Question

Which statements are true? Δ???? for an exothermic reaction is positive. Δ???? for an endothermic reaction is positive. The evaporation of water is an exothermic process. A combustion reaction is exothermic. When energy is transferred as heat from the surroundings to the system, Δ???? is negative. When energy is transferred as heat from the system to the surroundings, Δ???? is negative.

Answer 1

Answer: The true statements are $\Delta H$ for an endothermic reaction is positive, a combustion reaction is exothermic and when energy is transferred as heat from the system to the surroundings, $\Delta H$ is negative.

Explanation:

There are 2 types of chemical reactions categorized into heat change:

1. Endothermic reactions: These reactions are defined as the reactions in which energy is absorbed by the system from the surroundings. The $\Delta H$ for these reactions is always positive. For Example: Changing of water into water vapor.
2. Exothermic reactions: These reactions are defined as the reactions in which energy is released by the system to the surroundings. The $\Delta H$ for these reactions is always negative. For Example: Combustion reactions.

Combustion reactions are defined as the reactions in which a hydrocarbon reacts with oxygen gas to produce carbon dioxide and water. Heat is released during these reactions. Thus, it is considered as exothermic reactions.

Hence, the true statements are $\Delta H$ for an endothermic reaction is positive, a combustion reaction is exothermic and when energy is transferred as heat from the system to the surroundings, $\Delta H$ is negative.