Writing Chemical Equations, Is It a Gas or Aqueous Solution?

[Mazurka]

Hello,

I understand that this question may seem rather obvious for some. Nonetheless, it is something that seems to have thrown me off a number of times.

When writing chemical equations involving acidic or basic solutions, I know that the acid on the reactant side is written with the state (aq). i.e. HCl (aq) + H₂O (l) → H₃O⁺(aq) + Cl^-(aq)

However, if it is a gas that forms an acid upon contact with water, is the acid on the reactant side still written with the state (aq), or is it written with the state (g)? For example, let's say a question states "HCl (g) is dissolved in water to form an acidic solution..." What state would the hydrogen chloride gas be written as within the equation?

Ideas are welcome, explanations are greatly appreciated. :)

Thank you,

Eric.

 

[Simon Bridge]

HCl(aq) is the proper notation for Hydrochloric Acid (HCl dissolved in water).

HCl is the proper notation for the gas. You don't usually specify the state unless there are very unusual conditions.

HCl(g) + H20(l) ---> HCl(aq) but this is not really a chemical reaction. eg. you wouldn't normally use an equation to describe making salty water:

NaCl(s) + H2O(l) ---> NaCl(aq)

In general, you express the reactants in the state they start out in. You only need to do this if it is not obvious from the context. Your confusion is what happens when one of your reactants is the form of a gas that starts out dissolved in water (it's a solution - aq). This confusion is magnified by the common habit of leaving the (aq) off the acid.

In practice, these notes are a shorthand to keep track of things which get explained in the preamble to the equations.

 

[juanrga]

Hello,

I understand that this question may seem rather obvious for some. Nonetheless, it is something that seems to have thrown me off a number of times.

When writing chemical equations involving acidic or basic solutions, I know that the acid on the reactant side is written with the state (aq). i.e. HCl (aq) + H₂O (l) → H₃O⁺(aq) + Cl^-(aq)

However, if it is a gas that forms an acid upon contact with water, is the acid on the reactant side still written with the state (aq), or is it written with the state (g)? For example, let's say a question states "HCl (g) is dissolved in water to form an acidic solution..." What state would the hydrogen chloride gas be written as within the equation?

Ideas are welcome, explanations are greatly appreciated. :)

Thank you,

Eric.

You are asking about a dissolution reaction. The process is

$$\text{HCl(g)} \rightleftharpoons \text{HCl(aq)}$$

but $\text{HCl(aq)}$ is very unstable and dissociates as you correctly note above. See http://dl.clackamas.edu/ch105-03/dissolut.htm

 

[Highway]

Also, most gasses are things like N₂ / O₂ / H₂, and they are more than likely on the products side of an irreversible reaction.

Like, the reaction would form some product, and give off a gas as well.

 

[LudusRex]

Hi Eric,

Thank you for reaching out with your question. Writing chemical equations can be tricky, especially when it comes to differentiating between gases and aqueous solutions. Let me provide some clarification to help you understand this concept better.

Firstly, it is important to understand that the state of a substance in a chemical equation is not always determined by its physical state at room temperature. For example, water is typically a liquid at room temperature, but in a chemical equation, it is often written as (l) to indicate that it is in the liquid state. Similarly, a gas can be written as (g) or (aq) depending on its role in the reaction.

In your example, HCl (g) is dissolved in water, so it would be written as HCl (aq) in the equation. This is because when gaseous HCl dissolves in water, it forms an aqueous solution of HCl molecules surrounded by water molecules. Therefore, it is appropriate to write it as (aq) to indicate that it is in the aqueous state.

On the other hand, if the question stated that "HCl (g) reacts with water to form an acidic solution," then it would be written as HCl (g) in the equation. This is because in this case, HCl is acting as a gas and not as a dissolved species in the solution.

I hope this helps clarify things for you. If you have any further questions, please don't hesitate to ask.

Best,