In the chemistry lab, one often has to work with very reactive metals such as so

Question

In the chemistry lab, one often has to work with very reactive metals such as sodium and lithium. Unfortunately, these same metals can react vigorously with water or oxygen and cause fires. Given the following information:

Choose from the following substances that might be used in a fire-retarding agent for combating each of the following fires.

water,carbon dioxide,    sand (silicon dioxide), carbon tetrachloride,                for lithium burning in air
water, carbon dioxide,    sand (silicon dioxide)carbon tetrachloride                       sodium burning in air
water,carbon dioxide,      sand (silicon dioxide)carbon tetrachloride                      potassium burning in air

(kJ/mol) (kJ/mol) Li2O(s) -597.9 Li+(aq) -278.5 Na2O(s) -416 Na+(aq) -240 K2O(s) -361 K+(aq) -251 CO(g) -110.5 CO2(g) -393.5 H2O(l) -286 OH-(aq) -230 CCl4(l) -135 SiO2(s) -911 LiCl(s) -409 NaCl(s) -411 KCl(s) -436 Cl-(aq) -167

Explanation / Answer

fOR ANY REACTION the heat of raction is givenby

delta Hrxn = sum of deltaHf (products) - sum of delta H f of reactants.

1) for this reaction

delta Hrxn = 2 xLi2O - [4xLi + O2]

= 2 (-597.9) -0

=-1195.8 kJ

2) delta Hrxn = delta H of (Li2O +CO) - delta h of (Co2 + Li)

= (-597.9 -110.5 ) -(-393.5)

= -314.9 kJ

3) delta H rxn = [2xLiOH +H2] -[2xH2O +2Li]

= {2(-278.5 -230) +0} - 2(-286)

= -445 kJ

N

The alkali metal fires are not extinguished by water , as they react with water to give H2 which is combustible.

For Li fires CO2 is useful.

For sodium fires sand can be used.

For potasium ires sand or CCl4 can be used.

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