## Omitting solid/liquid

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Yeonjoo Kim 2B
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Joined: Wed Sep 18, 2019 12:17 am

### Omitting solid/liquid

Hello, can someone explain why we would not include solid or liquid in K expressions? Dr. Lavelle also mentioned about omitting H20 as well, but I didn't really get why. For example, in his lecture, he said to leave out CaCO3 and CaO in CaCO3⇌ CaO + CO2 so Kc=[CO2].

Joseph Hsing 2C
Posts: 91
Joined: Wed Sep 30, 2020 9:42 pm

### Re: Omitting solid/liquid

For liquids omit any solvents because it will be in excess so there would be no point tracking their molar concentration. Both solids and liquids are excluded because their effective concentrations will stay constant throughout the reaction.

David Chibukhchian 2G
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Joined: Wed Sep 30, 2020 9:50 pm

### Re: Omitting solid/liquid

I definitely agree with the other answer and would add that it's good to think about what exactly would happen if we did include liquid water (or any solvent) in the equilibrium expression. Because its concentration basically does not change, you would end up with a very similar value in both the numerator and the denominator, and as a result, they would cancel out of the expression. For this reason, we generally don't include them. That's how I think of it, I hope that helps!

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### Re: Omitting solid/liquid

We omit solids and liquids from equilibrium expressions due to the fact that their concentrations do not change. Therefore there's no use in putting them in the equation.

Samuel Flores 1E
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Joined: Wed Sep 30, 2020 9:51 pm

### Re: Omitting solid/liquid

Hello! In a chemical reaction, solids and liquids will have a constant concentration as we proceed through the chemical reaction. In other words, the reaction does not change the concentration of solids or liquids. As a result, even if we included their concentrations in the equilibrium expression, the concentration would be the same for liquids and solids, so it would cancel from the numerator and denominator. Therefore, we simply omit their concentrations.

Hope this helps!

Kelly Tran 1J
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### Re: Omitting solid/liquid

We do not include solids and liquids (solvents) in the expression for K because their concentrations essentially do not change. For example, the change in concentration of solvents are so small that the change is insignificant. In other words, if you take the initial concentration and subtract a really small change from it, you are essentially getting back the initial concentration. Therefore, the change is negligible, which is why you wouldn't include solids and liquids in the expression for K.

Lorraine Medina 3E
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Joined: Wed Sep 30, 2020 10:11 pm

### Re: Omitting solid/liquid

Hi! the concentrations of solids and liquids do not change, therefore we omit them. Hope this helps!

Serena Song 1A
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### Re: Omitting solid/liquid

The concentration for solids and liquids are fixed, unlike gases or aqueous solutions. Since the initial and final concentrations for solids and liquids are about the same, they would cancel out anyways in the K equation, so we omit them altogether.

Karl Yost 1L
Posts: 171
Joined: Thu Dec 17, 2020 12:19 am

### Re: Omitting solid/liquid

We omit solids as their concentration doesn't change. The concentrations of liquids can change, which means that we only omit them when they are in vast excess (e.g. when they are the solvent).

Namita Shyam 3G
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Joined: Tue Nov 17, 2020 12:20 am

### Re: Omitting solid/liquid

So the reason why we leave out solids is because solids don't have a concentration or pressure. However, gases and liquids do, which is why we can use them to find K.
Also, to answer your second question, the reason why we don't include H2O is because H2O is a solvent (the stuff we have more of). Since we essentially have the same amount of H2O before and after a reaction, when we put H2O concentrations into the K equation, they cancel out (which is why it doesn't make sense to include solvents!!).

Jason Knight - 1F
Posts: 102
Joined: Wed Sep 30, 2020 9:55 pm

### Re: Omitting solid/liquid

We leave out solids and liquids in our equilibrium expression because they do not play a factor in the calculations. The solids concentration is a fixed value, and the concentration of liquid from reactant to product is too insignificant to note or take into account for our calculation. Only include aqueous and gaseous state molecules in the expression.

Aarushi Verma 2I
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Joined: Wed Nov 18, 2020 12:32 am

### Re: Omitting solid/liquid

We leave solids and liquids out because they do not have a concentration so we cannot put them into our K expression. We also don't include H20 in the K expression when it acts as a solvent because its concentration is so high on both the reactants and products side that we would see no difference. If you also put H20 into K, it will cancel out on the top and the bottom, so we just skip that step and go straight to the cancelled out version.

Nishka Vipul 1J
Posts: 48
Joined: Thu Dec 17, 2020 12:18 am

### Re: Omitting solid/liquid

Yeonjoo Kim 2B wrote:Hello, can someone explain why we would not include solid or liquid in K expressions? Dr. Lavelle also mentioned about omitting H20 as well, but I didn't really get why. For example, in his lecture, he said to leave out CaCO3 and CaO in CaCO3⇌ CaO + CO2 so Kc=[CO2].

In addition to everything mentioned above, I just wanted to add that you can omit H2O when it is in the liquid state. However, you do have to consider it when it is in a gaseous state!

emwoodc
Posts: 81
Joined: Fri Aug 09, 2019 12:16 am

### Re: Omitting solid/liquid

the concentration of solids and liquids do not change! so when we calculate concentration they're omitted!

Alexandra Salata 2L
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Joined: Wed Sep 30, 2020 9:39 pm

### Re: Omitting solid/liquid

we omit them from the equations because their concentrations do not change, therefore they are not involved in the equilibrium expression.

Krish_Ajmani_3J
Posts: 16
Joined: Wed Nov 20, 2019 12:20 am

### Re: Omitting solid/liquid

Solids and liquids should not be factored into equilibrium problems because their concentrations do not change.

Javier Perez M 1H
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Joined: Wed Sep 30, 2020 9:52 pm

### Re: Omitting solid/liquid

Definitely, as stated above do not include nongases nor solvents such as the common H2O except when itls part of the reaction not acting as main solvent that you include the concentration of H20.

Will Skinner
Posts: 84
Joined: Wed Sep 30, 2020 9:28 pm

### Re: Omitting solid/liquid

We omit the solids and liquids because they are often present in excess and their concentrations change insignificantly, effectively they stay the same.

t_rasul2I
Posts: 98
Joined: Wed Sep 30, 2020 9:38 pm

### Re: Omitting solid/liquid

you would leave out solids and liquids in their pure form because their concentration is not over a volume. Therefore, their concentration is exponentially larger than other species in the equation and that will throw off your answer.

Vivian_Le_1L
Posts: 40
Joined: Thu Dec 17, 2020 12:19 am

### Re: Omitting solid/liquid

Solids and liquids are omitted from the equilibrium constant, because they have a constant concentration. They don't make a difference in the expression, and thus omitted.