Which is the Stronger Acid, HCl or H2SO4?

Do you still remember the HCl and sulfuric acid we talked about in chemistry class? They all have a certain degree of corrosiveness. Are you clear about the difference between these two drugs? Which is a stronger acid? Let’s explore the truth.

Introduction of HCl vs Sulfuric Acid

1.HCl

HCl is a solution formed by the dissolution of hydrogen chloride in water and is a mixture. According to the difference in concentration, it can be divided into concentrated HCl and dilute HCl. The former is a colorless liquid with a pungent odor and volatility. It volatilizes to produce hydrogen chloride gas. The latter is obtained by diluting it more than 50 times with concentrated HCl, and it is also a colorless liquid with basically no irritating odor and less obvious volatility. It has acidity and reducibility, a strong pungent odor, and high corrosiveness.

2.Sulfuric Acid

Sulfuric acid is the most active binary inorganic strong acid that can react with the vast majority of metals. 98% concentration sulfuric acid has strong oxidizing properties, as well as dehydration, strong corrosiveness, low volatility, acidity, and water absorption. It is an important industrial raw material widely used in industries such as environmental protection, chemical industry, and electronic smelting.

When concentrated sulfuric acid dissolves in water, it releases a large amount of heat. Therefore, when diluting concentrated sulfuric acid, it is not allowed to pour water into it. Instead, the concentrated sulfuric acid should be slowly injected into the water along the wall of the container and stirred continuously with a glass rod to quickly diffuse the generated large amount of heat.

HCl vs sulfuric acid, which is a stronger acid?

The first viewpoint states: “Sulfuric acid is a strong acid, and in most cases, we consider it as a binary acid that provides two hydrated hydrogen ions. The first step of the proton dissociation reaction is quantitative. The second step of proton dissociation is also feasible.”

The second viewpoint holds that sulfuric acid is not always stronger than HCl, and it is not absolute which acid is stronger. Firstly, sulfuric acid is also an oxidant. Secondly, sulfates are not always soluble like chlorides. Finally, hydrochloric acid is usually used as a gas to dissolve in water, and sulfuric acid itself is a liquid.

The third viewpoint believes the acid strength is commonly determined by chemists using its pKa value, which is the acid dissociation constant Ka’s negative log. An acid is stronger the lower its pKA value, which indicates how much more dissociated it is.  For example, the pKa of HCl is about -7, while the pKa of sulfuric acid is about -3.99, so HCl is much stronger than sulfuric acid

Which one is correct? The third viewpoint is correct: HCl is stronger than sulfuric acid.

Be careful when using HCl and Sulfuric Acid

HCl is prone to evaporation and should be stored in a closed container. If hydrochloric acid leaks, hydrogen chloride will evaporate into the air, forming a high concentration HCl aqueous solution with water in the air, which can corrode the eyes and skin of the human body. Long-term direct contact with HCl in the air can lead to diseases such as rhinitis and tracheitis. Not only does it harm human health, but the evaporated HCl can also corrode surrounding metal objects, causing instrument damage.

Sulfuric acid has water absorption and can be used as a desiccant. If concentrated sulfuric acid is exposed to the air for a long time, it will absorb water vapor in the air, causing the solute mass fraction of concentrated sulfuric acid to decrease. Concentrated sulfuric acid can dry gases that do not react with it, such as O2, H2, etc.

It also has dehydration properties. It can extract moisture from paper and skin, generating black charcoal. So when using concentrated sulfuric acid, extra caution should be taken. If concentrated sulfuric acid accidentally gets on the skin or clothes, it should be immediately rinsed with plenty of water and then coated with 3% to 5% sodium bicarbonate solution.

Conclusion

To sum up, HCl vs sulfuric acid, which is a stronger acid? The chemical community believes that HCl is stronger. It is volatile and has a strong irritating odor, so please pay attention to sealing and storage. What’s more, it can be used for rust removal on metal surfaces, manufacturing drugs, and human gastric juice contains HCl that can aid digestion.