Wednesday, 7 January 2015

4.3.1 - Laboratory Activity : Studying the reaction of alkali metals with oxygen

 Laboratory Activity 4.3.1: Studying the reaction of alkali metals with oxygen
Aim: To study the reaction of alkali metals with oxygen
Problem statement: How do lithium, sodium and potassium differ in reactivity with oxygen?
Hypothesis: Reactivity increases for the reaction between alkali metals and oxygen when going down Group 1
Variable:
 » Fixed variable : Oxygen » Manipulated variable : Type of alkali metal » Responding variable : Reactivity of the metal with oxygen

 Material: » Lithium » Sodium » Potassium » Phenolphthalein indicator » Oxygen » Water » Filter paper Apparatus: » Gas jar with cover » Gas jar spoon » Small knife » Test tube » Forceps » Bunsen burner
Procedure:

 1 The animation below shows the procedures and observation of the experiment. 2 A small piece of lithium is cut with a knife. Dry it with filter paper 3 The lithium is placed in the gas jar spoon by using a forceps. 4 The lithium is heated until it begins to burn, then it is plunged into a gas jar filled with oxygen. 5 When the reaction is completed, 10cm3 of water is added into gas jar. 6 Two drops of phenolphthalein indicator are then added into the gas jar. 7 The colour of the indicator with the solution formed in the gas jar is recorded. 8 The experiment is repeated using sodium and potassium.
Results:

 Metal Observation Colour of solution produced when added with phenolphthalein indicator. Lithium Burns with a red flame. Forms a white ash which dissolves in water to form a colourless solution. Pink Sodium Burns vigorously with a bright yellow flame. Forms a white ash which dissolves in water to form a colourless solution. Pink Potassium Burns very vigorously with a brilliant purple flame. Forms a white ash which dissolves in water to form a colourless solution. Pink
Discussion:

Lithium, sodium and potassium react with oxygen to produce a white metallic oxide.
 ○ $4Li\left(s\right) +{O}_{2}\left(g\right)\to {2Li}_{2}O\left(s\right)$ ○ $4Na\left(s\right) +{O}_{2}\left(g\right)\to {2Na}_{2}O\left(s\right)$ ○ $4K\left(s\right) +{O}_{2}\left(g\right)\to {2K}_{2}O\left(s\right)$

The white solids which are formed are alkali metal oxides.

The burning of lithium in oxygen is the least vigorous and the burning of potassium is the most vigorous. Thus, the reactivity of metals increases down Group 1.

The metallic oxide dissolves in water to form an alkali. Thus, the solution turns phenolphthalein indicator to pink colour.
 ○ ${Li}_{2}O\left(s\right) +{H}_{2}O\left(l\right)\to 2LiOH\left(aq\right)$ ○ ${Na}_{2}O\left(s\right) +{H}_{2}O\left(l\right)\to 2NaOH\left(aq\right)$ ○ ${K}_{2}O\left(s\right) +{H}_{2}O\left(l\right)\to 2KOH\left(aq\right)$
Conclusion:
 ► All Group 1 metals react with oxygen to form a metallic oxide which dissolves in water to produce an alkali. ► The reactivity increases down the Group 1. ► The hypothesis is accepted.