kinetic energy dependence of molecule on temperature and how these play a role in reactions.
Reactions of the Alkali Metals With Water: A Novel Demonstration Submitted by
M. Dale Alexander New Mexico State University, Las Cruces, NM 88003 Checked by
Warren Vlllaescusa Our Lady of the Lake University San Antonio, TX 78285
Figure 3. AH is positive. AS is wsitive . AG ~ d s w at e low T,negative at hlgn T; Reaclon 1s nonspontaneous at low temperatares b ~ becomes t sponraneoas as temperatare 1s rased
CaCO, (s)+ CaO(s)+ C02(g)
The reactions of the alkali metid s with water provide for some of the most popular and spectacular class&om chemistry demonstrations. Described herein is a novel variation that is much safer to perform than the standard demonstration of simply dropping a small piece of the alkali metal into a beaker of water. Moreover. the demonstration does a better job of illustrating certain aspects about the reaction than do the standard demonstrations. Materials sodium metal ' l~th~um metal (optional) pam thrnner (hydrocarbon mutturel phenolphthalein
' knife
elms cvlinder (500 mL) gas eohcting bottle (250 mL) ring stand clamp wocden splints Procedure The cylinder is clamped to the ring stand for support. Approximately 200 mL of water is added to the cvlinder aidng with a Iew drops of phenolphthalein solution. Approximately 200 mL of paint thinner is added forming a layer above the aqueous solution. A piece of sodium about the size of a grape is cut off a larger piece of sodium with a knife illustrating the softness of the metal. The piece of sodium is dropped into the cylinder. The sodium sinks in the paint thinner until it reaches the interface between the two layers at which time it reacts with water according - to the following equation. ZNa(s) + 2Hz0(1)+Hz (gl + ZNaOHCaq) ~~~~
Figure 4. AH is negative AS is negative AG Negative at low T; positive at high T Reaction is spontaneous at low temperatures: the reverse reaction becomes spontaneous at high temperatures CaO(s)+ CO,(g) + CaC03(s) become negative, thus making the process spontaneous. See Figure 3. Now tip this container so that all the "particles" are in the jar and none in the vial. Again hold it upright. Slowly shaking the container by just twisting your wrist (low temperature) allows the particles to return to the vial. This occurs because the entropy contribution to the overall driving force, (-TG) with QS being negative and T being small, is not able to ovemme the negative m v a l u e . AG is, therefore, negative a t low temperatures, and the process is spontaneous. See Figure 4. At any particular temperature (intensity of shaking), you should get the same ratio of urnducts to reactants a t equilibriumkhether you start with the material in the vial or out of the vial. (In practice it may take quite a while to reach the equilibrium condition.) Summary
We have found this demonstration useful in presenting a uhvsical model for the four oossible combinations of entmpy and enthalpy to predict the spontaneity of a reaction. It also utilizes the concepts of activation energy and the 418
Journal of Chemical Education
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The evolution of hydrogen gas is evident, and the hydrogen evolved carries the sodium into the hydrocarbon layer temporarily stopping the reaction. It is this behavior that makes the demonstration mite safe to uerform in suite of the large size of the piece of sodium. he piece of sodium repeatedly drops back down to the interface until the reaction is complete. During the reaction, the piece of sodium is largely devoid of corrosion allowng the students to view its gray metallic appearance. The aqueous layer turns red due to the production of NaOH, and the fact that no precipitate forms shows that NaOH is soluble in water. Variations The low density of the alkali metals can be demonstrated with the following experiment. The behavior of sodium with respect to its reaction with water is consistent with its very negative standard reduction potential (-2.71 V). The class is asked to predict the behavior of lithium under the same conditions (lithium's standard reduction poten-
tial is even more negative -3.05 V). A piece of lithium is dropped into the cylinder to test the prediction, and the students are surprised to see that the iithium floats upon the hydrocarbon layer as a result of the low density of the metal (0.54 e1mL). - . The hydrogen gas produced in the reaction of sodium with water can be collected, and its properties can be demonstrated. In order to do this, a cylinder without a spout is used. After the reaction with sodium 1s imtlated, the cylinder is fitted with a one-holed ruhber stoooer eauiooed with a short piece of glass tubing. With thii setup, hydrogen produced in the reaction can be collected by displacement of air in a n inverted wide mouth gas collecting bottle placed over the glass tubing. If the hydrogen is collected in this manner, the bottle will contain an explosive mixture of hydrogen and air in just a few seconds, and the mixture in the bottle can be ig&ted to demonstrate hydrogen's combustibility. To carry out the ignition, place the inverted bottle containing hydrogen and air on-the bench top a safe A
distanee from the apparatus (at least three feet away). Ignite a wooden splint, raise the inverted bottle approximately one foot above the bench top, and bring thg flame just below the mouth of the bottle holding the splint at a right angle to the bottle. Even though the-explosion is not violent, the flash can cause minor burns to the hand if the d i n t is not held ~ r o ~ e r lIfv the . bottle is allowed to collect hydrogen gas for a longer period of time such that the mixture is more than 909 hydrogen, the mixture will not be explosive but will exhibit smooth burning instead. If pure hydrogen is desired, the glass tubing can be equipped with rubber tubing that permits the hydrogen gas to be collected over water. Note: Paint thinner was chosen as the hydrocarbon mixture because of its relatively low volatility and relatively high flash point. Other hydrocarbon mixtures can be employed, bucif one more volatile than the paint thinner is used, there is a greater risk of fire should the auparatus he knocked over by accident.
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Volume 69 Number 5 May 1992
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