Video Transcript
Sodium atoms can be reacted with water molecules to produce sodium hydroxide and
hydrogen. Which of the following figures correctly shows how sodium atoms replace hydrogen
atoms during this substitution reaction? (A) Two Na solid plus two H2O liquid react to form two NaOH aqueous plus H2 gas. (B) Two Na solid plus two H2O liquid react to form two NaOH2 aqueous. (C) Two Na solid plus two H2O liquid react to form Na(OH)2 aqueous plus NaH2
solid. (D) Two Na solid plus two H2O liquid react to form NaOH aqueous plus NaOH3
aqueous. (E) Two Na solid plus two H2O liquid react to form two NaO solid plus two H2 gas.
This question is asking us to identify the correct substitution of atoms that occurs
during a reaction between sodium metal and water. We are told that sodium reacts with water. So these two substances are the reactants and should appear on the left side of the
reaction arrow. We are also told that the products will be sodium hydroxide and hydrogen. These two substances should appear on the right side of the reaction arrow. Now, let’s use this information to write a word equation.
We can write sodium plus water react to form sodium hydroxide plus hydrogen. Next, underneath our word equation, let’s write a chemical equation using the correct
symbols for the different elements and compounds in the reaction. Sodium has the symbol Na and water has the chemical formula H2O. Sodium hydroxide has the chemical formula NaOH, and hydrogen gas, which is diatomic,
has the formula H2. The fully balanced chemical equation includes a coefficient of two in front of Na,
H2O, and NaOH.
Now that we know that the chemical formula of the product sodium hydroxide is NaOH,
we can eliminate answer choices (B), (C), and (E), because NaOH is not a product in
these equations. Let’s remove these equations from the screen. We also know that diatomic hydrogen is a product. Therefore, we can also eliminate answer choice (D) because H2 is not a product in
this equation. This leaves only answer choice (A), which must be the correct answer. But, let’s confirm this by reviewing what we know about substitution reactions.
In a substitution reaction, a more active element replaces a less active element in a
compound. Sodium is more active than hydrogen, so it can replace hydrogen atoms in a
compound. We can see that this is exactly what is represented by the diagram in answer choice
(A). The sodium atoms each replace a hydrogen atom in the two water molecules. This produces two units of sodium hydroxide. The hydrogen atoms that were replaced by sodium join together to form hydrogen
gas.
In conclusion, the figure that correctly shows how sodium atoms replace hydrogen
atoms during a substitution reaction is answer choice (A). Two Na solid plus two H2O liquid react to form two NaOH aqueous plus H2 gas.
