The constant of balance characterizes the shift of reversible chemical reaction towards formation of products of reaction or initial substances. It is possible to calculate a balance constant in various ways, depending on statements of the problem.
It is required to you
- - handle;
- - note paper;
- - calculator.
Instruction
1. The constant of balance can be expressed through equilibrium concentration of participants of reaction – that is concentration of substances at the moment when the speed of direct reaction is equal to the speed of the return. Let reversible reaction of substances A and B in certain conditions with formation of substance C be given: nA +mB ↔ zC, where n, m, z – coefficients in the equation of reactions. The constant of balance can be expressed: Kc = [C] ^z/([A] ^n * [B] ^m) where [C], [A], [B] are equilibrium concentration of substances.
2. In the first type of tasks it is required to define a balance constant from equilibrium concentration of substances. Equilibrium concentration can directly be not set. At their decision at first write down the reaction equation, place coefficients.
3. Example: nitrogen monoxide at certain conditions reacts with oxygen with formation of NO2. Initial concentration of substances NO and O2 – 18 mol/l and 10 mol/l are given. It is known that 60% of O2 reacted. It is required to find a constant of balance of reaction.
4. Write down the reaction equation, place coefficients. Pay attention in what ratio substances react. Calculate concentration of O2 which reacted: 10mol*0.6 = 6 mol/l. From the equation of reaction find concentration of the reacted NO – 12 mol/l. And concentration of NO2 - 12 mol/l.
5. Define the number of not reacted NO: 18-12 = 6 mol. And not reacted oxygen: 10-6 = 4 mol. Calculate a balance constant: Kc = 12^2 / (6^2*4) = 1.
6. If in a statement of the problem constants of speed of direct and return reaction are specified, find a balance constant from a ratio: K = k1/k2 where k1, k2 are constants of speed of direct and return chemical reactions.
7. At isothermal process and isobaric process the constant of balance can be found from the equation of standard change of energy of Gibbs: ΔGр-и = - RT*lnKc = - 8.31T*2.3lgKc, where R – the universal gas constant equal 8.31; T – reaction temperature, To; lnKc is a natural logarithm of a constant of balance. For convenience it is transferred to decimal lgKc multiplication by coefficient 2.3.
8. To define change of standard energy of Gibbs of reaction, you can from the equation for isothermal isobaric process: ΔG = ΔH – T ΔS, where T – reaction temperature, To; ΔH - an enthalpy, kJ/mol; ΔS - entropy, J/(mol hail). Value of an enthalpy and entropy for 1 asking the main chemical compounds at a temperature of 25 wasps are given in reference books. If temperature of reaction differs from 25 wasps, values of an enthalpy and entropy have to be given in a statement of the problem.
9. Also ΔG reactions at 25 OS you can find size, having put education potentials ΔGобр each of products of reaction and having subtracted from the sum ΔGобр initial substances. Values of potentials of formation of 25 wasps for 1 asking various substances are provided in help tables.