1.7 Simple Equilibria and Acid-Base Reactions

• Reversible reactions are shown by the $\rightleftharpoons$symbol
• Characteristics of dynamic equilibrium:

  1. Can only be established in a closed system. Matter cannot be exchanged with the surroundings (this will effect the position of the equilibrium), but energy can be exchanged.
  2. Can be approached from either direction.
  3. Equilibrium is a dynamic state - at equilibrium, the rate in both directions must be the same.
  4. Dynamic equilibrium is stable under fixed conditions but is sensitive to changes in concentration, pressure, and temperature.
• Factors affecting the position of equilibrium

  1. Changing the concentration of reactants or products
  2. Changing the pressure (if gases involved)
  3. Changing temperature
• A reversible reaction is one that can proceed in both directions.
• Dynamic equilibrium is achieved when forward and reverse reactions occur at the same rate.

  Therefore there is no noticeable change in concentration of reactants and products.
• If a system is at equilibrium, and a change is made in any of the conditions, the system responds to counteract the change as much as possible.
• Le Chatelier's Principle - When a reaction at equilibrium is subject to a change in concentration, pressure or temperature, the position of the equilibrium will move to counteract the change.
• The position of the equilibrium shifts to oppose the change imposed on it. It is possible to make an equilibrium system shift in the direction we want.
• Increasing concentration - Equilibrium shits to the side with least moles to reduce the concentration of the added product/reactant.
• Changing pressure - Increase in pressure favours the side with fewer moles. Decrease in pressure favours the side with more moles.
• Changing temperature:

  Increasing temperature - Towards endothermic reaction - gets rid of heat into atmosphere

  Decreasing temperature - Towards exothermic side.
• Effect of catalyst on an equilibrium - A catalyst has no effect on the position of the equilibrium since it speeds up the forward and reverse reaction so will only increase the rate at which equilibrium is achieved.
• $\triangle H^\theta$= +58kjmol^-1

  +ve - endothermic

  -ve - exothermic
• As temperature is decreased, equilibrium moves in exothermic direction.
• As pressure is increased, the equilibrium shifts in the direction of the side with the smaller number of gas moles.
• Only temperature can effect Kc values
• At equilibrium, the concentrations of the reactant and products no longer change and are a fixed amount, a constant.

  The equilibrium constant is an expression of the relative fixed amounts of products : reactants at equilibrium.
• $K_c =$$\frac{[PRODUCTS]^p}{[REACTANTS]^r}$
• $aA +$$bB\leftrightharpoons cC+$$dD$

  $K_c=$$\frac{[C]^c[D]^d}{[A]^a[B]^b}$
• $2SO_2 +$$O_2\leftrightharpoons 2SO_3$

  $Kc=$$\frac{[SO_3]^2}{[SO_2]^2[O_2]}$
• The value of Kc stays the same even if you change the concentration of any substance because the equilibrium shifts to combat these changes.
• The units of Kc change depending on the equation so you need to do another calculation to find the units of Kc
• If all cancel out, you have to say "no units"
• Units of Kc:

  $K_c=$$\frac{[NO_2]^2}{[N_2O_4]^1}$

  $Kc=$$\frac{moldm^{-3}\times moldm^{-3}}{moldm^{-3}}$

  $Kc=$$\frac{moldm^{-3}\times \sout{moldm^{-3}}}{\sout{moldm^{-3}}}$

  $K_c=$$moldm^{-3}$
• Units for Kc:

  $K_c=$$\frac{[SO_3]^2}{[SO_2]^2[O_2]}$

  $K_c=$$\frac{(moldm^{-3})^2}{(moldm^{-3})^2 \times moldm^{-3}}$

  $K_c=$$\frac{\sout{(moldm^{-3})^2}}{\sout{(moldm^{-3})^2} \times moldm^{-3}}$

  $K_c=$$\frac{1}{moldm^{-3}}$

  $K_c=$$dm^3mol^{-1}$
• If Kc is greater than 1: more products, favours right side

  If Kc is less than 1: more reactants, favours left side
• Products favoured: Kc > 1
• Reactants favoured: Kc < 1
• Kc will increase if you get more products than reactants
• Acid: Proton donor - releases H+ ions when mixed with water
• Base: Proton acceptor - accepts H+ ions when mixed with water
• Alkali: A base that dissolves in water releasing OH- ions
• Strong acids fully dissociate - all split into max hydrogen
• Weak acids partially disociate
• pH - The pH of a solution is the negative logarithm to base 10 of the molar hydrogen ion concentration
• $pH=$$-\log_{10}[H^+]$(no units)
• $[H^+]=$$10^{(-pH)}$ moldm^-3
• Monobasic: releases 1 proton
• Dibasic: releases 2 protons
• Tribasic: releases 3 protons