Chemistry · MU Pleven

CRITERIA FOR PREDICTING THE DIRECTION OF CHEMICAL REACTIONS EQUILIBRIUM CONSTANT, ENTROPY AND FREE ENERGY

Covers how the equilibrium constant Kc and the reaction quotient Q indicate which way a reaction will proceed, including the magnitude of Kc and the ratio Q/Kc, followed by Le Chatelier's principle with a summary table of stresses and shifts. The second half moves to thermodynamics: systems and state functions, internal energy, work and heat, the first law, enthalpy, and spontaneous versus non-spontaneous processes. It then explains entropy, Gibbs free energy and the Gibbs-Helmholtz equation, temperature effects on spontaneity, and the statements of the second law.

Inside: 12 sections · 12 labelled pictures · 1 table · 29 formulas · 37 flashcards · 45 min

Topic 9 of 47 in the MU Pleven syllabus for Chemistry (Medicine, Year 1). The free AceMed note covers it in 12 sections, about 6,511 words (45 min of reading), with 37 flashcards.

In this note

  1. CRITERIA FOR PREDICTING THE DIRECTION OF CHEMICAL REACTIONS EQUILIBRIUM CONSTANT, ENTROPY AND FREE ENERGY
  2. I. CHEMICAL EQUILIBRIUM AND EQUILIBRIUM CONSTANT.
  3. II. THERMODYNAMIC CRITERIA FOR PREDICTING THE SPONTANEITY OF CHEMICAL REACTIONS
  4. Some thermodynamic quantities
  5. III. FIRST LAW OF THERMODYNAMICS
  6. IV. SECOND LAW OF THERMODYNAMICS
  7. Spontaneous and nonspontaneous processes
  8. The driving force for a spontaneous process
  9. Entropy
  10. Deriving the criteria from Gibbs-Helmholz equation
  11. Effect of temperature on the spontaneity of a process
  12. Statements of the SECOND LAW of thermodynamics

I. CHEMICAL EQUILIBRIUM AND EQUILIBRIUM CONSTANT.

Equilibrium constant is a quantitative criterion for predicting the direction of chemical reactions.

Chemical equilibrium is a state characteristic for reversible reactions. A reversible reaction is a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously.

reactants ⇄ products

All reactions can be regarded in principle as reversible so long as the reaction happens in a closed system.

At chemical equilibrium the rates of the forward and of the back reaction are equal and the concentrations of all substances involved in the reaction are constant with time (but not equal) – equilibrium concentrations.

rate of forward reaction = rate of backward reaction

Both forward and backward reactions occur but there are no observable changes in the amounts of reactants and products - dynamic equilibrium.

Equilibrium constant

The equilibrium constant represents the extent to which reactants are converted to products. For a reversible reaction:

The equilibrium constant, Kc, is given by:

The equilibrium constant is equal to the ratio of the product of the equilibrium concentrations of the reaction products and the product of the equilibrium concentrations of the reactants, each concentration raised to the power equivalent to the respective number of moles in the balanced chemical equation.

The equilibrium constant is a ”constant” only if the temperature does not change. Once we know the value of the equilibrium constant, we can use it to calculate unknown equilibrium concentrations. The equilibrium constant helps us predict the direction in which a reaction mixture will proceed to achieve equilibrium, and to calculate the concentrations of reactants and products once equilibrium has been reached.

Characteristics of Kc:

  • Kc is a constant for any given reaction as long as the temperature does not change.
  • Kc indicates the components that are in greater concentration at equilibrium – the products or the reactants.
  • Because Kc is determined by reactant and final product concentration it is independent of the number of intermediate steps in the reaction mechanisms.
  • The equilibrium constant Kc depends only on the temperature and on the nature of the substances involved in the equilibrium.
  • The equilibrium constant does not depend on the concentration of the compounds or on the presence of catalysts.
  • The Kc value is independent of initial concentrations of reactants or products.
  • The concentrations of solid substances are not included in the equilibrium constant since they are assumed constant.

Continue reading free

The whole note, all 12 sections, is free with an AceMed account, with its 37 flashcards. Premium adds flashcard recall, highlights and Ask AI.

Start freeAlready a member? Open the note