Question

Why does a reaction vessel feel warm during an exothermic reaction and feel cold during an endothermic reaction?

Short answer

Exothermic reactions release heat, making the vessel warm, while endothermic reactions absorb heat, making it cool.

Step-by-step solution

Understanding Exothermic Reactions

In an exothermic reaction, energy is released into the surroundings as heat. This happens because the total energy of the reactants is higher than that of the products, and the excess energy is liberated as heat.

Recognizing the Warmth from Exothermic Reactions

Since energy is released as heat, the temperature of the surroundings, including the reaction vessel, increases. This is why the vessel feels warm during an exothermic reaction.

Understanding Endothermic Reactions

In an endothermic reaction, energy is absorbed from the surroundings. The energy required for reactions to occur is greater than the energy present in the products, so the system absorbs heat from the surroundings.

Recognizing the Coolness from Endothermic Reactions

Because heat is absorbed, the temperature of the surroundings, including the reaction vessel, decreases. This is why the vessel feels cool during an endothermic reaction.

Key concepts

Exothermic Reaction

Exothermic reactions are fascinating because they release energy in the form of heat. This energy release occurs when the bonds formed in the products are stronger than the bonds broken in the reactants.
The released energy is often visible or tangible, as it increases the temperature of the surroundings, including the reaction vessel itself. Just think of a reaction like combustion: when wood burns, it releases heat. That heat is what you feel when you're near a campfire, providing warmth.

• The energy of the reactants is greater than that of the products.
• Excess energy is dispersed into the environment as heat.
• This process makes the reaction vessel feel warm.

The reason you feel heat is due to the energy flow from the system (reaction) to the surroundings. As this heat disperses, our skin detects the temperature increase, explaining the warmth you encounter.

Endothermic Reaction

On the other hand, endothermic reactions work in the opposite manner. They absorb energy from the surroundings, pulling heat into the system.
In these reactions, the energy needed to break the bonds of reactants is greater than the energy released when new bonds form in the products. Think of mixing baking soda and vinegar. This reaction absorbs heat, cooling the container.

• Endothermic reactions draw energy into the system.
• The energy goes from the surroundings into the reaction.
• The reaction vessel feels cold as heat is absorbed.

The energy flow is essential for the reaction to proceed, and as the surroundings lose heat, you perceive the decrease in temperature. Hence, the sensation of coldness when touching the reaction vessel becomes evident.

Energy Transfer

Energy transfer is the key concept that underlines both exothermic and endothermic reactions. It describes how energy moves between a system and its surroundings.
For exothermic reactions, energy moves out of the system, warming the surroundings. This creates a noticeable temperature change as heat is added to the environment. In contrast, endothermic reactions involve energy flowing into the system, extracting heat from the environment.

• Energy transfer is movement of energy in or out of a system.
• In exothermic, energy is transferred to the surroundings.
• In endothermic, energy is taken from surroundings into the system.

Understanding energy transfer helps explain why these temperature changes happen in the surroundings of chemical reactions. It's all about how energy decides to flow, which you can feel as warmth or coolness in the reaction vessel.