Michael S. answered • 08/07/26
B.S. in Chemistry, Indiana University; Organic Chem Teaching Intern
The answer is A.
Two of the five options die before you do any chemistry at all. Molecular motion and kinetic energy are not two separate dials. Kinetic energy IS the measure of that motion, and temperature is the measure of its average value, so the two can never move in opposite directions. That eliminates B (motion down, KE up) and D (motion up, KE down) on inspection.
C goes next, on the direction of the energy flow. The substance is releasing energy, meaning it is handing energy to its surroundings, so its own internal energy goes down and not up. C describes a substance absorbing energy. Careful with the sign convention here: the surroundings get warmer, and it is easy to read the temperature rise off the beaker instead of off the sample.
That leaves A and E, and E is the one worth understanding.
Why E is a trap rather than simply a wrong answer. E is genuinely true in one situation: a phase change. When water freezes at 0 C it releases 6.01 kJ per mole, and the thermometer does not move for the entire time it is freezing. That energy comes out of potential energy, from intermolecular attractions forming, and not out of kinetic energy, so the average particle speed is unchanged. If this question came from a unit on phase changes, E is what the option is fishing for.
But the word in E is only, and that is what makes it false as written. A substance can release energy with no phase change anywhere in sight. A hot mug of coffee cooling on the counter releases energy the whole time and stays liquid throughout. The question does not restrict itself to a transition, so the generally true statement is A.
A heating or cooling curve holds both facts in one picture:
SEGMENT WHAT IS CHANGING TEMPERATURE MATCHES OPTION ------- ------------------ ----------- ------------- sloped kinetic energy changes A flat potential energy constant E
So the full reading of A: energy leaves the substance, the particles slow down, the average kinetic energy falls, and the temperature drops with it. Exothermic to the surroundings means colder for the sample.
The habit worth keeping. Whenever a set of choices offers you molecular motion and kinetic energy as if they were independent, that pairing is usually the whole test. Sort the options by that single rule first and you are typically down to two before any real work starts.
