Question

What is meant by the term lower in energy? Which is lower in energy, a mixture of hydrogen and oxygen gases or liquid water? How do you know? Which of the two is more stable? How do you know?

Short answer

The term "lower in energy" refers to a state of a system where its overall potential energy is lower compared to another state, generally making it more stable. In this case, liquid water is lower in energy and more stable than a mixture of hydrogen and oxygen gases. This is because the formation of water from hydrogen and oxygen gases is an exothermic reaction, releasing energy (\(\Delta _rH^{\circ} = -285.8\:kJ/mol\)), and systems in lower energy states are considered more stable due to their decreased reactivity.

Step-by-step solution

Define "Lower in Energy"

The term "lower in energy" refers to a state of a system where its overall potential energy is lower compared to another state. In chemistry, compounds or mixtures with lower energy states are generally more stable, as they release energy in the form of heat or light during the process of achieving that state.

Comparing Energy States of the Two Systems

Here, we have two systems: (1) a mixture of hydrogen and oxygen gases, and (2) liquid water. In the first system, hydrogen and oxygen exist as individual gas molecules. In the second system, hydrogen and oxygen have combined to form water molecules. To compare the energy states of these two systems, we can investigate their relative energy release when they form the other system.

Formation of Water from Hydrogen and Oxygen Gases

When hydrogen and oxygen gases react to form water, the following chemical reaction takes place: \[2H_2(g) + O_2(g) \rightarrow 2H_2O(l)\] This reaction is exothermic, meaning it releases energy (in the form of heat) during the process. The negative standard enthalpy change (ΔH) for this reaction indicates that the system is losing energy: \[\Delta _rH^{\circ} = -285.8\:kJ/mol\] The fact that energy is released during this reaction means that liquid water has a lower energy state than a mixture of hydrogen and oxygen gases.

Stability Comparison

In general, systems in a lower energy state are considered more stable because they tend to be less reactive and require an input of energy to undergo changes. Since liquid water has a lower energy state than a mixture of hydrogen and oxygen gases, it is considered more stable. This stability can be observed in daily life, as liquid water is far less prone to spontaneous combustion or explosion than a mixture of hydrogen and oxygen gases. In summary, liquid water is lower in energy and more stable than a mixture of hydrogen and oxygen gases. This conclusion is based on the energy released during the reaction between hydrogen and oxygen gases to form water molecules and the general concept that lower energy states correlate with increased stability.

Key concepts

Energy Stability

Energy stability in chemical reactions refers to how stable a compound is after a reaction compared to before. If a system is in a lower energy state, it means the system is more stable. In other words, it requires less energy to maintain its form.

When substances react, they seek to reach a state where they use up the least possible energy. This is similar to how a ball rolls down to the bottom of a hill to rest safely and securely. Liquid water, for instance, is more stable than a combination of hydrogen and oxygen gases because it resides in a lower energy state.

• Lower energy states equate to increased stability.
• Energy stability prevents spontaneous reactions in stable compounds.
• Systems "prefer" to be in the configuration that requires minimal energy.

Understanding energy stability helps in predicting the behavior of chemical substances and how likely they are to react with each other under normal conditions.

Enthalpy Change

Enthalpy change is fundamentally about the heat involved during chemical reactions. During a reaction, energy is either absorbed or released. We denote this with the Greek letter delta followed by an "H": \( \Delta H \).

When a reaction releases heat, it is called "exothermic," and the enthalpy change is negative. This negative value tells us that the system has released energy to its surroundings and is now in a lower energy, more stable state. For instance, when hydrogen and oxygen react to form water, the process is markedly exothermic.

• Exothermic reactions release energy, shown by a negative \( \Delta H \).
• A negative enthalpy change indicates a shift to a more stable state.
• The more negative \( \Delta H \), the more energy released, signifying greater stability post-reaction.

This principle helps us understand why some reactions, like the formation of water, occur easily and spontaneously.

Chemical Reactions

Chemical reactions are processes where reactants transform into products. During this transformation, bonds are broken and new ones are formed. The process often involves energy changes due to the making and breaking of these bonds.

In our specific example, hydrogen and oxygen gases are the reactants, and they undergo a reaction to form liquid water, which is the product. The energy released signals that the initial reactants are in a higher energy state than the resultant product, thus confirming that the product, water, is more stable.

• Reactions involve bond-breaking and bond-making.
• Energy changes are key to understanding reaction mechanisms.
• Products in lower energy states are generally more stable than their reactant counterparts.

Grasping the flow of energy in chemical reactions gives insight into reactant stability and potential hazards, like why some mixtures can be extremely volatile while others are stable.