Problem 73 - Chemistry Matter and Change

Question

What is the sign of \(\Delta H\) for an exothermic reaction? An endothermic reaction?

Step-by-Step Solution

Answer

Exothermic: \(\Delta H < 0\); Endothermic: \(\Delta H > 0\).

Step 1: Understand the Concept of 9;9;9;9;Exothermic Reaction

An exothermic reaction is a chemical reaction that releases energy in the form of heat to its surroundings. In such reactions, the products have lower energy than the reactants, leading to the release of excess energy.

Step 2: Examine 9;9;9;9;9;Exothermic Reaction and 9;9;\(\Delta H\)

For an exothermic reaction, the enthalpy change (\(\Delta H\)) is negative, indicating that the system has lost energy to the surroundings.

Step 3: Understand the Concept of 9;9;9;9;Endothermic Reaction

An endothermic reaction is a chemical reaction that absorbs energy from its surroundings, usually as heat. In these reactions, the products have higher energy than the reactants, and energy is taken from the surroundings.

Step 4: Examine 9;9;9;9;9;Endothermic Reaction and 9;9;\(\Delta H\)

For an endothermic reaction, the enthalpy change (\(\Delta H\)) is positive, indicating that the system has gained energy from its surroundings.

Key Concepts

Exothermic Reaction

Exothermic reactions are fascinating because they release heat into their surroundings. This means the reaction gives off energy, usually making the area around it warmer. Imagine lighting a campfire: the wood burns and releases heat, warming your hands.
During an exothermic reaction, the energy stored in the chemical bonds of the reactants is greater than the energy stored in the bonds of the products. This difference is released as heat energy. So, the products end up with less energy compared to the reactants. Let's break this down:

• Reactants have high energy.
• Products have low energy.
• Energy difference is released as heat.

If you know a reaction is exothermic, you can tell it because the enthalpy change (9; \(\Delta H\)) is negative. The system "loses" heat to the surroundings, because less energy is needed to keep the products stable.

Endothermic Reaction

Endothermic reactions absorb energy from their surroundings. Think of melting ice – it takes in heat from the environment to change from solid to liquid. During these reactions, more energy is needed to create the products' chemical bonds than what was present in the reactants.
So, what is the process like?

• Reactants have lower energy compared to the products.
• To form products, more energy is absorbed.
• Surroundings become cooler as energy is consumed.

When observing an endothermic reaction, you can notice that the enthalpy change (9; \(\Delta H\)) is positive. This indicates energy being absorbed by the system – think of it as the system "gaining" heat.

Chemical Reaction Energy

Energy in chemical reactions is all about the bonds. During a chemical reaction, bonds between atoms are broken and formed. This determines whether energy is released or absorbed. Two key types of reactions, exothermic and endothermic, show this energy transfer.

• In exothermic reactions, breaking bonds in reactants releases more energy than is used to form new bonds in products. Result: energy release.
• In endothermic reactions, more energy is used forming bonds in products than is released by breaking reactants' bonds. Result: energy absorption.

This transfer of energy can be tracked using enthalpy change (9; \(\Delta H\)), which tells you if the reaction absorbed or released heat. This is why understanding enthalpy is crucial for studying how energy changes in reactions affect our surroundings.