Elements and Their Oxides An oxide is a. Oxides can form when an element reacts with oxygen, often in air. This reaction can be rapid with the release of a great deal of energy, as in the combustion of magnesium. Alternatively it can be slow, like the gradual tarnishing of a freshly cleaned piece of magnesium. These two reactions are actually the same: Review of Acids and Bases An acid is a substance that. These hydrogen ions cause the acidic properties of the solution. Hydrogen chloride is a gaseous molecular compound. When hydrogen chloride is bubbled into water, it forms hydrochloric acid, HCl (aq). This compound breaks down in water to produce hydrogen ions and chloride ions: A base is a substance that. The properties of basic solutions are due to the presence of hydroxide ions. Sodium hydroxide, NaOH (s), is a solid, ionic compound used in drain-opening products. Sodium hydroxide produces sodium ions and hydroxide ions when it dissolves in water:
The ph Scale The ph scale is a numerical scale ranging from to. It is used to compare the acidity of solutions. One solution is more acidic than another because the dissolved compound produces hydrogen ions. A solution with a ph of 7 is. Solutions with a ph above 7 are ; solutions with a ph below 7 are. Properties of Oxides Non-metals, such as suifur and nitrogen, may react to produce non-metallic oxides. Most non-metallic oxides such as sulfur dioxide or nitrogen monoxide are gases at room temperature and produce acidic solutions when dissolved in water. Oxides that form acidic solutions are called. Metals, such as magnesium and calcium, react to produce metallic oxides. Metallic oxides tend to be ionic solids that produce basic solutions when dissolved in water. Oxides producing basic solutions are called. Acidic Oxides A non-metallic oxide like carbon dioxide, CO 2, does not contain any hydrogen. Yet bubbling carbon dioxide into water can lower the ph of water from 7 to. Therefore, the reaction of carbon dioxide with water must result in a substance that produces hydrogen ions. CARBON DIOXIDE The acidification of water by carbon dioxide most likely occurs in a two-step process: 1. Carbon dioxide undergoes a synthesis reaction with water to form carbonic acid:
2. Carbonic acid immediately breaks down in water, producing a hydrogen ion: The reaction of carbon dioxide with water droplets in the atmosphere makes rain naturally mildly acidic. As a result, "normal rain" has a ph of about 5.6 rather than a neutral ph of 7. Oceanographers have evidence that unusually high levels of carbon dioxide in the atmosphere are affecting the chemistry of the oceans. In addition to contributing to global warming, carbon dioxide appears to be making the oceans more acidic. Increasing acidity hampers the growth of the shells of many marine organisms. At a certain ph, some animals may not be able to make shells at all. NITROGEN OXIDES Oxides of nitrogen contribute to the acidification of rain below its normal ph of 5.6. Some of the nitrogen oxides in the atmosphere form as a result of natural processes like. Most, however, are the by-product of the combustion of fuels. Normally, most of the nitrogen in the atmosphere is made up of the diatomic N 2 molecule. This molecule is stable and unreactive because of its triple bond. However, nitrogen makes up much of the air that is drawn into cars' engines. The high temperatures in these engines cause nitrogen to undergo a synthesis reaction with oxygen. The product is nitrogen monoxide (also called nitric oxide): Nitrogen monoxide is released into the atmosphere through the car's exhaust pipe. It mixes with the air and undergoes a further reaction with oxygen to form nitrogen dioxide: Nitrogen dioxide is a toxic respiratory irritant and is involved in the formation of smog. It also combines with water droplets in the atmosphere to form nitric acid, HNO 3(aq), and nitrous acid, HNO 2(aq) : Fortunately, modern vehicles are equipped with a device called a catalytic converter that prevents most of the nitrogen oxides and other pollutants from being discharged
in the exhaust. The interior of the converter is coated with beads of platinum, palladium, or rhodium catalyst. As nitrogen oxides come in contact with these beads, they decompose into harmless nitrogen and oxygen: A catalyst is ideal for this purpose because, unlike a reactant, it is not consumed (used up) during the reaction. A well-maintained catalytic converter can often last the life of the automobile. Despite the catalytic converter, however, some pollutants are still released into the atmosphere. SULFUR OXIDES Sulfur oxides are a major contributor to acid precipitation. Some sulfur oxides are produced naturally as a result of volcanic activity. However, the bulk of sulfur oxide in the atmosphere results from the combustion of fossil fuels. Sulfur is a common impurity in many fossil fuels like coal and crude oil. Sulfur dioxide is produced when these fuels are burned: Once released into the atmosphere, sulfur dioxide reacts with additional oxygen to produce sulfur trioxide: Finally, sulfur trioxide combines with water in the atmosphere to form droplets of aqueous sulfuric acid. This mixture falls to Earth as acid precipitation: As we will see in later, many industries have devised clever technological solutions to treat sulfur-containing emissions before they become an environmental problem. Some industries follow the principles of green chemistry and capture the "waste" sulfur for use in other products. Basic Oxides Most metallic elements form basic metallic oxides. Metallic oxides generally react with water to form metal hydroxides. A considerable quantity of thermal energy is often released as well. The word equation for this reaction is:
Many metals form oxides simply by being exposed to the air. A freshly cut piece of sodium, for example, tarnishes in a matter of seconds to form a coating of sodium oxide: Adding sodium oxide to water produces sodium hydroxide, a base: Sodium hydroxide is one of the most widely used industrial chemicals. Its applications include the production of detergents and paper. Calcium is another metal whose oxide, CaO (s), is an important industrial chemical. Calcium oxide is more commonly known as lime. Calcium oxide reacts with water to form calcium hydroxide, Ca(OH) 2 also known as "slaked lime:" Calcium oxide and calcium hydroxide are very useful substances. When a lake is affected by acid precipitation, either compound can be added to raise the water's ph. Farmers use the compounds to treat acid soil, and hazmat (hazardous materials) teams use them to neutralize acid spills. Calcium oxide and calcium hydroxide are also important ingredients in the production of glass, cement, and paper.
Worksheet 2.6: Oxides 1. What effect would the addition of a small quantity of an acid to a basic solution have on the ph of the solution? Why? 2. What type of solution will each of these compounds form when it is mixed with water? (a) barium oxide (b) potassium oxide (c) chlorine dioxide (d) arsenic trioxide 3. Copy and complete the following pairs of chemical equations. Note that, in each pair, the product of the first reaction is a reactant in the second reaction. (a) 4 K (s) + O 2(g) - + H 2 O (l) 2 KOH (aq) (b) + - CI 2 O (g) + 2 HCIO (aq) (c) + 2 P 2 O 5(s) + 2 H 3 PO 4(aq) 4. Adding potassium oxide to water causes the ph of the water to rise to 10. Write a chemical equation to explain this observation. 5. Calcium oxide is a key ingredient in cement. What safety precautions do you think should be observed while mixing cement? Why? 6. Nitrogen oxide emissions from a coal-burning electricity generating station can be neutralized by either bubbling them through a mixture of calcium oxide and water or passing them through a large catalytic converter. Which option do you think is more sustainable? Why? 7. Plants generally do not grow near volcanoes that have recently erupted. Use what you know about oxides to explain this observation. 8. (a) Describe the reaction that would occur if a small piece of calcium oxide were placed in a dish of water. (b) Write a chemical equation for the reaction in (a). (c) What additional observation would you expect to make if the water contained phenolphthalein indicator? 9. Find out more about the effects of ocean acidification. What effects might marine ecosystems experience as a result of this phenomenon?