 mol/L. From the reaction equations, find the concentration of unreacted NO – 12 mol/L. the NO2 concentration is 12 mol/L.
Determine the amount of unreacted NO: 18-12 = 6 mol. And unreacted oxygen: 10-6 = 4 mol. Calculate the constant of equilibrium: KS = 12^2/(6^2*4) = 1.
If the clause specified the rate constants of direct and reverse reactions, find the constant of equilibrium from the relation: K = k1/k2 where k1, k2 – the rate constants of the forward and reverse chemical reactions.
In an isothermal process and Isobaric process a constant equilibrium can be found from the equation of the standard changes of Gibbs energy: ΔGр- = - RT*lnKc = -8,31 T*2,3 lgKc, where R is the universal gas constant, equal to 8,31; T – reaction temperature, K; lnKc – the natural logarithm of the constants of equilibrium. For convenience, convert it into decimal lgKc by multiplying by a factor of 2.3.
To determine the change in standard Gibbs energy of reaction, you can from the equation for isothermal Isobaric process: ΔG = ΔH – T ΔS, where T is the reaction temperature, K; ∆ H - enthalpy, kJ/mol; ΔS is entropy, j/(mol-deg). The value of enthalpy and entropy for 1 mole of basic chemical compounds at a temperature of 25 ° C are given in reference literature. If the reaction temperature differs from 25 ° C, the values of enthalpy and entropy should be given in the problem statement.
Magnitude of the ΔG of the reaction at 25 ° C you can find the folding potentials of education ΔGобр each of the reaction products and subtracting from the sum ΔGобр starting materials. The potential values of education 25oC for 1 mole of various substances are given in reference tables.
In the case that the participants in the reaction are in different States of aggregation, in the formula for determination of the equilibrium constant includes the concentrations of substances in more a mobile (gas or liquid) state.