CHEMISTRY COMMUNITY

Posted: **Mon Jan 20, 2020 8:35 pm**

when a chemical equation is written and all the reactants and products are gases, do you use Kp rather than Kc? in what cases do you use Kc rather than Kp?

Posted: **Mon Jan 20, 2020 8:42 pm**

You use Kp when all the products and reactants are gases and the question includes units like bar/atm/pa. You use Kc when the products and reactants are given in moles or molarity, even if the products and reactants are gases. If the question asks for Kp and the question only gives you information about moles/molarity, use the ideal gas law and solve for pressure (P= (n/v)RT). Vice versa if they ask for Kc and they only give you information about pressure. Also, keep in mind there are different gas constants, so use the one that fits the units you have. Hope this helps.

Posted: **Mon Jan 20, 2020 8:48 pm**

Kc is used when you have the molarity of a solution. Kp is always used with gases.

Posted: **Tue Jan 21, 2020 10:13 am**

To add on, you use Kp when the reactants and products are gases. If your reaction has gases and/or aqueous substances, then use Kc. However, if you're still having trouble deciding, you should check the units to ensure you're using the right K, for example Barr (pressure) vs M (concentration).

Posted: **Tue Jan 21, 2020 10:16 am**

If all the reactants are gas then use Kp. You can calculate Kc for gas too but they would have to explicitly tell you so

Posted: **Thu Jan 23, 2020 12:37 pm**

It really depends what information the problems gives you. If the concentrations of the reactants and products are given, then you you Kc, on the other hand, if the pressure of the reactants and products are given then you use Kp.

Posted: **Thu Jan 23, 2020 2:18 pm**

It depends on what asked and given. If you are calculating molar concentration then you would use Kc.

Posted: **Thu Jan 23, 2020 2:52 pm**

You can use kp when the reactants and products are gases and the partial pressures of the gases are given rather than their concentrations

Posted: **Thu Jan 23, 2020 3:35 pm**

You calculate Kc if given concentration. Kp if partial pressures are given.

Posted: **Thu Jan 23, 2020 4:34 pm**

You use Kc when you are given a concentration, while Kp is used when given partial pressures. However, you can use Kp to find Kc if needed.

Posted: **Thu Jan 23, 2020 6:30 pm**

connie 2C wrote:when a chemical equation is written and all the reactants and products are gases, do you use Kp rather than Kc? in what cases do you use Kc rather than Kp?

You can use whatever is convenient when they're all gases. You cannot use KP if it's not a gas.

Posted: **Thu Jan 23, 2020 7:01 pm**

Kp can be used when all the concentrations are in pressure, and Kc when they are concentrations. Pressure can be converted to concentration by using PV=nRT (and vice versa), but all the species must be in the gas phase to use Kp.

Posted: **Thu Jan 23, 2020 7:16 pm**

From what I understand, pressures can be used with Kc or Kp but concentrations can only be used with Kc. Right?

Posted: **Thu Jan 23, 2020 7:57 pm**

Kc is used when you're dealing with molar concentrations, or when all the substances in your reaction are aqueous. Kp is used for partial pressures, or when the substances in your reaction are all gases.

Posted: **Fri Jan 24, 2020 12:04 pm**

kp is used when dealing with gases and is usually given with units like atm or bar. kc is for molar concentrations and will be given as aqueous solutions.

Posted: **Fri Jan 24, 2020 12:23 pm**

The difference here just has to do with units used, though Kp will include all gases yes.

Posted: **Fri Jan 24, 2020 8:23 pm**

Kp is used when dealing with partial pressures, so typically gases. Kc is used when dealing with concentration, so aqueous species.

Posted: **Fri Jan 24, 2020 9:32 pm**

You use Kp unless it specifically asks for Kc

Posted: **Fri Jan 24, 2020 10:09 pm**

Kp should be used with gases since you're dealing with partial pressures rather than concentrations.

Posted: **Fri Jan 24, 2020 10:15 pm**

It all depends on the information given. You would use Kp when the partial pressures are given, but you use Kc when the concentration or moles or grams are given even when the reactants and products are gases.

Posted: **Fri Jan 24, 2020 11:55 pm**

For a reaction that is entirely gasses, Kp can be used if the pressure is given. Kc can be used for gasses as well if the pressure is converted to concentration, or concentration is given initially.

Posted: **Sat Jan 25, 2020 12:20 am**

kp is used for gases since they deal with Pressure. kc is used for questions involving molarity, generally solutions, however you can find the concentration of gases too. you would have to use the ideal gas law, pv=nrt

Posted: **Sun Jan 26, 2020 9:53 pm**

Kp is for partial pressures of gasses, Kc on the other hand is the equilibrium constants

Posted: **Sun Jan 26, 2020 10:11 pm**

If the reaction consists of all gases then you would use Kp. Kc can be used for gases as well if the gases are given in concentration rather than pressure.

Posted: **Sun Jan 26, 2020 10:16 pm**

Kp is used with partial pressures like gases such as ATM. But Kc is used when you have things like aq concentrations.

Posted: **Sun Jan 26, 2020 10:25 pm**

Kp should be used for gases since you are dealing with partial pressures

Posted: **Sun Jan 26, 2020 11:23 pm**

Kp is for gases and uses partial pressure. Kc uses concentrations.

Posted: **Tue Jan 28, 2020 4:09 pm**

Kp is when you deal with partial pressure so it is used for gasses. Use Kc when you see aq.

Posted: **Tue Jan 28, 2020 6:41 pm**

connie 2C wrote:when a chemical equation is written and all the reactants and products are gases, do you use Kp rather than Kc? in what cases do you use Kc rather than Kp?

Kc should be used normally, but the question will clarify if it wants molar concentrations or partial pressures. Therefore pay attention to the question, also Kc will be in units of mol.L-1 and Kp will be in bar or atm.

Posted: **Tue Jan 28, 2020 7:12 pm**

You use Kp because K=kp for all cases. Kc=K when certain conditions are met

Posted: **Sun Feb 02, 2020 6:26 pm**

Kp is used when all products and reactants in the equation are gases. Another indication to use Kp is if the question uses units like bar or atm. Kc is used when the molarity of the products and reactants is given, even though the products and reactants involved in the reaction are gases.

Posted: **Sun Feb 02, 2020 6:31 pm**

Kp is used when the quantity of gases are given in context of pressure. Kc is used when the molarity of solutions or gases are given.

Posted: **Mon Feb 10, 2020 8:53 am**

Kc is concentration Kp is partial pressure.

Posted: **Mon Feb 10, 2020 9:29 am**

You use Kc when you have concentrations in molarity snd use Kp when you have partial pressures.

Posted: **Mon Mar 09, 2020 10:59 pm**

In that instance, you would use Kp, Kc is used for molar concentration.

Posted: **Wed Mar 11, 2020 10:12 pm**

If the reactants and products are given in units of pressure (bars, atmospheres, etc.), then use kp. If reactants and products are given in units of concentration (mol/L), then use kc.

Posted: **Thu Mar 12, 2020 6:50 pm**

Kp is used with gases and Kc is used when you are dealing with concentrations.

Posted: **Thu Mar 12, 2020 9:59 pm**

Kc deals with concentrations and Kp deals with atm and torr.

Posted: **Sun Mar 15, 2020 7:34 am**

Kp is used in the event that you products and reactants all exists in a gas phase. Therefore we use the partial pressures of the gases and make sure the units we use as a result also take that into inclusion. For Kc we use that when we are trying to determine concentration rather than the pressure.

Posted: **Sun Mar 15, 2020 7:37 am**

It depends on if the give you pressures or concentrations. I am pretty sure that you can use both if its a gas though.

Posted: **Sun Mar 15, 2020 7:41 am**

You use Kp for gases if you use partial pressure for gases. You can also change pressure to concentration using PV=nRT and in that case, you would use Kc since you’re now using concentrations.

Posted: **Sun Mar 15, 2020 7:42 am**

Kc is used if you are given concentrations and Kp is used when you are given partial pressures.

Posted: **Sun Mar 15, 2020 7:45 am**

Kc is used when you are given concentrations, and Kp is used when you are given partial pressures of gases. When Kp is given, the products and reactants are both gases. When Kc is given, the products and reactants can consist of gases or aqueous solutions

Posted: **Tue Mar 17, 2020 10:57 pm**

Kc is used when molarity is given. When it comes to gas you only use Kp when given partial pressures or asked to convert to partial pressures.

Posted: **Mon Jan 04, 2021 11:48 pm**

You use Kc when having the molar concentrations of the particles and Kp for when given the pressures (I think only gases have kp values).

Posted: **Tue Jan 05, 2021 10:55 pm**

Depends on what the problem gives you. If it gives you units in bar/atm, it will most likely be Kp. If it gives you units in M or x moles in x L, it will probably be Kc.

Posted: **Wed Jan 06, 2021 6:57 pm**

Kc should be used when questions give the concentration in mol/liter and Kp is for partial pressure. However, there is an equation to convert between the two if need be.

Posted: **Thu Jan 07, 2021 2:21 pm**

If given a chemical equation and some of the reactants and products are in a solution (designated by aq), then you must use Kc. This makes sense as you are dealing with solutions and in a lab setting, concentration is important to know. (you wouldn't randomly measure/convert the reagents to P). If given a chemical equation and all the reactants/products inputted into the K equation are gases, then it is standard to use partial pressure. In a lab, it's more natural to find the pressure of a gas rather than its concentration.

Posted: **Thu Jan 07, 2021 2:32 pm**

Use Kp when all products and reactants are in the gas phase, and you Kc in any other circumstance. But remember, you never include solids and liquids when determining the equilibrium constant, only molecules in aqueous or gaseous form.

Posted: **Thu Jan 07, 2021 4:33 pm**

yeah as all the others said you would use Kc for concentration and Kp for pressure

Posted: **Thu Jan 07, 2021 4:35 pm**

It depends on what the question gives you and what it is asking for. If it gives you the concentration of a reactant or product, use Kc. If it gives you a pressure, use Kp.

Posted: **Thu Jan 07, 2021 5:04 pm**

when the reactants and products use Kp which is for partial pressure and use Kc when the reactants or products are in aqueous form. when the reactants or products are solids or liquids we disregard them.

hope this helps!! :)

Posted: **Thu Jan 07, 2021 6:31 pm**

Kc is an equilibrium constant for gases and is found through the use of partial pressure; Kp is an equilibrium constant which is found through the use of concentrations

Posted: **Thu Jan 07, 2021 7:01 pm**

Kc is used for concentrations while Kp is used for pressures. If you are given the concentrations of reactants and products, use Kc. If you are given partial pressures of reactants and products, use Kp. It's also important to note that you should use Kp if all products and reactants in the reaction are in the gas phase.

Posted: **Thu Jan 07, 2021 7:05 pm**

Yeonjoo Kim 2B wrote:Kc is an equilibrium constant for gases and is found through the use of partial pressure; Kp is an equilibrium constant which is found through the use of concentrations

Someone correct me if I am wrong, but I believe it is switched! Kp is the equilibrium constant used for when all products and reactants are gases while Kc is related to concentration and is used for reactions that have products and reactants in aqueous phases.

Posted: **Thu Jan 07, 2021 7:31 pm**

If the problem gives partial pressures of reactants or products or it asks for the partial pressures, use Kp. Otherwise use Kc. Also in lecture, Lavelle talks about the way to get the conc of a gas from knowing its partial pressure. So if need be you could do that.

Posted: **Thu Jan 07, 2021 7:35 pm**

Kelly Ha 1K wrote:
Yeonjoo Kim 2B wrote:Kc is an equilibrium constant for gases and is found through the use of partial pressure; Kp is an equilibrium constant which is found through the use of concentrations

Someone correct me if I am wrong, but I believe it is switched! Kp is the equilibrium constant used for when all products and reactants are gases while Kc is related to concentration and is used for reactions that have products and reactants in aqueous phases.

I agree. Kc is for equilibrium constant found from concentrations, while Kp is the equilibrium constant found using the partial pressures.

Posted: **Thu Jan 07, 2021 8:53 pm**

Kc refers to concentration while Kp refers to pressure. You calculate either or depending on what values you were given in the problem.

Posted: **Thu Jan 07, 2021 8:55 pm**

I agree with the post above regarding the use of Kp with gases and Kc with concentration. You can also make note if you were given the concentration whether if it's in MOLS (Kc) or BARS (Kp).

Posted: **Fri Jan 08, 2021 8:25 pm**

You use Kp when you are dealing with gases, but use Kc when you are dealing with molarity.

Posted: **Fri Jan 08, 2021 11:00 pm**

Hi! I was also confused by this initially because in AP chem we brushed over this concept pretty quickly. But the difference is mainly that Kp deals with partial pressures of gases while Kc is in consideration of the concentrations of gases. Both are equilibrium constants of gas though.

Posted: **Fri Jan 08, 2021 11:16 pm**

To me Kc makes the most sense so I would tend to use it, but when a problem dealing with gases explicitly gives you partial pressures(atm) us Kp:)

Posted: **Sat Jan 09, 2021 12:40 am**

Like others have said above, it depends on the problem given. Sometimes, an equation would only contain gases, yet they would calculate Kc. Typically, the problem would hint whether they want you to solve for Kc or Kp based on the units given.

Posted: **Sat Jan 09, 2021 1:58 am**

kp is used if it is calculated using partial pressure while kc is used if it is calculated using concentration

Posted: **Sat Jan 09, 2021 12:08 pm**

Leila_4G wrote:From what I understand, pressures can be used with Kc or Kp but concentrations can only be used with Kc. Right?

Yes that's right. Pressures can be used with either Kc or Kp and is dependent on the information provided in the problem. Concentrations can only be used with Kc and the concentrations required to solve the problem will be given.

Posted: **Sat Jan 09, 2021 1:04 pm**

If you are using concentrations, then use Kc. If you are using pressures, then use Kp.

Posted: **Sat Jan 09, 2021 1:16 pm**

The version of K used is dependent on how the value of K is calculated. If you used concentration of products and reactants to calculate K, then use Kc to denote this. If you used partial pressures to calculate K, them use Kp.

Posted: **Sat Jan 09, 2021 1:21 pm**

Kc and Kp are both equilibrium constants for mixtures of gases, but Kc depends on molar concentrations whereas Kp depends on the partial pressures of gasses inside a closed system.

Posted: **Sat Jan 09, 2021 3:11 pm**

Kc is used for concetrations while Kp is used for partial pressurse.

Posted: **Sat Jan 09, 2021 3:17 pm**

After reviewing the question you will have a better sense if Kc or Kp is being used. By definition, Kc is used when working with concentrations and Kp is used when working with partial pressure. It is important to note that Kp only applies to reactions with all (R) and (P) molecules in the gas phase.

Posted: **Sat Jan 09, 2021 3:50 pm**

If the question does not specifically indicate which K is wants, look at the units given. If they are pressure units like bar, you are finding Kp... if not, find Kc

Posted: **Sat Jan 09, 2021 4:19 pm**

From my understanding, Kp is when all the products and reactants are gases. If the reaction is heterogenous, then you would use Kc.

Posted: **Sat Jan 09, 2021 4:31 pm**

You would use Kp.

Posted: **Sat Jan 09, 2021 4:59 pm**

Kc is always used when you are given the molar concentrations or the moles of the products or reactants. Kp is used when you have only gases, or if, in the equation, it mentions atm/bar for the units.

Posted: **Sat Jan 09, 2021 5:36 pm**

Kc is for molarity
Kp is more atms/bars

Posted: **Sat Jan 09, 2021 10:47 pm**

if they are all gases, you would most likely use Kp because gases aren't usually given in concentration. It also depends on the units you are given, so if you are given a value in concentration, you can convert everything to mol/l using the ideal gas equation PV=nRT

Posted: **Sat Jan 09, 2021 11:17 pm**

Kc and Kp are the equilibrium constants of gaseous mixtures. However, the difference between the two constants is that Kc is found by obtaining by molar concentrations, whereas Kp is defined by obtaining the partial pressures of the gasses inside a closed system.

Posted: **Sun Jan 10, 2021 10:19 am**

Hi! The difference is that you use Kc with concentrations/aqueous mixtures and Kp is used for pressure/ gases.

Posted: **Sun Jan 10, 2021 1:44 pm**

The c in Kc refers to concentration and the p in Kp refers to pressure so for equations referring to gases you would use Kp as it refers to the pressure of the gas.

Posted: **Sun Jan 10, 2021 1:53 pm**

Kp is for homogeneous gaseous reactions. Otherwise you Kc. According to UA session: he will specify whether to use Kp or Kc when both are applicable.

Posted: **Sun Jan 10, 2021 1:55 pm**

When the problem contains all gases, you can use Kp if the pressure measurements are given. If you are given measurements for concentration rather than pressure, however, it is okay to use Kc.

Posted: **Sun Jan 10, 2021 2:29 pm**

You only use Kp for gases/calculating partial pressure

Posted: **Sun Jan 10, 2021 2:43 pm**

Kc is used for problems with molarity and Kp for calculating pressures of gas.

Posted: **Sun Jan 10, 2021 2:50 pm**

Hi! It really depends on what units are given to you and the context of the problem. But generally speaking, you would use Kp when working with gases because they are the only ones that have partial pressures. If you need to convert, use the ideal gas law (PV = nRT) to determine the concentration when given atm/bar/pa or to determine pressure when given something in moles or molarity. I hope this helps!

Posted: **Sun Jan 10, 2021 2:59 pm**

I believe Kp is used for gasses, but if conversion is necessary using PV = nRT is really helpful! n/V being concentration.

Posted: **Sun Jan 10, 2021 3:22 pm**

like others have previously said, the way you can tell if you use Kc or Kp is by looking at the units you are given in the equation. Kc is when you use mols whereas Kp is when you see atms/bars

Posted: **Sun Jan 10, 2021 5:34 pm**

You use Kp when all the reactants and products are gases. If the reactants and products are given in molar concentration, then you should use Kc.

Posted: **Sun Jan 10, 2021 5:35 pm**

if everything (rectants/products) are in gaseous states then KP is used, otherwise KC

Posted: **Sun Jan 10, 2021 5:58 pm**

You would use Kc if the moles are given, and if the partial pressures are given, use Kp. If both are given then you would have to use the Ideal Gas Law along with given information to convert between the two. Hope this helps!

Posted: **Sun Jan 10, 2021 6:24 pm**

Well to begin it has to be said that Kc refers to concentration and Kp is referring to pressure. With that, the first note is that Kp sorely can be used for gases. Whereas, Kc can be used universally. so one asks when do you pick which one? Well, remember it is based on the equation states and what you are given. If they give pressure such barr or atm and all are gases then it makes sense that you are going to be finding Kp. If you are given molarity well then it makes sense you will be finding concentration and be looking for Kc.

Posted: **Sun Jan 10, 2021 6:36 pm**

ayushibanerjee06 wrote:You use Kp when all the products and reactants are gases and the question includes units like bar/atm/pa. You use Kc when the products and reactants are given in moles or molarity, even if the products and reactants are gases. If the question asks for Kp and the question only gives you information about moles/molarity, use the ideal gas law and solve for pressure (P= (n/v)RT). Vice versa if they ask for Kc and they only give you information about pressure. Also, keep in mind there are different gas constants, so use the one that fits the units you have. Hope this helps.

This was very helpful thank you!

Posted: **Sun Jan 10, 2021 8:01 pm**

Kc is used when given concentration, and Kp is used when given partial pressures.

Posted: **Sun Jan 10, 2021 8:22 pm**

Kp should be used when dealing with gases. Hope this helps!

Posted: **Sun Jan 10, 2021 8:36 pm**

Use KP when you have anything to do with pressure. If you see any given partial pressure you want to use KP while KC is for varying molarity/ amount of the product/reactants you have.

Posted: **Sun Jan 10, 2021 8:45 pm**

Kp is used when the reactants and products are in gaseous form (make sure they're all in the same units before you input into the formula!). Kc is used usually when the reaction occurs in solution, so you use the concentrations of aqueous reactants and products. Be sure not to include liquid or solids in the equation! Hope this helps!

Posted: **Sun Jan 10, 2021 9:12 pm**

Kp just indicated that you used partial pressures (gases) in your equilibrium equation, while Kc indicated that you used concentrations in the equilibrium equation.

Posted: **Sun Jan 10, 2021 9:15 pm**

Thank you to everyone for the explanations, I was also having trouble with this!

Posted: **Sun Jan 10, 2021 10:29 pm**

One thing I wasn't completely clear on is when we calculate Kc our notation for setting up the equation should use the brackets to denote concentration but when we calculate Kp we use "P" to denote partial pressure?

Posted: **Sun Jan 10, 2021 10:45 pm**

You use Kc when the concentrations of reactants and products are given, and you use Kp when partial pressures are given

Posted: **Sun Jan 10, 2021 10:49 pm**

Also will we need to have the formula that converts Kc to Kp memorized? Or will that be given to us on an equations sheet?

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