Answer the questions.[Che-3M]

Question 13 Marks

The atomic number of an element is 20 . Write its electronic configuration. State whether this element is a metal or a non-metal. What is its valency? Write the name and formula of the compound which this element forms with chlorine.

Answer

The element with an atomic number of 20 is calcium (Ca).
The electronic configuration of calcium is: $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4 s^{2}$.
Calcium is a metal because it is located in Group 2 (or Group IIA) of the periodic table, which consists of metals known as alkaline earth metals.
The valency of calcium is +2 .
The compound that calcium forms with chlorine is calcium chloride. The chemical formula for calcium chloride is $\mathrm{CaCl}_{2}$.

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Question 23 Marks

Define a chemical reaction. State four observations which help us to determine that a chemical reaction has taken place. Write one example of each of the observations with a balanced chemical equation.

Answer

Chemical reaction is the transformation of chemical substance into another chemical substance. Only a rearrangement of atoms takes place in a chemical reaction. Old bonds are broken and new bonds are formed. Some of the characteristics of chemical reactions are:
i. Change in colour: In some reactions, there is a change in colour after the reaction. For example, a chemical reaction between citric acid and purple-coloured potassium permanganate solution is characterised by a change in colour of potassium permanganate from purple to colourless.
$\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+2 \mathrm{KI} \longrightarrow 2 \mathrm{KNO}_{3}(\mathrm{aq})+\mathrm{PbI}_{2}(\mathrm{~s})$
$\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+2 \mathrm{KI}$ - Colourless
$\mathrm{PbI}_{2}(\mathrm{~s})$ - Yellow
ii. Formation of precipitate: The chemical reaction between sulphuric acid barium chloride solution. $\mathrm{BaCl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq})$
$\longrightarrow 2 \mathrm{HCl}(\mathrm{aq})+\mathrm{BaSO}_{4}(\mathrm{~s})$
iii. Change in temperature: Temperature change is the characteristic of many reactions. For example, the chemical reaction between quicklime and water to form slaked lime. In this reaction temperature of the reaction is increased.
$\mathrm{CaO}(\mathrm{s})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \longrightarrow \mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{aq})+$ Heat
iv. Evolution of gas: Some reactions are characterised by evolution of gas as a result of chemical reaction. For example, the chemical reaction between sodium carbonate and hydrochloric acid is characterised by the evolution of carbon dioxide gas.
$\mathrm{Zn}+\mathrm{H}_{2} \mathrm{SO}_{4} \longrightarrow \mathrm{ZnSO}_{4}+\mathrm{H}_{2}$

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Question 33 Marks

Identify the type of reactions taking place in each of the following cases and write the balanced chemical equation for the reactions.
a. Zinc reacts with silver nitrate to produce zinc nitrate and silver.
b. Potassium iodide reacts with lead nitrate to produce potassium nitrate and lead iodide.

Answer

a. It is a displacement reaction.
$
\mathrm{Zn}+2 \mathrm{AgNO}_{3} \rightarrow \mathrm{Zn}\left(\mathrm{NO}_{3}\right)_{2}+2 \mathrm{Ag}
$
b. It is a double displacement reaction.
$
2 \mathrm{Kl}+\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2} \rightarrow 2 \mathrm{KNO}_{3}+\mathrm{Pbl}_{2}
$

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