Decomposition Reactions Using Heat, Light and Electrical Energy

To decompose water, copper (II) carbonate and silver chloride into their respective elements or simpler compounds, while observing signs of chemical change using electrical energy, heat and light.

Practical VII – Decomposition Reactions Using Heat, Light and Electrical Energy

Aim

To decompose water, copper (II) carbonate and silver chloride into their respective elements or simpler compounds, while observing signs of chemical change using electrical energy, heat and light.

Method

Refer to pg. 20 & 21 of Year 11 Chemistry Stage 6 Preliminary Course Practical Manual

Diagram

Electrolysis of Water

 

 

 

 

 

 

 

 

 

 

Heating of Copper (II) Carbonate

 

 

 

 

 

 

 

Decomposition of Silver Chloride

 

 

 

 

 

 

 

 

 

Results – Tabulated Observations

Electrolysis of Water

Gas Produced

Colour Change

Temperature Change

Precipitate Formed

Solid Dissolved

Hydrogen – ‘Pop’ test

No

No

No

No

Oxygen – Glowing splint Test

Clear & Colourless

Constant Temperature

 

 

 

Heating of Copper (II) Carbonate

Gas Produced

Colour Change

Temperature Change

Precipitate Formed

Solid Dissolved

Carbon Dioxide produced from copper carbonate as limewater turned cloudy, milky and opaque.

Copper (II) Carbonate turned from a green colour to a black colour.

Lime water changed from a clear liquid to an unclear, opaque, milky textured liquid.

Copper (II) Carbonate heated by the Bunsen burner, the copper carbonate appeared to be moving in the test tube when heat was applied.

Calcium Carbonate turning limewater milky

No

 

Composition of Silver Carbonate

Gas Produced

Colour Change

Temperature Change

Precipitate Formed

Solid Dissolved

No

Are exposed to light on filer paper turned from white to a purple/black.

Area covered by object (rubber), which was unaffected by light, remained white.

No, constant temperature

Silver Chloride white to purple/black

No

 

Discussion

Electrolysis of Water

In the electrolysis of water, hydrogen and oxygen, both in the form of gas, were separated from water (in liquid form). This was evidence of a chemical change, despite the fact that there was no evidence in a change of colour, new precipitate or disappearance of a solid.  

The chemical reaction was a result of the intramolecular forces of the Hydrogen and Oxygen atoms covalent bond in the H2O molecule breaking apart due to the passing of an electric current through the water. The electrical current and sulphuric acid aided in the process of decomposition of H2O into Hydrogen and Oxygen.

To prove that the gas produced in the anode was oxygen, a glowing splint was placed in the area filled with oxygen where it ignited again, proving that the gas was oxygen. At the cathode electrode, hydrogen gas was determined through the ‘pop’ test where a match was placed underneath a test tube that contained oxygen. Subsequently a loud ‘pop’ sound was omitted from the test tube and the flame of the match was extinguished, confirming the presence of hydrogen gas.

Water -> Hydrogen Gas + Oxygen Gas

2H2O (L) -> 2H2 + O2 (g)

Heating of Copper (II) Carbonate

In the heating of Copper (II) Carbonate, copper (II) oxide and carbon dioxide was produced. Consequently, in the lime water, calcium carbonate and water was produced. All of this was evidence for two chemical changes that occurred during this decomposition reaction as a gas was produced, colour change, temperature change and formation of precipitation.

The first chemical reaction occurred as the chemical bonds in Copper (II) carbonate were broken by hear to produce the new chemical substance chopper oxide and carbon dioxide. Copper (II) Oxide was black in colour as Copper (II) Carbonate changed from green to black.

Copper Carbonate –(heat)> Copper oxide + Carbon Dioxide gas

CuCO3(s) –(heat)> CuO (s) + CO2

The second reaction that occurred was in the lime water, here the limewater started to become milky and white. This was a result of carbon dioxide gas produced from the heating of Copper (II) Carbonate reacting with the limewater. As a result, a precipitate was formed – calcium carbonate.

Composition of Silver Chloride

The decomposition of silver chloride ‘broke up’ the silver and chlorine atoms in the compounds. This was evidence of a chemical reaction as it possessed a change in colour and formation of a precipitate. The chemical reaction occurred as a result because silver nitrate was mixed with sodium chloride, allowing silver chloride to be produced on the filter paper.

 Silver Nitrate + Sodium Chloride -> Silver Chloride + Sodium Nitrate

AgNO3 (aq) + NaCL (aq) -> AgCl (s) + NaNO3 (aq)

Silver Chloride –(Light)> Silver + Chlorine

2AgCl –(Light)> 2Ag(s) + Cl2 (g)

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