Directorate of Education, GNCT of Delhi Subject : Science Worksheet : 4 Date : 13-07-2020 Class : X Name of the Student ________________ Name of the Class Teacher _______________ Chemical Reactions and Equations Decomposition reactions plays a significant role in our day to day life. Previously, you read about decomposition reactions and its various types. Let’s discuss one more interesting decomposition reaction today. Almost everyone of you have seen or heard about Black and White photography. But, a few of us know the Science behind it. Actually, black and white pictures are created by making use of a decomposition reaction that is as follows: 2AgCl(s) -------Sunlight---à 2Ag(s) + Cl2(g) v Silver Chloride decomposes into Silver and Chlorine by light. Thus, the white colour reactant turned into a grey colour product. This is how Black and White photographs are made. v Since this reaction involves use of light energy, so this Decomposition reaction is also known as Photochemical Decomposition reaction. v You can perform this experiment by putting 2 gm of Silver Chloride in a china dish under the sunlight. v Note: Silver Bromide behaves the same way as Silver Chloride. 2AgBr(s) --------Sunlight--à 2Ag(s) + Br2(g) v It is advisable to store Silver Chloride or Silver Bromide in dark color bottles. 1. What is the role of Sunlight in Photochemical decomposition reactions? _________________________________________ 2. Which type of Chemical reaction occurs in black and white photography? ________________________________________________ 3. Silver Chloride turns grey to form Silver metal. Is it true or false? ______________________ 4. A photographer did not found Silver Chloride to develop black and white photographs. Name the other chemical which can be used by the photographer to develop the photographs ? ______________________________________ 5. Open your family’s old album. You may locate some of the black and white photos in it. Cherish those moments with your family and tell them how it works! _______________________________________________________
Answers
Decomposition reaction examples
Decomposition reaction happens all throughout, but we often cease to see it. Let us look at some decomposition reaction examples in real life.
KClO3 decomposition reaction
H2O2 decomposition reaction
Even Though the decomposition reaction concept is simple, these are some of the common doubts people have about the concept.
Why are decomposition reaction called the opposite of combination reaction?
We know a combination reaction to be a chemical reaction in two or more substances combine together to produce a new compound. Whereas in a decomposition reaction, the combination between constituents are broken down to form multiple products.
Are all decomposition reactions endothermic?
No, not all decomposition reactions are endothermic. A decomposition reaction can be both endothermic or exothermic, even though the latter is more predominant
Examples:
Decomposition of NO to N2 & O2 is exothermic.
Decomposition of ozone (O3) to oxygen (O2) is exothermic.
State an important use of decomposition reaction?
One major application of the decomposition is the extraction of metals from its ore. Like obtaining zinc from calamine and Sodium from sodium from sodium chloride.
Double decomposition reaction
A double decomposition reaction is a type of decomposition reaction in which two constituent reactants interchange positive and negative ions and form two new compounds.
Decomposition Reaction
Example
HCl(aq) + NaOH(aq) → NaCl(aq) + HOH(l)
Types of decomposition reaction
Let us look at the three main types of decomposition reactions which we see around us.
Thermal decomposition reaction A thermal decomposition reaction can be defined as a decomposition reaction which is activated by thermal energy. The reaction is generally endothermic as energy is required to break chemical bond in order to separate the constituent elements.
example of thermal decomposition reaction
CaCO3 → CaO + CO2 When heated Calcium Carbonate,decomposes into Calcium Oxide and Carbon Dioxide. This process is employed in the manufacturing of quick lime which is an important component in industries.
Electrolytic decomposition reaction
An electrolytic decomposition reaction is a type of decomposition reaction in which the activation energy for decomposition. Example of electrolytic decomposition reaction
Dissociation of water:2H2O→ 2H2 + O2
Photo decomposition/ photolytic decomposition/ photochemical decomposition
A photo decomposition is the type of decomposition reaction in which the reactant is broken down to its constituents by photons.Example of photo decomposition reaction
O3 + hν → O2 + O(1D)Photolysis occurs in the atmosphere wherein ozone molecules is broken down into oxygen molecule and atomic oxygen.
Uses of decomposition reaction
Manufacture of cement or calcium oxide
For metallurgical processes: Extraction of metals from its oxides,chloride etc
The relief from acid indigestion
Thermite welding
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