Chemical Reactions and Equations

Decomposition Reactions of Silver Halides and Lead Nitrate

The decomposition of silver bromide ($AgBr$) follows a specific photochemical pathway when exposed to light energy. The chemical equation for this process is represented as $2AgBr(s) \xrightarrow{\text{Sunlight}} 2Ag(s) + Br_2(g)$. This reaction, alongside the decomposition of silver chloride, is fundamentally significant in the field of analogue imaging, as these reactions are used in black and white photography. The energy required to trigger these specific decomposition reactions is provided by sunlight.

In another instance of thermal decomposition, lead nitrate ($Pb(NO_3)_2$) reacts when heat is applied. The balanced chemical equation for this reaction is $2Pb(NO_3)_2(s) \xrightarrow{\text{Heat}} 2PbO(s) + 4NO_2(g) + O_2(g)$. In this process, solid lead nitrate decomposes into solid lead oxide, gaseous nitrogen dioxide, and gaseous oxygen. This reaction illustrates how thermal energy can break down a complex compound into simpler substances.

Electrolysis of Water (Activity 1.7)

Activity 1.7 details the process of the electrolysis of water, a method of decomposition using electrical energy. The apparatus required for this experiment includes a plastic mug with two holes drilled at its base, fitted with rubber stoppers. Carbon electrodes, specifically graphite rods, are inserted into these stoppers and connected to a $6\text{ volt}$ battery. The mug is filled with water until the electrodes are fully immersed, and a few drops of dilute sulphuric acid are added to the water to enhance conductivity. Two test tubes, also filled with water, are inverted over the carbon electrodes.

Upon switching on the electrical current and leaving the apparatus undisturbed, bubbles begin to form at both electrodes, which subsequently displace the water in the test tubes. Observations reveal that the volume of gas collected is not the same in both tubes. Hydrogen gas ($H_2$) is collected at the cathode, while oxygen gas ($O_2$) is collected at the anode. The volume of the hydrogen gas collected is exactly double the amount of the oxygen gas. This is consistent with the chemical composition of the water molecule, where hydrogen and oxygen are present in a $2:1$ ratio. These gases can be tested by bringing a burning candle close to the mouth of the test tubes, a procedure that must be performed carefully by a teacher due to the flammable nature of the gases.

Photochemical Decomposition of Silver Chloride (Activity 1.8)

In Activity 1.8, approximately $2\,g$ of silver chloride ($AgCl$) is placed in a china dish. Initially, silver chloride is white in colour. When the china dish is placed in sunlight for a period of time, the white silver chloride turns grey. This change in colour is the result of the decomposition of silver chloride into silver metal and chlorine gas by the action of light. The chemical reaction is expressed as $2AgCl(s) \xrightarrow{\text{Sunlight}} 2Ag(s) + Cl_2(g)$. This serves as a primary example of how light energy can facilitate the decomposition of a chemical compound.

Single Displacement Reactions

Displacement reactions occur when a more reactive element displaces or removes another element from its compound. An experimental observation of this involves placing an iron nail into a blue copper sulphate ($CuSO_4$) solution. Over time, the iron nail becomes brownish in colour due to the deposition of copper, and the blue colour of the copper sulphate solution fades as it transforms into iron sulphate ($FeSO_4$). The chemical reaction for this experiment is $Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$. In this instance, iron has displaced copper from the solution.

Additional examples of displacement reactions include the reaction of zinc ($Zn$) and lead ($Pb$) with copper compounds. Zinc reacts with copper sulphate to form zinc sulphate and copper: $Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)$. Similarly, lead reacts with copper chloride to form lead chloride and copper: $Pb(s) + CuCl(aq) \rightarrow PbCl_2(aq) + Cu(s)$. These reactions occur because zinc and lead are more reactive elements than copper, allowing them to displace copper from its compounds.

Double Displacement and Precipitation Reactions (Activity 1.10)

Activity 1.10 explore double displacement reactions through the mixing of two salt solutions. In this experiment, approximately $3\,mL$ of sodium sulphate ($Na_2SO_4$) solution is taken in one test tube, and approximately $3\,mL$ of barium chloride ($BaCl_2$) solution is taken in another. When these two solutions are mixed, a white substance is formed that is insoluble in water. This insoluble substance is known as a precipitate. Any chemical reaction that produces such an insoluble solid can be classified as a precipitation reaction.

The specific chemical reaction between sodium sulphate and barium chloride is represented by the equation $Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)$. In this reaction, the white precipitate formed is barium sulphate ($BaSO_4$), while sodium chloride ($NaCl$) remains in the solution. This process demonstrates the exchange of ions between the two reactants, characteristic of double displacement reactions.