4.7 - Thermal Stability of Carbonates and Nitrates

• thermal stability


  indication of how easily compounds decompose on heating
• polarising power


  the extent to which a positive ion is able to distort the electron cloud around a neighbouring anion

  the larger the positive charge & smaller ion (greater charge density) the greater the polarising power
• trend in thermal stability of group 1 nitrates & carbonates

  increases down the group because atoms get bigger so charge density decreases so polarising power decreases so nitrate & carbonate ions are distorted less so are more thermally stable
• thermal decomposition of group 1 Carbonates

  thermally stable under a Bunsen burner - decompose at higher temperatures

  exception: lithium carbonate decomposes into lithium oxide and carbon dioxide
• thermal decomposition of group 1 nitrates

  decompose into nitrite & Oxygen

  2MNO3 → 2MNO2 + O2

  exemption: lithium nitrate decomposes into lithium Oxide, nitrogen dioxide and Oxygen
• trend in thermal stability of group 2 nitrates and carbonates

  increases down the group because cations get bigger so have a lower charge density so have a lower polarising power so distorts anions less so more thermally stable
• thermal decomposition of group 2 carbonates

  MCO3 → MO + CO2
• thermal decomposition of group 2 nitrates

  2M(NO3)2 → 2MO +4NO2+O2
• beryllium carbonate


  doesn't exist because it's so unstable
• thermal stability of group 1 vs group 2

  group 2 nitrates and carbonates are less thermally stable than those of group 1 because cations have a +2 charge against a +1 charge

  so they have a greater charge density so have a greater polarising power so distort anions more so are less thermally stable
• testing thermal stability of nitrates

  measure how long it toakes for a specific amount of oxygen to be produced using gas syring or how long it takes to relight a glowing splint

  measure how long it takes for brown NO2 to be produced - must be done in a fume cupboard or gas is toxic
• testing thermal stability of carbonates

  measure how long it takes for a specific amount of carbon dioxide to be produces using a gas syringe or seeing how long it takes for limewater to turn cloudy