Question

The "Chemistry in Focus" segment Coffee: Hot and Quick(lime) discusses self- heating cups of coffee using the chemical reaction between quicklime, \(\mathrm{CaO}(s),\) and water. Is this reaction endothermic or exothermic?

Short answer

The chemical reaction between quicklime (calcium oxide, \(\mathrm{CaO}(s)\)) and water is an exothermic reaction, as it generates heat which is used to warm up the coffee in the self-heating cup.

Step-by-step solution

Understand Endothermic and Exothermic Reactions

Endothermic reactions are chemical reactions that absorb heat from the surroundings, resulting in a decrease in temperature. On the other hand, exothermic reactions release heat to the surroundings, causing an increase in temperature.

Analyze the given information

The exercise mentions a self-heating cup of coffee using the chemical reaction between quicklime, \(\mathrm{CaO}(s),\) and water. Since the cup is self-heating, this means that the chemical reaction occurring in the cup is generating heat, which is then transferred to the coffee to warm it up.

Determine the type of reaction

As mentioned in step 2, the reaction generates heat, which is a characteristic of exothermic reactions. Therefore, the chemical reaction between quicklime (calcium oxide, \(\mathrm{CaO}(s)\)) and water is an exothermic reaction.

Key concepts

Exothermic Reactions

Chemical reactions can release heat, making the surrounding area warmer. These are called exothermic reactions. When an exothermic reaction occurs, energy is released usually in the form of heat. The temperature around the reaction increases because heat flows from the reaction to the surroundings.

Examples of exothermic reactions include:

• The burning of wood in a fireplace, which releases heat and keeps you warm on a cold night.
• The rusting of iron, which slowly releases heat over time.

When you think about self-heating products, like the quicklime reaction used for heating coffee, it's the exothermic process at work. The quicklime (Calcium Oxide) and water reaction releases heat, offering practical and immediate warmth. Understanding the nature of these reactions helps us recognize why they are important for everyday uses.

Endothermic Reactions

Some reactions absorb heat rather than release it. These are termed endothermic reactions. In an endothermic reaction, heat from the surrounding environment is consumed, leading to a drop in temperature around the reaction site. This is often experienced when the reaction feels cold to the touch.

Consider these typical endothermic reactions:

• The process of photosynthesis in plants, where sunlight is absorbed to convert carbon dioxide and water into glucose and oxygen.
• The melting of ice cubes, which requires heat absorption to change from solid to liquid.

These reactions are crucial in nature and technology. While they might not be as immediately felt as their exothermic counterparts, understanding their function helps in fields like energy storage and culinary arts, where controlling temperature through chemical reactions is essential.

Calcium Oxide (Quicklime)

Calcium oxide, commonly known as quicklime, is a widely used chemical compound. It appears as a white, caustic, crystalline substance. Quicklime is produced by the thermal decomposition of calcium carbonate (limestone), a process known as calcination. This transformation is remarkable for its simplicity and effectiveness in a range of environments.

Calcium oxide's practical applications are numerous:

• In building materials, quicklime is used in the creation of lime mortar and plaster.
• It serves as a reagent in various industrial processes, including sewage treatment and cement production.

When quicklime reacts with water, it forms calcium hydroxide and releases a substantial amount of heat in an exothermic reaction. This unique property is leveraged for heating purposes, such as self-heating cans and emergency warming packs, demonstrating the utility and importance of understanding its chemical behavior.