Endothermic Reactions

The test tube feels cold, which means the temperature of the surroundings has decreased. We need to connect this temperature change to the type of reaction occurring.

An endothermic reaction absorbs energy from the surroundings, causing the temperature of the surroundings to decrease. If the test tube feels cold, energy is flowing from the surroundings (including your hand) into the reaction.

Since the surroundings are getting colder, energy is being absorbed by the reaction. This matches the definition of an endothermic reaction — energy enters the reaction from the surroundings.

We need to determine the type of reaction based on the observation that the test tube feels cold. This means the surroundings have lost energy to the reaction.

The choices “The reaction is exothermic because it is releasing energy to the surroundings” and “The reaction is exothermic because it is absorbing energy from the surroundings” both describe exothermic reactions. Exothermic reactions release energy to the surroundings, which would make the test tube feel hot, not cold. Both can be eliminated.

The choice “The reaction is endothermic because it is releasing energy to the surroundings” states the reaction is endothermic but says it is releasing energy to the surroundings — this is a contradiction. Endothermic reactions absorb energy, not release it. The choice “The reaction is endothermic because it is releasing energy to the surroundings” is eliminated.

The choice “The reaction is endothermic because it is absorbing energy from the surroundings” correctly states the reaction is endothermic and that it is absorbing energy from the surroundings, which explains why the test tube feels cold.

Energy required to break bonds = 850 kJ (energy input). Energy released when forming bonds = 620 kJ (energy output). We need to find the net energy change.

$\Delta H=\text{Energy to break bonds}-\text{Energy released forming bonds}$ $\Delta H=850-620=+230\text{ kJ}$

Since more energy is needed to break bonds (850 kJ) than is released when forming bonds (620 kJ), the reaction absorbs net energy from the surroundings. This makes it endothermic, confirmed by the positive ΔH value.

We need to calculate ΔH using bond energies and classify the reaction. Breaking bonds requires energy (850 kJ in); forming bonds releases energy (620 kJ out).

Since more energy is absorbed (850 kJ) than released (620 kJ), there is a net absorption of energy. This means ΔH must be positive (+230 kJ). This eliminates the choices “ΔH = −230 kJ; exothermic reaction” and “ΔH = −230 kJ; endothermic reaction”, which show negative ΔH values.

A positive ΔH means energy is absorbed from the surroundings — this is the definition of an endothermic reaction. The choice “ΔH = +230 kJ; exothermic reaction” incorrectly labels a positive ΔH as exothermic, so it is eliminated.

The choice “ΔH = +230 kJ; endothermic reaction” correctly states ΔH = +230 kJ and classifies the reaction as endothermic, matching both the calculation and the definition.