Thursday, September 8, 2011

10th SA-1-Acid Base and Salt Trend Setter Question Paper

1: Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?

Ans: Hydrogen gas is usually liberated when an acid reacts with a metal.fig 2.1 page 19  
Take few pieces of zinc granules and add 5 ml of dilute H2SO4. Shake it and pass the gas produced into a soap solution. The bubbles of the soap solution are formed. These soap bubbles contain hydrogen gas.                 2H2SO4            +     Zn      →  Zn (SO4)2   +  2H2  
We can test the evolved hydrogen gas by its burning with a pop sound when a candle is brought near the soap bubbles.

2: Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.

                        CaCO3  (s)   +    HCI(aq)    →   CaCI2(aq) + CO2   + HO (I)

3 : Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

Ans: The dissociation of HCl or HNO3 to form hydrogen ions always occurs in the presence of water. Hydrogen ions (H+) combine with H2O to form hydronium ions (H3O+).     HCI --------à   H+  +  CI –      H+     +     HO →   HO+
Although aqueous solutions of glucose and alcohol contain hydrogen, these cannot dissociate in water to form hydrogen ions. Hence, they do not show acidic character.

4  Why does an aqueous solution of an acid conduct electricity?

Ans.:   Acids dissociate in aqueous solutions to form ions. These ions are responsible for conduction of electricity.

5.   Why does dry HCl gas not change the colour of the dry litmus paper?

Ans. : Colour of the litmus paper is changed by the hydrogen ions. Dry HCl gas does not contain H+ ions. It is only in the aqueous solution that an acid dissociates to give ions. Since in this case, neither HCl is in the aqueous form nor the litmus paper is wet, therefore, the colour of the litmus paper does not change.

6. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Ans.:  Since the process of dissolving an acid in water is exothermic, it is always recommended that acid should be added to water. If it is done the other way, then it is possible that because of the large amount of heat generated, the mixture splashes out and causes burns

7: How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?

Ans. : When an acid is diluted, the concentration of hydronium ions (H3O+) per unit volume decreases. This means that the strength of the acid decreases.

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