A chemical change in which one type of substance changes into two or more different substances is called decomposition. We will study thermal decomposition, electrolysis, and representative examples.
Basic Knowledge
Decomposition is a reaction in which one type of substance changes into several different substances. When energy is added, bonds break and decomposition occurs.
① Thermal decomposition (decomposition by heating)
・Thermal decomposition of sodium hydrogen carbonate: 2NaHCO3 → Na2CO3 + H2O + CO2
(sodium hydrogen carbonate→sodium carbonate＋water＋carbon dioxide)
・Thermal decomposition of silver oxide: 2Ag2O → 4Ag + O2
(silver oxide→silver＋oxygen)
② Electrolysis (decomposition by electric current)
・Electrolysis of water: 2H2O → 2H2 + O2
Hydrogen is produced at the cathode (−), and oxygen at the anode (+). Volume ratio H2:O2 = 2:1
・Electrolysis of aqueous copper chloride: CuCl2 → Cu + Cl2
Copper precipitates at the cathode, and chlorine gas is produced at the anode
Decomposition（a chemical change in which one type of substance changes into two or more substances）
Thermal decomposition（decomposition caused by adding heat）
Electrolysis（decomposition caused by passing electric current）
Cathode and anode（the cathode (−) is the side into which electrons flow. Hydrogen is produced there during water electrolysis）
Sodium hydrogen carbonate（NaHCO3, baking soda. Decomposes when heated and produces CO2）
Silver oxide（Ag2O. Decomposes into silver and oxygen when heated）
Further Study
The thermal decomposition of sodium hydrogen carbonate is a frequent middle-school chemistry experiment. Important points:
・Heat the test tube with its mouth slightly lower (to prevent produced water from flowing to the hot part and breaking the test tube)
・Passing the gas produced through limewater makes it turn cloudy white → confirmation of carbon dioxide
・The white powder left behind (sodium carbonate Na2CO3) turns deep red with phenolphthalein solution (it is more strongly alkaline than sodium hydrogen carbonate)
In the electrolysis of water, a small amount of sodium hydroxide or sulfuric acid is added to make water conduct electricity more easily. The volume ratio of the collected gases, H2:O2 = 2:1, agrees with the ratio of hydrogen and oxygen atoms in the chemical formula H2O, supporting the correctness of the atomic and molecular model.
- Decomposition = one type → two or more types of substances
- Heating sodium hydrogen carbonate → sodium carbonate＋water＋CO2
- Confirmation of CO2 = limewater turns cloudy white
- Water electrolysis = H2 from the cathode, O2 from the anode
- Volume ratio of H2:O2 = 2:1
- Thermal decomposition of silver oxide → silver (shiny white)＋oxygen
- Lowering the mouth of the test tube during heating prevents water from flowing back
Points to Note
① In water electrolysis, be careful not to reverse hydrogen from the cathode (−) and oxygen from the anode (+). ② Sodium hydrogen carbonate (baking soda) and sodium carbonate are different substances. Sodium carbonate remains after heating. ③ Decomposition is a type of chemical change and differs from a physical change (change of state). ④ The volume ratio 2:1 applies to pure water. Even when an electrolyte is dissolved to improve electrical conductivity in an experiment, the ratio does not change.
Practice
- How can you confirm the gas produced when sodium hydrogen carbonate is heated?
- State the gases produced at the cathode and anode, respectively, during water electrolysis.
- Write the chemical equation for heating silver oxide.