Boiling point is the temperature at which the vapor pressure of the liquid is equal to the external pressure on the liquid and the liquid changes into a vapor. Melting point is the temperature at which the lattice is collapses and the solid is converted into liquid. The trend in structure is from the metallic oxides containing giant structures of ions on the left of the period via a giant covalent oxide in the middle to molecular oxides on the right. Sodium peroxide, magnesium oxide and aluminum oxide are metallic oxide which have high melting and boiling point compared to other period 3 oxide. This is due to their giant ionic structure and the strong ionic bond. Strong ionic bond in the molecule need more energy to overcome it.
Therefore, the more energy required, the higher the melting and boiling point. Aluminum oxide will have higher melting and boiling point than magnesium oxide because Al3+ ions having high charge density than Mg2+, which enable it to polarize the electron cloud of O2- to give it a high degree of covalent structure in ionic bond. Silicon (IV) oxide has higher melting and boiling point compared to phosphorus (V) oxide and chlorine (I) oxide due to the strong covalent bond and the giant covalent structure. More energy is needed to break the strong covalent bond and in turns result the high melting point of silicon oxide. The trend of period 3 oxide is fluctuate across the period 3 and the degree of melting and boiling point is MgO > Al2O3 > SiO2 > Na2O2 > P4O10 > Cl2O .
Besides, there are different actions of water of period 3 oxide. Sodium peroxide is soluble in water. When sodium peroxide is put into water, it will dissolve in the water and form an alkaline solution. This reaction increase the temperature of the solution, therefore it is an exothermic reaction. The chemical equation for this reaction is Na2O2 + 2H2O 2NaOH + H2O2 .
Magnesium oxide is partially soluble in water. When it is put into water, it will slightly dissolve in the water and form a basic solution which the pH is around 9. This reaction increase the temperature of the solution, therefore it is an exothermic reaction. The chemical equation for this reaction is MgO + H2O Mg(OH)2 .
Aluminum oxide and silicon(IV) oxide will not react with water because they are insoluble in water. Aluminum oxide is an ionic compound with covalent properties giving rise to a strong intermediate force. Strong intermediate force and the high charge density of aluminum ions give rise to high lattice energy of a compound. The higher the lattice energy, the higher the amount of energy required to break the ionic bond, thus the lower the solubility in water. Silicon (IV) oxide cannot dissolve in water because of its giant covalent structure and the strong covalent bond which is difficult to break down.
Even though they are insoluble in water, but when they are added into the water, heat will release and the reaction is exothermic. Phosphorus (V) oxide and chlorine (I) oxide is soluble in water. When they are added into water, they will react violently and totally dissolve in the water. This reaction will make the solution become acidic. This reaction will release heat to the surrounding which is an exothermic reaction. The product of the reaction between water and phosphorus (V) oxide is phosphoric acid. The product of the reaction between water and chlorine (I) oxide is hypochlorous acid. The chemical equation of these two reaction are : P4O10 + 6H2O 4H3PO4
Cl2O + H2O 2HOCl
Moreover, the trend of pH value of the period 3 oxide is decrease from sodium peroxide to phosphorus (IV) oxide and the pH value of chlorine (I) oxide is slightly higher than phosphorus (IV) oxide which is pH 3. The pH value for sodium peroxide, magnesium oxide, aluminum oxide, silicon (IV) oxide and phosphorus (V) oxide are 14, 9, 7, 7, and 1 respectively.
The trend of acid-base nature of period 3 oxide is change from basic to amphoteric to acidic. This is due to the atomic number and the differences in electronegativity between the element and oxygen. When the atomic number of the element increase across the period, the number of electrons also increase across the period, the differences in electronegativity between the element and oxygen decrease. Ionic oxides are usually basic anhydrides, whereas covalent oxides are usually acidic anhydrides. There are large electronegativity difference of sodium and magnesium with oxygen. Sodium atom and magnesium atom will lose their valance electrons to oxygen atom to form metal ions and basic oxide ions, O2-. Therefore, sodium peroxide and magnesium oxide are ionic and basic compound.
The electronegativity difference of silicon, phosphorus and chlorine with oxygen is small because silicon, phosphorus and chlorine are non-metal. Non-metal atom will share electrons with oxygen atom by forming covalent bond to form covalent compound. Since silicon (IV) oxide, phosphorus (V) oxide and chlorine (I) oxide is covalent compound, they will show acidic properties. On the other hand, the electronegativity difference between aluminum and oxygen is large, this makes aluminum oxide an ionic compound. However, it also has significant covalent character as electron cloud of O2- ions is distorted towards Al3+ due to its high charge density. Therefore, aluminum oxide is ionic with partial covalent character which contribute its amphoteric nature that has both acidic and basic properties.
Beside, metallic oxide which is sodium peroxide, magnesium oxide and aluminum oxide are good conductor of electricity in molten state. They will not conduct electricity in solid state due to the absence of free mobile ions. They can conduct electricity in molten state because the free mobile ions is present. The molten metallic oxide are good electrolyte when they are in molten state. Silicon (IV) oxide, phosphorus (V) oxide and chlorine (I) oxide cannot conduct electricity either in solid state or in molten state. This is because they do not contain any free mobile electrons and ions.
In addition, some of the period 3 oxides are soluble in organic solvent such as hexane. Sodium peroxide, magnesium oxide and aluminum oxide is ionic compound which held by ionic bond. These three ionic compound are soluble in water but insoluble in organic solvent such as hexane. This is because organic solvent are organic compounds which cannot hydrate ions. As a result, ionic compounds cannot dissolve in hexane.
On the other hand, covalent compound such as silicon (IV) oxide, phosphorus (V) oxide and chlorine (I) oxide are soluble in organic solvent, hexane. This is because the covalent molecules in silicon (IV) oxide, phosphorus (V) oxide and chlorine (I) oxide and organic molecule in hexane are both held by weak intermolecular forces of attraction which is Van der Waals forces. As the result, the covalent molecules in silicon (IV) oxide, phosphorus (V) oxide and chlorine (I) oxide are easily miscible with the organic molecules in the hexane because they have the same type of weak intermolecular forces of attraction. Hence, silicon (IV) oxide, phosphorus (V) oxide and chlorine (I) oxide can dissolve in hexane.
During the experiment, there are some safety and precaution steps that must be done to minimize the danger and error that will occur. During the experiment, do not handle the phosphorus (V) oxide and sodium peroxide with bare hand. This is because phosphorus (V) oxide is corrosive and irritates eyes, skin and lungs. Sodium peroxide is also corrosive and it is a powerful oxidant. The first precaution step is the appearance of the period 3 oxides must be observed before the experiment start.
The mixture is stirred immediately and evenly after adding the period 3 oxides by using a glass rod to make sure that the period 3 oxide and distilled water is completely react. The reading of thermometer is taken when the reading is constant. Moreover, the eye must be placed parallel to the scale of the thermometer to avoid the parallax error when taking the readings. Besides, before taking the powder of period 3 oxides, the spatula used must be clean and dry to avoid the dirt or water drop into the container of period 3 oxides.
Conclusions: when going across period 3 from left to right, the properties of the oxides of elements change from basic to acidic; sodium peroxide, magnesium oxide, silicon (IV) oxide and phosphorus (V) oxide are in white powder form; the metallic oxides which are sodium peroxide and magnesium
oxide have giant ionic structure and held by strong ionic bond; silicon (IV) oxide has giant covalent structure and held by strong covalent bond; phosphorus (V) oxide has simple molecular structure which held by weak covalent bond. Procedure :
1. The physical state and the colour of the period 3 oxide samples at room temperature were examined and recorded in Table 1. 2. Four test tubes were set up side by side.
3. 5cm3 of distilled water was poured into each boiling tubes. 4. A thermometer was placed in the boiling tube to measure the initial temperature of distilled water. 5. Half a spatula-tip of sodium peroxide was added into the boiling tube and the mixture was stirred carefully with a glass rod. 6. After 30 seconds, the final temperature of the solution was measured and the solution was observed.
7. 3 drops of universal indicator solution was added into the boiling tube, the colour of the solution was observe and compared with the chart provided. The pH value of the solution was recorded. 8. Steps 4 to 7 were repeated by using magnesium oxide, silicon (IV) oxide and phosphorus (V) oxide to replace sodium peroxide respectively. 9. The pH of the distilled water in the fifth boiling tube was measured by adding 3 drops of universal indicator solution for comparison with the above.