Video Transcript
Which of these statements does not
describe the conservation of energy in a chemical reaction? (A) Energy is neither created nor
destroyed during a chemical reaction. (B) If the energy of a system
increases, then the energy of the surroundings decreases by the exact same
amount. (C) Energy can only be transferred
from one form to another. (D) If the energy of a system
decreases, then the energy of the surroundings increases by the exact same
amount. Or (E) the energy contained in the
bonds of reactant molecules always equals the energy contained in the bonds of
product molecules.
The question’s asking us to
identify the one statement out of the five that does not describe the conservation
of energy in a chemical reaction. The law of conservation of energy
can be stated in the form that energy cannot be created or destroyed. But energy can be converted between
its various forms, like thermal energy, sound, light, and chemical potential
energy.
The first statement says that
energy is neither created nor destroyed during a chemical reaction. This is perfectly true. And in fact, it doesn’t even need
to apply to chemical reactions. It’s a universal law. So since this does describe the
conservation of energy in a chemical reaction, it’s not a correct answer.
The second statement says that if
the energy of a system increases, then the energy of the surroundings decreases by
the exact same amount. What this statement is suggesting
is if the system energy goes up, then it must be drawing energy from the
surroundings. So the energy of the surroundings
must go down. This is an application of the law
of conservation of energy. The total energy must be
constant. So this statement is true. The only things in the entire
universe are the system and the surroundings. And the total energy of the
universe must be constant. We can substitute the term energy
and enthalpy when it pertains to the system, and it’s still true.
The third statement says that
energy can only be transferred from one form to another, for instance, the
conversion of electrical energy into heat and light in a tungsten bulb. This squares with our conservation
law perfectly. Energy can only be transferred or
converted from one form to another.
Statement (D) says that if the
energy of a system decreases, then the energy of the surroundings increases by the
exact same amount. This is just statement (B) the
other way around. And we can see that it’s perfectly
true, which leaves us with (E) the energy contained in the bonds of reactant
molecules always equals the energy contained in the bonds of product molecules. We know this is false because we
often see chemical bond energy changing throughout a reaction. In our example, when we form
calcium oxide and carbon dioxide from calcium carbonate, the products have weaker
bonds than the reactants.
