Question

Classify the following as an endothermic or exothermic reaction: (a) making popcorn in a microwave oven (b) a burning match (c) boiling water (d) burning rocket fuel (e) the reaction inside a heat pack

Short answer

(a) Endothermic, (b) Exothermic, (c) Endothermic, (d) Exothermic, (e) Exothermic.

Step-by-step solution

Definition of Endothermic and Exothermic Reactions

Understand the definitions of endothermic and exothermic reactions. An endothermic reaction requires the absorption of heat, resulting in a drop of temperature in the surroundings. Conversely, an exothermic reaction releases heat, causing the surroundings' temperature to increase.

Classifying Making Popcorn

Decide if making popcorn in a microwave is an endothermic or exothermic process. Microwave ovens heat the popcorn kernels, causing them to absorb energy and pop. This is an example of an endothermic reaction because the kernels absorb energy (heat) to cause the reaction.

Classifying a Burning Match

Determine if a burning match is an endothermic or exothermic reaction. A burning match releases heat and light, which indicates that it is an exothermic reaction.

Classifying Boiling Water

Ascertain if boiling water is an endothermic or exothermic process. Water absorbs heat from a heat source to transition from a liquid to a gaseous state, making this an endothermic process.

Classifying Burning Rocket Fuel

Evaluate whether burning rocket fuel is an endothermic or exothermic reaction. Burning rocket fuel releases a large amount of heat and energy, propelling the rocket forward. This is an exothermic reaction.

Classifying the Reaction Inside a Heat Pack

Determine if the reaction inside a heat pack is an endothermic or exothermic process. Heat packs generate heat as part of their reaction, which makes the process exothermic because heat is being released to the surroundings.

Key concepts

Thermochemistry

Thermochemistry is a branch of chemistry that examines the heat energy associated with chemical reactions and phase changes. When it comes to understanding whether a particular process, like making popcorn or burning rocket fuel, is endothermic or exothermic, it's essential to think about the energy flow.

For any substance to change its state or to undergo a chemical transformation, energy must be either absorbed from or released into the surroundings. In the kitchen, for instance, when you are making popcorn in a microwave, the kernels absorb microwave energy to pop, reflecting the core principle of thermochemistry: energy can neither be created nor destroyed, only transformed.

Energy in Chemical Reactions

The concept of energy in chemical reactions is central to understanding why certain processes release heat while others absorb it. Take a burning match; the matchstick combines with oxygen in the air in a combustion reaction, releasing energy in the form of heat and light.

Every chemical bond has a specific amount of energy associated with it, and when those bonds are broken or formed during a reaction, energy is exchanged in the form of heat. In thermochemical terms, this energy is what we observe as a temperature change in the surrounding environment. During a chemical reaction such as the one inside a heat pack, the formation of new bonds releases energy, warming up your hands with exothermic delight.

Enthalpy Changes

Enthalpy, symbolized as \( H \), is the measure of total heat content in a system at constant pressure. Enthalpy changes (\( \Delta H \) are critical for indicating whether a reaction is endothermic or exothermic. A positive \( \Delta H \) value signifies that heat is absorbed from the surroundings—this is characteristic of an endothermic process, such as boiling water, where energy is needed to break the intermolecular forces holding water molecules together.

In contrast, a negative \( \Delta H \) represents an exothermic reaction, where energy is released into the surroundings. For example, burning rocket fuel involves an immense release of heat, which not only propels the rocket but also results in a large negative \( \Delta H \), reinforcing the concept that exothermic reactions often involve a 'hot' exchange!