Endothermic & Exothermic Reactions: Videos & Practice Problems

Endothermic reactions are characterized by the absorption of thermal energy, resulting in molecules gaining enough energy to break bonds. This can be observed in phase changes such as melting (fusion), vaporization, and sublimation, where heat causes molecules to spread apart from solid to liquid to gas, respectively. These reactions feel cold to the touch due to the heat absorption from the surroundings. In contrast, exothermic reactions release thermal energy, causing molecules to slow down and form bonds, observable in condensation and freezing, or deposition in the case of substances like dry ice. Exothermic reactions feel warm as they release heat to the surroundings. On an energy diagram, endothermic reactions show a positive change in enthalpy (ΔH), with products at a higher energy level than reactants, while exothermic reactions depict a negative ΔH, indicating products with less energy than reactants.

$\Delta H$

Endothermic & Exothermic Reactions

Endothermic reactions are characterized by the absorption of thermal energy from the surroundings, leading to an increase in molecular motion. As the system absorbs heat, the molecules gain sufficient energy to break their bonds, making these reactions primarily heat-absorbing and bond-breaking processes. A common example of an endothermic reaction is the melting of ice. When an ice cube absorbs heat, it transitions from a solid state to a liquid state, demonstrating the concept of fusion. Further heating of the liquid water can lead to vaporization, where it transforms into a gas. This process involves breaking the intermolecular connections between water molecules as they change from solid to liquid and then to gas.

In thermochemistry, the phase changes associated with endothermic reactions include:

• Melting (Fusion): Transition from solid to liquid.
• Vaporization: Transition from liquid to gas.
• Sublimation: Transition from solid directly to gas.

In practical terms, if you were to touch a substance undergoing an endothermic reaction, it would absorb heat from your hand, resulting in a sensation of coldness. This occurs because the substance is drawing thermal energy from its surroundings, including your skin.

Energy diagrams are useful tools for visualizing the energy changes during a reaction. In an endothermic reaction, the energy diagram illustrates that the reactants start at a lower energy level and progress to a higher energy level as they transform into products. The change in enthalpy, denoted as ΔH, is positive, indicating that energy is absorbed. The y-axis of the energy diagram represents energy, while the x-axis typically represents the progress of the reaction from reactants to products.

In summary, endothermic reactions are essential processes that involve the absorption of heat, leading to significant changes in molecular structure and energy states, as evidenced by phase transitions and energy diagrams.

Endothermic & Exothermic Reactions Example 1

Endothermic reactions are characterized by the absorption of heat, typically involving bond breaking processes. In contrast, exothermic reactions release heat and often involve bond formation. To identify an endothermic reaction, it is essential to analyze the phase changes and the nature of the bonds involved.

For instance, when steam condenses, it transitions from a gas to a liquid. This process involves the formation of bonds rather than breaking them, thus it is not endothermic. Similarly, when molten lava solidifies, it changes from a liquid to a solid, again forming bonds and not breaking them, which disqualifies it as an endothermic reaction.

On the other hand, when water boils, it undergoes a phase change from liquid to gas. This process requires energy input to break the intermolecular bonds in the liquid, making it an endothermic reaction. Lastly, water freezing involves a transition from liquid to solid, where bonds are formed, indicating that this process is also not endothermic.

In summary, among the processes discussed, only the boiling of water represents an endothermic reaction, as it involves the breaking of bonds and the absorption of heat.

Endothermic & Exothermic Reactions

Exothermic reactions are characterized by the release of thermal energy from the system to the surroundings. As molecules in the system lose heat, they slow down, and with sufficient energy loss, they can form bonds. This process can be understood through phase changes, particularly with water molecules. In a gaseous state, water molecules are energetic and move freely. When they release heat, they lose energy and begin to come closer together, leading to condensation, where gas transforms into liquid. If this liquid water is then placed in a freezer, it continues to lose heat and solidifies, a process known as freezing.

Interestingly, some substances can bypass the liquid phase entirely under certain conditions. For example, carbon dioxide, commonly referred to as dry ice, can transition directly from a gas to a solid through a process called deposition when it is removed from a cold environment. This phenomenon illustrates the versatility of phase changes in exothermic reactions.

When considering the thermal sensations associated with exothermic reactions, one can feel warmth when touching a container holding an exothermic reaction, as it releases heat to the surroundings. This is a direct consequence of the energy dynamics at play.

In an energy diagram representing an exothermic reaction, the y-axis indicates energy levels while the x-axis represents the progress of the reaction from reactants to products. In this context, the reactants possess higher energy compared to the products. As the reaction proceeds, excess energy is released, resulting in a decrease in energy levels, which is depicted by a downward slope in the diagram. The change in enthalpy, denoted as ΔH, is negative, reflecting the energy released during the transition from reactants to products. This visualization helps to reinforce the concept of exothermic reactions and their energy transformations.

Endothermic & Exothermic Reactions Example 2

In chemistry, understanding the distinction between exothermic and endothermic reactions is crucial. An exothermic reaction is characterized by the release of heat, typically associated with bond formation, while an endothermic reaction involves the absorption of heat and is linked to bond breaking.

For example, when steam condenses into water, it transitions from a gas to a liquid, which involves the formation of bonds. This process releases heat, making it an exothermic reaction. In contrast, burning carbon dioxide (CO₂) suggests breaking down substances, which is indicative of an endothermic reaction, as energy is absorbed to break bonds.

Similarly, a cold pack feels cold because it absorbs heat from the surroundings, indicating an endothermic process. On the other hand, a hot pack releases heat, confirming it as an exothermic reaction. Additionally, the sublimation of dry ice, where it transitions from a solid to a gas, also absorbs heat, further categorizing it as endothermic.

Thus, among the options provided, steam condensing is the only process that exemplifies an exothermic reaction, as it involves the formation of bonds and the release of heat.

Here’s what students ask on this topic:

What is an endothermic reaction?

An endothermic reaction is a chemical process in which the system absorbs energy from its surroundings in the form of heat. This means that the total enthalpy, or heat content, of the reacting system increases. During an endothermic reaction, the energy required to break the bonds in the reactants is greater than the energy released when new bonds form in the products. As a result, the reaction vessel or environment often feels cold to the touch, as heat is being taken in rather than given off.

A common example of an endothermic process is the reaction of barium hydroxide octahydrate with ammonium thiocyanate. When mixed, these chemicals absorb heat from their surroundings and become much colder, demonstrating the endothermic nature of the reaction. Photosynthesis in plants is another example of an endothermic reaction, where plants absorb sunlight to convert carbon dioxide and water into glucose and oxygen.

What is an exothermic reaction?

An exothermic reaction is a chemical process that releases energy in the form of heat or light. In such reactions, the total energy of the products is less than the total energy of the reactants. This difference in energy is released to the surroundings. The energy is often transferred as heat, making the surroundings warmer, which is why exothermic reactions are commonly associated with warmth or even flames in cases like combustion. Examples of exothermic reactions include the burning of fuels, such as wood or gasoline, rusting of iron, and many metabolic processes that occur in living organisms. These reactions can be represented in chemical equations with heat as a product. For instance, when natural gas (methane) burns in a furnace, it combines with oxygen to produce carbon dioxide, water, and heat, which warms your house.

How is enthalpy used to predict whether a reaction is endothermic or exothermic?

Enthalpy, represented as $H$, is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.

A reaction is endothermic if it absorbs heat from its surroundings, which results in a positive change in enthalpy ($\mathrm{\Delta H}>0$). In an endothermic reaction, the enthalpy of the products is greater than the enthalpy of the reactants, indicating that energy is taken in during the reaction.

Conversely, a reaction is exothermic if it releases heat to its surroundings, leading to a negative change in enthalpy ($\mathrm{\Delta H}<0$). In an exothermic reaction, the enthalpy of the products is less than the enthalpy of the reactants, showing that energy is given off during the reaction.

To predict whether a reaction is endothermic or exothermic, you can calculate the change in enthalpy ($\mathrm{\Delta H}$) by subtracting the enthalpy of the reactants from the enthalpy of the products. The sign of $\mathrm{\Delta H}$ will indicate the nature of the reaction.

How can you tell if a reaction is endothermic or exothermic?

To determine if a reaction is endothermic or exothermic, you need to consider the flow of energy during the reaction. An exothermic reaction releases energy to the surroundings, usually in the form of heat, and can often be identified by a temperature increase in the reaction mixture or surroundings. Common signs of an exothermic reaction include the reaction container feeling warm or hot to the touch and the reaction emitting light or sound.

On the other hand, an endothermic reaction absorbs energy from the surroundings, which typically results in a temperature decrease in the reaction mixture or surroundings. This type of reaction might feel cold to the touch as it proceeds. Additionally, you can use a calorimeter to measure the heat absorbed or released by a reaction, which can provide a quantitative indication of whether the process is endothermic or exothermic.

Chemical equations can also give you clues: if the energy term is written on the products' side, it's exothermic; if it's on the reactants' side, it's endothermic. However, the most accurate method is to measure the heat change directly using calorimetry.