Q37 of 44 Page 5

Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why?


Zinc reacts with hydrochloric acid to liberate Hydrogen gas as it is more reactive than Hydrogen and hence, displace it. Copper is less reactive when compared to Hydrogen and hence cannot displace it.
The same can be understood by looking at the activity series of metals. The activity series of metals refers to the list of metals ranked in the decreasing order of their reactivity in order to displace Hydrogen gas from water or acid solutions. It helps us in predicting which metal will displace which metal in Acidic solutions.
Zinc lies above Hydrogen whereas Copper lies below Hydrogen.


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35

Balance the following chemical equations and identify the type of chemical reaction.

(a) Mg(s) + Cl2(g) → MgCl2(s)


(b) HgO(s) ⎯⎯⎯⎯ (Heat) → Hg(l) + O2(g)


(c) Na(s) + S(s) ⎯⎯⎯⎯ (Fuse) → Na2S(s)


(d) TiCl4 (l) + Mg(s) ⎯→ Ti(s) + MgCl2 (s)


(e) CaO(s) + SiO2(s) ⎯→ CaSiO3(s)


(f) H2O2 (l) ⎯⎯⎯⎯ UV → H2O(l) + O2 (g)

 36

A magnesium ribbon is burnt in oxygen to give a white compound X accompanied by emission of light. If the burning ribbon is now placed in an atmosphere of nitrogen, it continues to burn and forms a compound Y.

(a) Write the chemical formulae of X and Y.


(b) Write a balanced chemical equation, when X is dissolved in water.

 38

A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.

(a) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.


(b) Name the black substance formed and give its chemical formula.

 39

On heating blue coloured powder of copper (II) nitrate in a boiling tube, copper oxide (black), oxygen gas and a brown gas X is formed

(a) Write a balanced chemical equation of the reaction.


(b) Identity the brown gas X evolved.


(c) Identity the type of reaction.


(d) What could be the pH range of aqueous solution of the gas X?