Video Transcript
When excess hydrochloric acid
reacts with calcium carbonate, carbon dioxide is produced. If five grams of calcium carbonate
is consumed in three minutes and 20 seconds, what is the average rate of
reaction?
Hydrochloric acid, HCl, is
obviously an acid. And we can see that calcium
carbonate, CaCO3, is a carbonate. What might spring to mind is the
classic reaction between an acid and a carbonate producing a salt, carbon dioxide,
and water. And indeed, the question tells us
that carbon dioxide is one of the products. It’s not necessary to answer the
question, but I’m going to draw out the chemical equation. Here, we have HCl reacting with
solid calcium carbonate. I’ve assumed it’s solid because
calcium carbonate isn’t soluble in water. Our products are the soluble salt,
calcium chloride; the gas, CO2; and H2O in its liquid form. And we’ll need two HCl in order to
balance it.
The question also tells us that
we’re consuming five grams of calcium carbonate in three minutes and 20 seconds. Our job is to use this information
to calculate the average rate of reaction. When we’re dealing with masses, the
average rate of our reaction is equal to the change in mass divided by the time
taken. But we’ve been given the time taken
in a mixture of units, minutes and seconds. So we should convert this all to
minutes or all to seconds. I’m going to do it all to
seconds.
There are 60 seconds in one minute,
so our time taken is equal to three minutes multiplied by 60 seconds per minute add
the 20 seconds which is equal to 180 seconds plus 20 seconds, which is 200
seconds. So our average rate with respect to
the calcium carbonate is five grams divided by 200 seconds, which works out as 0.025
grams of calcium carbonate per second.
