Equilibrium — JEE Mains Chemistry

Formulas (3)

• Writing K from the equation · Law of mass action
  $$K_c=\frac{[\mathrm{C}]^c[\mathrm{D}]^d}{[\mathrm{A}]^a[\mathrm{B}]^b}$$
• Reversing, scaling and adding equations · Combining equilibria
  $$K_{\text{reverse}}=\frac1K,\qquad K_{n\times}=K^n,\qquad K_{1+2}=K_1K_2$$
• Kp and Kc through the change in gas moles · Kp and Kc
  $$K_p=K_c\,(RT)^{\Delta n}$$

Watch out for (6)

• A solid left in the expression
  → Writing K from the equation
• Equal rates, not equal amounts
  → Writing K from the equation
• Added equations multiply their K
  → Reversing, scaling and adding equations
• Scaling is a power, not a factor
  → Reversing, scaling and adding equations
• The sign of Δn
  → Kp and Kc through the change in gas moles
• Reading Δn from a ratio
  → Kp and Kc through the change in gas moles

Formulas (2)

• ICE tables in moles and concentration · Reaction quotient
  $$Q=\frac{[\mathrm{C}]^c[\mathrm{D}]^d}{[\mathrm{A}]^a[\mathrm{B}]^b}\ \text{(any moment)},\qquad Q<K\Rightarrow\text{forward}$$
• ICE tables in partial pressures · Partial pressure
  $$p_i=\frac{n_iRT}{V}=x_i\,P$$

Watch out for (4)

• The change row follows the coefficients
  → ICE tables in moles and concentration
• Check the direction before you write the table
  → ICE tables in moles and concentration
• The inert gas is in the total, not in K
  → ICE tables in partial pressures
• Carbon adds nothing to the pressure
  → ICE tables in partial pressures

Formulas (2)

• Degree of dissociation and Kp · A ⇌ B + C
  $$K_p=\frac{\alpha^2P}{1-\alpha^2}\qquad\Longleftrightarrow\qquad \alpha=\sqrt{\frac{K_p}{K_p+P}}$$
• K, ΔG° and temperature · Gibbs energy and K
  $$\Delta G^\circ=-2.303\,RT\log K$$

Watch out for (4)

• Pressure pushes α down, not up
  → Degree of dissociation and Kp
• The fraction is K over the rest
  → Degree of dissociation and Kp
• K belongs to the equation as written
  → K, ΔG° and temperature
• Multiply the slope by 2.303
  → K, ΔG° and temperature

Formulas (1)

• Finding the new equilibrium · Same K before and after
  $$K=\frac{[\mathrm{B}]_{\text{old}}}{[\mathrm{A}]_{\text{old}}}=\frac{[\mathrm{B}]_{\text{new}}}{[\mathrm{A}]_{\text{new}}}\quad(\mathrm{A\rightleftharpoons B},\ \text{same }T)$$

Reference tables (1)

Watch out for (4)

• Pressure moves the mixture, not K
  → Which way the equilibrium shifts
• Read how the inert gas is added
  → Which way the equilibrium shifts
• Start from the disturbed mixture
  → Finding the new equilibrium
• The change is only partly undone
  → Finding the new equilibrium

Formulas (2)

• Strong acids, bases and their mixtures · pH of a strong acid-base mixture
  $$\mathrm{pH}=-\log\frac{n_{\mathrm{H^+}}-n_{\mathrm{OH^-}}}{V_{\text{total}}}$$
• Weak acids and bases from Ka and Kb · Weak acid
  $$[\mathrm{H^+}]=\sqrt{K_aC},\qquad \mathrm{pH}=\tfrac12\left(\mathrm{p}K_a-\log C\right)$$

Watch out for (5)

• Sulphuric acid counts twice
  → Strong acids, bases and their mixtures
• Dilution does not cross 7
  → Strong acids, bases and their mixtures
• Hot water is neutral at pH below 7
  → Strong acids, bases and their mixtures
• Take the square root
  → Weak acids and bases from Ka and Kb
• In strong acid, divide by the strong acid's $\mathrm{H^+}$
  → Weak acids and bases from Ka and Kb

Formulas (2)

• The Henderson equation · Henderson equation
  $$\mathrm{pH}=\mathrm{p}K_a+\log\frac{[\text{salt}]}{[\text{acid}]}$$
• Buffers made by part-neutralisation · Weak acid part-neutralised by strong base
  $$\mathrm{pH}=\mathrm{p}K_a+\log\frac{n_b}{n_a-n_b}$$

Watch out for (4)

• The ratio flips for a basic buffer
  → The Henderson equation
• Salt over acid, not acid over salt
  → The Henderson equation
• Base left is y minus x, not y
  → Buffers made by part-neutralisation
• Excess strong acid is not a buffer
  → Buffers made by part-neutralisation

Formulas (1)

• pH of a salt solution · Salt of a weak acid and a strong base
  $$\mathrm{pH}=7+\tfrac12\,\mathrm{p}K_a+\tfrac12\log C$$

Reference tables (1)

Watch out for (4)

• Use the anion's concentration
  → pH of a salt solution
• Exact neutralisation leaves a salt, not a buffer
  → pH of a salt solution
• Phenolphthalein is an acid, and it ionises in base
  → Acid-base indicators and titrations
• Redox and acid-base indicators are not swapped
  → Acid-base indicators and titrations

Formulas (3)

• Ksp and molar solubility · Solubility product
  $$K_{sp}=x^x\,y^y\,s^{x+y}\qquad(\mathrm{A_xB_y})$$
• Common ion and solubility · Solubility with a common ion
  $$s=\frac{K_{sp}}{C^{\,n}}\qquad(n=\text{count of the common ion in the formula})$$
• Will it precipitate, and in what order · Precipitation condition
  $$Q>K_{sp}\ \Rightarrow\ \text{precipitate}$$

Watch out for (6)

• Each ion is its count times s
  → Ksp and molar solubility
• Divide first, then take the root
  → Ksp and molar solubility
• Square the common ion for AB₂
  → Common ion and solubility
• Convert to grams only at the end
  → Common ion and solubility
• Mixing halves both concentrations
  → Will it precipitate, and in what order
• The smaller Ksp is not always first
  → Will it precipitate, and in what order