A sample of substance X (molar mass = ) undergoes two separate experiments at

CHEMISTRY · Single correct · AP · Chemistry · Chemical Reactions · Chemical Reactions

Problem

1.00\,\text{g}

sample of substance X (molar mass =

18.0\,\text{g/mol}

) undergoes two separate experiments at

1\,\text{atm}

. • Experiment A: The sample melts at its melting point while absorbing

0.33\,\text{kJ}

of heat. • Experiment B: The same mass of X reacts completely with excess

O_2

, releasing

13\,\text{kJ}

of heat. Which statement most accurately classifies each experiment? (a) Both experiments are chemical changes because energy is transferred in each case. (b) Experiment A is a chemical change and Experiment B is a physical change because only A requires heat input. (c) Experiment A is a physical change and Experiment B is a chemical change because the per-mole energy for B is two orders of magnitude larger than for A. (d) Both experiments are physical changes because substance X remains present after each process.

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