Magnesium reaction with steam

Accidental contact of the molten alloy (26% Al) with a wet rusty iron surface caused violent explosions with brilliant light emission. Initial evolution of steam, causing fine dispersion of the alloy, then interaction of the fine metals with rust in a thermite reaction, were postulated as likely stages. Direct interaction of the magnesium (74%) with steam may also have been involved. 

The moderately reactive metals, magnesium, zinc and iron, react slowly with water. They will, however, react more rapidly with steam (Figure 10.3). In their reaction with steam, the metal oxide and hydrogen are formed. For example, magnesium produces magnesium oxide and hydrogen gas. 

Knowledge of the relative chemical reactivities of the elements helps us predict the course of many chemical reactions. For example, calcium reacts with cold water to produce hydrogen, and magnesium reacts with steam to produce hydrogen. Therefore, calcium is considered a more reactive metal than magnesium ... 

Describe how magnesium reacts with steam. Write a balanced equation for the reaction. 

Figure 11.6 An experiment showing magnesium reacting with steam. The steam is given off from mineral wool soaked in water at the right-hand end of the test tube. The white magnesium oxide formed is visible inside the test tube and the hydrogen gas produced in the reaction has been ignited at the end of the straight tube.

Magnesium-rich brine is also used for the production of magnesium oxide. The process involves the decomposition of magnesium chloride (MgCl2) present in brine in the temperature range of 600°C-800°C. The decomposition takes place by reaction with steam, and the reaction is represented in Equation 12.6. The Mg(OH)2 produced after the decomposition reaction is calcined as earlier to get MgO. 

Evidence for the solvated electron e (aq) can be obtained reaction of sodium vapour with ice in the complete absence of air at 273 K gives a blue colour (cf. the reaction of sodium with liquid ammonia, p. 126). Magnesium, zinc and iron react with steam at elevated temperatures to yield hydrogen, and a few metals, in the presence of air, form a surface layer of oxide or hydroxide, for example iron, lead and aluminium. These reactions are more fully considered under the respective metals. Water is not easily oxidised but fluorine and chlorine are both capable of liberating oxygen ... 

In a 5-I. flask, placed on a steam bath and fitted with a mechanical stirrer, a separatory funnel, a thermometer well (Note 1) and a calcium chloride tube, is placed 182 g. (7.5 moles) of magnesium turnings. To this is added a crystal of iodine and 100 cc. of a solution of 1133 g. (7.5 moles) of 2-bromo-pentane (Note 2) in 750 g. of -butyl ether (Note 3). The stirrer is started and the flask is heated with steam until the reaction starts. This may take from fifteen minutes to one hour, but the flask must be watched quite closely because the reaction, when once started, is very vigorous and evolves a large amount of heat. As soon as the reaction has started, 750 g. of w-butyl ether is added and then the balance of the solution of 2-bromo-pentane in w-butyl ether is added at such a rate that the temperature is kept at 50-60°. External cooling is used in order to allow more rapid addition of the 2-bromopentane. After addition is complete (about three hours), the mixture is heated on a steam bath for one hour. 

The molten magnesium liberated at the cathode floats to the surface and is dipped out periodically, while the chlorine released at the anodes is collected and reacted with steam at high temperatures to produce hydrochloric acid. This is recycled for further reaction with magnesium hydroxide. 

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