Basic Chemistry PBL Problem: Explosions and Brominator

Written by: Elsa Krisanti, Ph.D. at Chem. Engineering Dept. Universitas Indonesia

The first column of the periodic table (exclude hydrogen) contains elements that will show you spectacular view when you throw these elements into a lake. Of course if you take into account the amount and the speed of these elements you throw into a lake.
In one episode of Brainiac program, they claimed, it show what happens when approximately two grams of each of the five alkali metals (lithium, sodium, potassium, rubidium, and cesium) are thrown into a tub of water. The explosion of lithium, sodium and potassium were probably real, but the explosions of rubidium and cesium turned out to have been completely a bogus. The last two explosions were pretending came from two grams of rubidium and cesium, but actually it came from a explosive materials that blew in a bathtub. It is only for show, they didn’t actually explode like that.
1. Do you have any ideas why there are explosions when alkali metals are thrown into water? Why it is accused as being a fake when rubidium and cesium produce explosions when they are thrown into water?
2. Predict the products you would expect from a reaction between five alkali metals with water. Write balanced equations for each of the reactions considered in this question. If two grams of each of the five alkali metals were used, how much amount of the products yields?
3. How elements are grouped in the periodic tables? For example is for alkali metals group.
4. Does the reactivity of alkali metals correlate with the size and spectacle of the resulting explosion?
Part B:
In the second column from the right of the periodic table, you will find group of halogen family of elements. These elements are reactive and usually found bonding with metals or elements in the first column from the left of the periodic table or the alkali metals. Halogen elements are also reactive with some elements or metals from rare alkali metal group in the periodic table. When halogen elements combined with other elements or metals, the products are called halide.
5. If you have one kilogram of bromine (Br2) in the laboratory that should be sent to other place. Do you have any idea why bromine should be turned into a bromide salt by reacted with magnesium, calcium or strontium before you send it?
6. What factors can impact the reactivity of halogen?
7. Why in form of halide, halogen is more stable than as element?
8. How do you compare the the chemical property of each element in halogen family?
9. If you have magnesium, calcium and strontium what products obtained as the results from the reaction with Br2? Write balanced equations for each of the reactions considered in this question.

After some works in the laboratory, you found out that reaction of each metal with Br2 is not the same. In each trial a known amount of Mg, Ca, and Sr is added to a flask containing a fixed mass of bromine. After the reaction was finished, the metal bromide produced from each flask was isolated and weighed. The plotted of the mass of product against the mass of metal used is shown in following figure.

10. How do you explain the behavior recorded in the figure, that comes from reaction of fixed mass of bromine with mass of metals?
11. Why the mass of metal bromide products are constant at different mass of metals?
12. In the labs you have 100 g of magnesium, 300 g of calcium, and 500 g of strontium. How much amount of bromine required to form each metal bromide? If you have to use up one kilogram of bromine to make only one type of metal bromide, which of these metals will be the best choice? why


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