Question

Which of the following reactions does not produce hydrogen? (a) \(\mathrm{CH}_{4} \stackrel{\text { Cracking } 1000^{\circ} \mathrm{C}}{\longrightarrow}\) (b) \(\mathrm{CH}_{4}+\) steam \(\stackrel{\mathrm{Ni}-\mathrm{Cr}, 820^{\circ} \mathrm{C}}{\longrightarrow}\) (c) \(\mathrm{C}+\mathrm{H}_{2} \mathrm{O} \rightarrow\) (d) Water gas \(+\) steam \(\rightarrow\)

Short answer

All reactions listed typically produce hydrogen with standard interpretations and setups.

Step-by-step solution

Understand each reaction

We need to determine whether each reaction produces hydrogen. It's crucial to examine each reaction independently and interpret the reactions carried out.

Break down reaction (a)

Reaction (a) is the cracking of methane (\( \mathrm{CH}_4 \) at \( 1000^{\circ} \mathrm{C} \)). Cracking methane can result in various by-products such as hydrogen gas (\( \mathrm{H}_2 \)), acetylene (\( \mathrm{C}_2\mathrm{H}_2 \)), and carbon (\( \mathrm{C} \)). Thus, this reaction typically does produce hydrogen.

Analyze reaction (b)

Reaction (b) involves methane and steam over a nickel-chromium catalyst at \( 820^{\circ} \mathrm{C} \), known as steam reforming. The general reaction is \( \mathrm{CH}_4 + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{CO} + 3\mathrm{H}_2 \), producing hydrogen.

Review reaction (c)

Reaction (c) is carbon reacting with water (steam), known as the water-gas reaction: \( \mathrm{C} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{CO} + \mathrm{H}_2 \), which produces hydrogen.

Examine reaction (d)

Reaction (d) involves water gas plus steam reacting together. Typically, water gas (which includes hydrogen and carbon monoxide) when reacted with steam is used to shift the water-gas equation: \( \mathrm{CO} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{CO}_2 + \mathrm{H}_2 \). This still results in producing hydrogen.

Identify the reaction that does not produce hydrogen

All reactions listed result in the production of hydrogen either directly through equations or indirectly as a by-product. However, since each stated reaction can potentially generate hydrogen under typical conditions, re-evaluate if a definitive condition or setup prevents hydrogen formation in option (a) specifically. Reaction (a), while typically producing hydrogen, can theoretically yield products like solid carbon if conditions vary, hence making it the less probable consistent source under unchecked variations.

Key concepts

Methane Cracking

Methane cracking is a fascinating process that is used for hydrogen production. This method involves breaking down methane molecules ( \( \mathrm{CH}_4 \) ) at very high temperatures, often around ( 1000^{\circ} \mathrm{C} ). This cracking action splits the methane into hydrogen gas and solid carbon. People often refer to the solid carbon as 'carbon black.'

Key benefits of methane cracking include:

• Produces hydrogen without carbon dioxide emissions.
• Provides solid carbon, which can be used in other industrial applications.

However, controlling the process to consistently produce hydrogen can sometimes be challenging. The temperature and conditions must be precisely maintained to ensure hydrogen production rather than other potential by-products. This makes it a technique suited for places where energy is not a concern, given the high heat requirement.

Steam Reforming

Steam reforming is another vital method used to produce hydrogen on a large scale. In this process, methane ( \( \mathrm{CH}_4 \) ) reacts with steam in the presence of a nickel-chromium (Ni-Cr ) catalyst at high temperatures and pressures, usually around ( 820^{\circ} \mathrm{C} ). The reaction typically seen is:

\( \mathrm{CH}_4 + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{CO} + 3\mathrm{H}_2 \)

• This process is efficient and effective for large-scale hydrogen production.
• It is widely used in the industrial sector, including for ammonia production.

One downside to steam reforming is that it also produces carbon monoxide as a by-product, which is then often converted to carbon dioxide. This makes it less environmentally friendly compared to some alternative hydrogen production methods.

Water-Gas Reaction

The water-gas reaction involves the conversion of water steam and carbon into useful gases. Let's look at this reaction: carbon (\( \mathrm{C} \) ) reacts with water (\( \mathrm{H}_2 \mathrm{O} \) ), forming carbon monoxide (\( \mathrm{CO} \)) and hydrogen (\( \mathrm{H}_2 \) ):

\( \mathrm{C} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{CO} + \mathrm{H}_2 \)

This reaction is sometimes referred to as a precursor step in the production of water-gas or syngas (synthesis gas), a mixture of hydrogen and carbon monoxide. It can be further processed to increase hydrogen generation:

• The reaction contributes to the production of synthetic fuels and chemical feedstocks.
• It's often used to create hydrogen for various industrial applications.

This process's flexibility makes it a popular choice when carbon sources are readily available.

Catalytic Reactions

Catalytic reactions play a crucial role in many industrial processes, especially in making chemical reactions faster and more efficient. In the context of hydrogen production, catalysts help lower the energy barriers so that reactions like steam reforming or methane cracking can proceed at faster rates with less energy input. For instance, the nickel-chromium ( Ni-Cr ) catalyst in steam reforming is essential. Some advantages of using catalytic reactions for hydrogen production include:

• Enhanced reaction rates without requiring exceedingly high temperatures.
• Improved product distribution, often yielding a higher proportion of desired outcomes such as hydrogen.

Catalysts are primarily metal-based and can sometimes be expensive, but their benefits, in terms of improving process efficiency and output, often justify these costs. In many settings, optimizing catalytic material can significantly impact the overall cost and environmental footprint of the hydrogen production process.