Video Transcript
Which of these products is usually
formed during the thermal decomposition of a metal hydroxide such as calcium
hydroxide? (A) O2, (B) H2, (C) O3, (D) H2O,
(E) H2O2.
In chemistry, a decomposition
reaction is typically described as the breaking down of a compound into simpler
compounds or even elements. It’s often necessary to heat
substances to cause decomposition to occur. We refer to this type of reaction
as thermal decomposition. So, we can define a thermal
decomposition reaction as the breaking down of a substance when heated to form two
or more other substances.
This question is asking us about
the thermal decomposition of a metal hydroxide such as calcium hydroxide. Metal hydroxides are composed of a
metal cation and one or more hydroxide anions. In the compound calcium hydroxide,
the calcium ion has an ionic charge of two plus. A hydroxide ion contains one oxygen
atom and one hydrogen atom and has an ionic charge of one minus. Therefore, the calcium ion will
combine with two hydroxide ions to form the neutral compound calcium hydroxide. The chemical formula of calcium
hydroxide is Ca(OH)2 with the OH− ion written inside parentheses.
In general, the thermal
decomposition of a metal hydroxide produces a metal oxide and water in the form of
steam. Let’s begin writing a chemical
equation for the thermal decomposition of calcium hydroxide. The reactant on the left side of
the reaction arrow is calcium hydroxide solid, or Ca(OH)2, with the state symbol
s. The metal oxide produced is solid
calcium oxide, which has the chemical formula CaO and the state symbol s. The oxide anion has an ionic charge
of two minus. One oxide ion balances the charge
of one calcium ion. This is why the chemical formula of
the compound calcium oxide is CaO. The second product of the reaction
is steam. We know that the chemical formula
of water is H2O, but we need to use the state symbol for a gas here.
When looking at the equation we
wrote, we can see that the number of calcium ions on both sides of the equation is
equal. The number of hydrogen atoms is
also equal. And, we can see that we also have
the same number of oxygen atoms on both sides of the equation. Therefore, the chemical equation
that we wrote is balanced. We’ve learned that one of the
products formed during the thermal decomposition of a metal hydroxide is water in
the form of steam.
Therefore, the correct answer is
answer choice (D) H2O.
