Video Transcript
When a piece of sodium metal is
placed into a large trough of cold water, a chemical reaction takes place. Which of the following statements
best explains why this is the case? (A) Sodium is more active than
hydrogen. (B) Sodium is less active than
hydrogen. (C) Sodium is more active than
water. (D) Sodium is less active than
water.
In this question, we are told that
a chemical reaction occurs when sodium metal is placed into cold water. Sodium metal is a metal found in
group one on the periodic table. Group one metals are very active
metals. Sodium and potassium metal, which
are both group one metals, appear at the very top of the series of chemical
activity. Metals found at the top of the
series are the most active, and metals found toward the bottom of the series are the
least active.
Group one metals can replace
hydrogen in substitution reactions because they are more active than hydrogen. In a single-substitution reaction,
a more active metal element will replace a less active metal element in a chemical
compound. It is also possible for an active
metal to replace hydrogen in a compound as long as hydrogen is less active.
In this question, we know that the
reactants of the single-substitution reaction are sodium, the group one metal, and
water. The products of this type of
single-substitution reaction are a group one hydroxide and hydrogen gas. Let’s write a word equation to
represent the reaction between sodium and water. We can write sodium plus water
before the reaction arrow and sodium hydroxide plus hydrogen gas to represent the
products. Now, let’s write a chemical
equation for the reaction. The chemical formulas of the
reactants are Na followed by the state symbol “s” for sodium solid and H2O followed
by the state symbol “l” for liquid water. The product sodium hydroxide is a
solution. The chemical formula is NaOH
followed by “aq” to represent an aqueous solution. And the chemical formula for
hydrogen gas is H2 followed by the state symbol “g.”
Now that we have this basic
unbalanced equation, let’s take a closer look at what’s going on in this
reaction. We know from the series of chemical
activity that sodium is more active than hydrogen. During the reaction, the more
active sodium atoms replace the less active hydrogen atoms in the water
molecules. And the hydrogen atoms that got
replaced combine to form hydrogen gas. We’ve confirmed that a
single-substitution reaction takes place because sodium is a more active element
than hydrogen. The statement which best explains
why sodium metal reacts with water is answer choice (A). Sodium is more active than
hydrogen.
