Question

The electrolysis of \(25.0 \mathrm{~g}\) of water requires \(335 \mathrm{~kJ}\) of energy to produce hydrogen and oxygen gases. How much energy is released when hydrogen and oxygen gases react to produce \(25.0 \mathrm{~g}\) of water?

Short answer

335 kJ of energy is released when hydrogen and oxygen gases react to form 25.0 g of water.

Step-by-step solution

Understanding Electrolysis

Electrolysis of water involves using electrical energy to split water (\( ext{H}_2 ext{O} \)) into hydrogen (\( ext{H}_2 \)) and oxygen (\( ext{O}_2 \)) gases. For the given exercise, \( 335 ext{ kJ} \) of energy is required to electrolyze \( 25.0 ext{ g} \) of water into these gases.

Understanding Recombination Reaction

When hydrogen and oxygen gases react, they undergo a recombination reaction to form water. This is essentially the reverse process of electrolysis. The energy change associated with recombination is equal in magnitude but opposite in sign to the energy required for electrolysis.

Calculating Energy Release

Since the energy required to electrolyze \( 25.0 ext{ g} \) of water is \( 335 ext{ kJ} \), the same amount of energy (\( 335 ext{ kJ} \)) will be released when \( 25.0 ext{ g} \) of water is formed by recombining hydrogen and oxygen gases.

Key concepts

Electrolysis Process

Electrolysis is a fascinating chemical process where electrical energy is used to drive a non-spontaneous reaction. In simple terms, electrolysis splits water (\( \text{H}_2\text{O} \)) into its component gases: hydrogen (\( \text{H}_2 \)) and oxygen (\( \text{O}_2 \)). Naturally, water doesn't just decompose into hydrogen and oxygen without assistance. It needs energy in the form of electricity to break the strong bonds holding the water molecules together.
In the exercise, it required \(335\ \text{kJ}\) of energy to electrolyze \(25.0\ \text{g}\) of water. During this process, electrodes are placed in water, and when electricity flows, it separates the water molecules into hydrogen and oxygen.

• The positive electrode, or anode, attracts the oxygen molecules.
• The negative electrode, or cathode, attracts the hydrogen molecules.

This process exemplifies energy input to create new substances. Without this energy, water would remain unchanged.

Energy Conservation in Electrolysis

Energy conservation is a key principle in thermochemistry and the process of electrolysis perfectly showcases this principle. When we speak about energy conservation, we mean that energy cannot be created or destroyed; it can only be transferred or transformed.
In the context of electrolysis, the energy you put in (\(335\ \text{kJ}\) for the given example) to break apart the water molecules is conserved even when reversed. This means when the hydrogen and oxygen recombine to form water, the same amount of energy (\(335\ \text{kJ}\) in this case) is released.

• This energy release during the recombination of hydrogen and oxygen to form water is why these gases are highly reactive together.
• The energy remains the same in magnitude but merely swaps forms from electrical potential to thermal energy.

Understanding energy conservation helps in determining how reactions behave, especially when considering potential uses of the energy produced.

Chemical Reactions: From Water Splitting to Formation

Chemical reactions are processes where substances change their chemical properties or composition. In the case of electrolysis, the reaction is straightforward: water decomposes into hydrogen and oxygen. This decomposition is an example of a chemical reaction where bonds are broken and energy is absorbed.
Conversely, when hydrogen and oxygen gases are brought back together, they undergo a recombination reaction to form water once again. This is the reverse of the electrolysis process. In terms of thermochemistry, the chemical reaction during recombination releases energy. The magnitude of this release is exactly equal to the energy required for decomposition.

• The decomposition of water (using \(335\ \text{kJ}\)) and the recombination to form water (releasing \(335\ \text{kJ}\)) demonstrate how chemical reactions often compliment each other energetically.
• The reactions illustrate important chemical principles such as bond breaking requiring energy, and bond formation releasing energy.

Through this chemical dance, the reactants and products shift back and forth, showcasing the elegance of chemical reactions and energy transitions.