Video Transcript
Lithium carbonate decomposes at
1,000 degrees Celsius according to the equation shown. Li2Co3 solid decomposes to form
Li2O solid plus CO2 gas. A student attempts a similar
experiment using sodium carbonate but does not get a positive result when using
limewater to test for carbon dioxide. Which statement correctly explains
why this is the case? (A) Sodium carbonate does not
thermally decompose. (B) Sodium carbonate produces
carbon monoxide when it decomposes. (C) Sodium carbonate produces
carbon, in the form of soot, and oxygen gas when it decomposes. Or (D) sodium carbonate reacts with
water vapor in the air when heated, producing sodium oxide and carbonic acid.
In this question, we wanna find out
why the student did not get a positive test result for carbon dioxide.
Sodium is a very well-known element
in group one on the periodic table. And since we know that groups on
the periodic table tend to have similar properties, we can understand why the
student may have expected to see a similar result. Sodium is also incredibly common on
Earth and is the sixth most abundant element. Its positively charged ions are
found in everyday items, such as table salt.
Sodium carbonate, a compound of
sodium metal, is used in many manufacturing processes including for glass, paper,
and soap. Sodium carbonae is industrially
made using the Solvay process. Here, sodium chloride is reacted
with carbon dioxide, ammonia, and water to form sodium hydrogen carbonate and
ammonium chloride. Sodium hydrogen carbonate
precipitates out of solution and is heated until it decomposes into sodium
carbonate, which is a thermally stable compound.
When comparing the relative
stability of sodium and lithium compounds, it’s important to recall reactivity
trends of the periodic table. Elemental reactivity increases as
you move from the top to bottom down the periodic table, meaning elemental sodium is
more reactive than lithium. Furthermore, the stability of the
compounds formed from the reaction of these metals is inversely proportional to the
initial reactivity. And so from these trends, we know
that sodium compounds are relatively more stable than lithium compounds with similar
atomic makeup.
And since we know that sodium
carbonate is a thermally stable compound that does not decompose at high
temperatures, we can revisit and answer the question. Which statement correctly explains
why the student does not get a positive result when using limewater to test for
carbon dioxide with sodium carbonate? And because we know that sodium
carbonate is a thermally stable compound, we can correctly choose answer choice
(A). Sodium carbonate does not thermally
decompose.
