Video Transcript
In this video, we will learn how to describe and explain the properties of the element sodium and its compounds.
Sodium is a reactive metal found in group one of the periodic table. The metals in group one are known as the alkali metals. Although common in the Earth’s crust, sodium is never found in nature as a pure metal. Sodium reacts with many other elements to form salts. Sodium chloride is a very common salt. Using sodium chloride as a starting material, sodium metal may be extracted using electrolysis. To extract sodium metal from sodium chloride, the sodium chloride must first be in a molten state. To lower the melting point of sodium chloride, it’s mixed with calcium chloride. This mixture melts at about 600 degrees Celsius.
Sodium is produced at the negative electrodes, or cathodes, in this cell. These cathodes are made from iron. The sodium is molten at the temperature of the cell. And as it’s less dense than the molten sodium chloride and calcium chloride in the cell, it floats upwards and out of the cell. The anode, or positive electrode, in this cell is made from graphite, and this is where chlorine gas is produced. At the negative electrode, or cathode, in this cell, a sodium ion gains one electron to become a sodium atom.
The half equation shown here represents a reduction process. This is easily remembered using the mnemonic OILRIG, where oxidation is loss and reduction is gain of electrons. At the anode, or positive electrode, in this electrolysis cell, two chloride ions will arrive. Each chloride ion will lose one electron. So a total of two electrons are lost. A chlorine molecule, Cl2, is formed in this process. Since electrons are being lost in this process, the anode is the site of oxidation in this cell.
Now, we’ll look at some of the properties of sodium metal. As a metal, sodium has some rather unusual properties when compared with the properties of other typical metals. Physically, sodium is a soft metal with a low density and a low melting point and low boiling point. Sodium is in fact slightly less dense than water. So when placed into water, sodium floats. Sodium also reacts with water, displacing hydrogen gas in the reaction. The reaction is very vigorous, and it releases a lot of heat energy to the surroundings. In this exothermic reaction, the sodium will be seen to melt into a small ball that floats on the surface of the water.
Many of these physical properties can be attributed to the fact that sodium is a relatively large atom. In the metallic lattice of sodium, you won’t find that many sodium atoms per unit of volume. This gives sodium a low density. Also in the metallic lattice, we find that there is only one valence electron per sodium ion. This gives rise to relatively weak electrostatic attraction between these delocalized electrons and the sodium ions in the lattice. Therefore, the metallic bonding is correspondingly weak also.
Now, we’ll look at the reaction of sodium metal with oxygen gas. Sodium metal is so chemically reactive it’s normally stored under oil. This prevents any freshly cut sodium metal reacting with oxygen in the air. Sodium metal will react with oxygen very easily. When large pieces of it are heated in oxygen, it burns with an orange flame. When sodium burns in excess oxygen, sodium peroxide is the main product that’s formed. Unusually for a group one metal compound, sodium peroxide has a faint yellowish color. Sodium peroxide has the chemical formula Na2O2. We therefore need two moles of sodium atoms to react with a mole of oxygen molecules to make a mole of sodium peroxide.
When sodium is heated with oxygen in this way, a small amount of the superoxide of sodium, NaO2, may also be formed. Both the peroxide and superoxide of sodium react with water. They are strong oxidizing agents. Sodium peroxide reacts with water to yield both sodium hydroxide and hydrogen peroxide solutions. Sodium superoxide also reacts with water to yield solutions of sodium hydroxide, hydrogen peroxide, and oxygen gas. Sodium peroxide can also react with dilute acids. When sodium peroxide reacts with acids, the corresponding salt and the hydrogen peroxide are formed. For example, the reaction with hydrochloric acid would produce sodium chloride and hydrogen peroxide solutions.
Now, we’ll look at the reaction of sodium with water. As we’ve already seen, sodium metal must be kept away from water. This is because it reacts vigorously with water and produces hydrogen gas. The other product of this reaction is sodium hydroxide. Sodium hydroxide dissolves very easily in water and produces a strongly alkaline solution. Since sodium hydroxide is widely described as a base, this would be a basic solution. The sodium will be seen to fizz around and melt on the surface of the water. Sometimes the sodium catches fire in this very exothermic reaction. The hydrogen produced in this reaction will burn with a popping sound.
Now, we’ll look at the reaction of sodium with acids. Sodium metal will react very violently and exothermically with solutions of dilute acids. In the case of the reaction of sodium metal with hydrochloric acid, nitric acid, and sulfuric acid, the corresponding sodium salt and hydrogen gas are formed. The reaction of sodium metal and concentrated hydrochloric acid is not as violent as may be expected. In this case, the sodium chloride formed is not very soluble in the concentrated acid. It forms a barrier between the sodium metal and the concentrated acid, preventing any further reaction occurring.
Now, we’ll look at the reaction of sodium with nonmetals. Sodium also reacts readily with other nonmetal elements, such as hydrogen, sulfur, and phosphorus. Direct combination of the elements is possible if the sodium is heated first. Liquid sodium reacts with hydrogen gas at around 300 degrees Celsius to form sodium hydride. This contains the negatively charged hydride ion. Hydrides are reducing agents, and sodium hydride will react with water to produce sodium hydroxide and hydrogen gas. For this reason, hydrides have been investigated as a means of storing hydrogen gas for use in fuels or vehicles in a potential future hydrogen economy.
Sodium sulfide is easily formed in the laboratory by heating sodium metal with elemental sulfur. Sulfur is a yellow powder. Sodium sulfide is a toxic compound, and it reacts with moist air to give off hydrogen sulfide gas. Hydrogen sulfide gas smells of rotten eggs.
Sodium phosphide can be formed by heating white phosphorus with sodium metal. Unusually for a group one metal compound, sodium phosphide is a black solid. When sodium metal is heated directly with the halogens, the sodium halide salts are formed in some very exothermic reactions. Sodium burns very brightly in chlorine gas to produce the solid white salt sodium chloride. Vast deposits of sodium chloride exist underground and also dissolved in lakes and oceans.
The sodium halides are all soluble in water. Sodium halides are important sources of sodium ions. Sodium ions are the main cation found in extracellular fluid and blood plasma in the body. Sodium ions play an important role in nerve cell function. Commonly, our food provides us with plenty of sodium ions to stay healthy. Sodium ions are found in many vegetables and milk products.
Now, we’ll look at the properties of sodium hydroxide. Sodium hydroxide is a corrosive white solid in its pure form. It dissolves very easily in water, the process being highly exothermic. The solution is highly alkaline, and it has a soapy feel to touch. Sodium hydroxide solution can be heated carefully with fats or oils to produce soap. Industrially, it’s also used in making synthetic silk and paper.
A solution of sodium hydroxide would neutralize acidic solutions to produce a sodium salt and water. If the acid chosen was hydrochloric acid, the salt formed would be sodium chloride. In a similar way, if the acid chosen was sulfuric acid, the salt formed would be sodium sulfate.
Another interesting use for sodium hydroxide solution is for detecting metal cations in solutions. Metal cations are positively charged metal ions. Metal cations are found in solutions containing metal salts. These tests are simple lab bench tests, and they work on the basis that insoluble metal hydroxide precipitates are formed. To test for copper two plus ions in solution, we would add a few drops of sodium hydroxide solution to the test tube. Immediately, a blue precipitate will form. The blue precipitate is caused by the formation of copper(II) hydroxide in this reaction. If our solution of copper two plus ions at the start had been copper(II) sulfate, it would’ve been a blue solution. Solutions of copper two plus ions are frequently blue. The blue solution at the start is not to be confused with the blue precipitate of copper(II) hydroxide, which confirms the presence of copper two plus ions.
If the blue precipitate of copper(II) hydroxide is collected and heated strongly, it begins to turn black. In this reaction, the blue copper(II) hydroxide decomposes, producing water vapor and black copper(II) oxide. In the reaction of aqueous aluminum three plus ions with aqueous sodium hydroxide, a white precipitate of aluminum(III) hydroxide is seen to form. This is not a unique result, as many other metal cations produce white precipitates with sodium hydroxide solution. In this case though, the aluminum(III) hydroxide will dissolve if excess sodium hydroxide is added to it. Aluminum(III) hydroxide reacts with the excess sodium hydroxide to form a solution of sodium aluminate. Sodium aluminate is both soluble and colorless.
Next, we will look at the properties of sodium carbonate. Sodium carbonate is a white solid. Sodium carbonate is thermally stable. It does not decompose when heated. The thermal stability of sodium carbonate is unlike that of lithium carbonate and group two metal carbonates. Lithium carbonate and group two metal carbonates decompose on heating. When sodium carbonate is dissolved in water, it produces an alkaline or basic solution. The resulting solution would have a pH of about 11. That would turn universal indicator paper blue.
Sodium carbonate will neutralize acidic solutions, such as hydrochloric acid solution. Sodium salts will be produced in these reactions. In these reactions, fizzing will also be observed as carbon dioxide gas is generated. In fact, sodium carbonates are relatively safe way to neutralize an acidic spill in the laboratory. Industrially, sodium carbonate is important in making glass, paper, and textiles. Sodium carbonate can also be used to soften hard water that contains dissolved calcium and magnesium ions. These ions react with the sodium carbonate to form insoluble carbonates that can be filtered out. Sodium carbonate was sold as washing soda exactly for this purpose.
Sodium carbonate is manufactured in an industrial process called the Solvay process. Ammonia gas and carbon dioxide gas are passed into saturated sodium chloride solution. This reaction produces sodium bicarbonate. Upon heating, the sodium bicarbonate solution decomposes into sodium carbonate solution, carbon dioxide, and water vapor.
Next, we’ll look at what happens when some sodium compounds are heated strongly. Sodium nitrate decomposes when heated strongly. In this reaction, oxygen gas is released and sodium nitrite solid remains. Notice the subtle difference in the formulas of these two compounds. Sodium nitrate is deliquescent. Left exposed to air, it will absorb water vapor. Because of this deliquescence, sodium nitrate is not used in pyrotechnic devices. In gun powder, potassium nitrate is used instead.
As mentioned earlier, sodium carbonate is thermally stable. It does not decompose when heated. Do not confuse sodium carbonate with sodium bicarbonate, which has a different formula. Sodium bicarbonate decomposes when heated at 180 degrees Celsius. At this temperature, sodium bicarbonate releases carbon dioxide gas. Sodium bicarbonate is used in baking soda. At the temperature at which cakes are baked, the baking soda decomposes. The carbon dioxide gas helps the cakes rise.
We will now review the key points from this video. Sodium is a very reactive metal that is extracted using electrolysis. Sodium reacts with oxygen to form sodium peroxide, which has the formula Na2O2. Sodium reacts with many nonmetals, including sulfur, phosphorus, and hydrogen. Sodium reacts with water to form an alkaline solution of sodium hydroxide. Sodium hydroxide can be used to test for metal ions in solutions. Solid precipitates are formed.
