What is Constant pressure: Definition and 84 Discussions

The molar gas constant (also known as the gas constant, universal gas constant, or ideal gas constant) is denoted by the symbol R or R. It is the molar equivalent to the Boltzmann constant, expressed in units of energy per temperature increment per mole, i.e. the pressure–volume product, rather than energy per temperature increment per particle. The constant is also a combination of the constants from Boyle's law, Charles's law, Avogadro's law, and Gay-Lussac's law. It is a physical constant that is featured in many fundamental equations in the physical sciences, such as the ideal gas law, the Arrhenius equation, and the Nernst equation.
The gas constant is the constant of proportionality that relates the energy scale in physics to the temperature scale and the scale used for amount of substance. Thus, the value of the gas constant ultimately derives from historical decisions and accidents in the setting of units of energy, temperature and amount of substance. The Boltzmann constant and the Avogadro constant were similarly determined, which separately relate energy to temperature and particle count to amount of substance.
The gas constant R is defined as the Avogadro constant NA multiplied by the Boltzmann constant k (or kB):





{\displaystyle R=N_{\rm {A}}k.}
Since the 2019 redefinition of SI base units, both NA and k are defined with exact numerical values when expressed in SI units. As a consequence, the SI value of the molar gas constant is exactly 8.31446261815324 J⋅K−1⋅mol−1.
Some have suggested that it might be appropriate to name the symbol R the Regnault constant in honour of the French chemist Henri Victor Regnault, whose accurate experimental data were used to calculate the early value of the constant. However, the origin of the letter R to represent the constant is elusive. The universal gas constant was apparently introduced independently by Clausius’ student, A.F. Horstmann (1873)
and Dmitri Mendeleev who reported it first on Sep. 12, 1874.

Using his extensive measurements of the properties of gases,

he also calculated it with high precision, within 0.3% of its modern value.

The gas constant occurs in the ideal gas law:





{\displaystyle PV=nRT=mR_{\rm {specific}}T}
where P is the absolute pressure (SI unit pascals), V is the volume of gas (SI unit cubic metres), n is the amount of gas (SI unit moles), m is the mass (SI unit kilograms) contained in V, and T is the thermodynamic temperature (SI unit kelvins). Rspecific is the mass-specific gas constant. The gas constant is expressed in the same units as are molar entropy and molar heat capacity.

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  1. chocopanda

    Mixing two ideal gases with different V, T at constant pressure

    To be honest, thermodynamics is really not my strong suit and I get confused when and how to apply formulas. My thought process is as follows: - there are two ideal gases (ideal gas law applies) - the pressure remains constant (isobaric process), so p1= p2 = p - I imagine there being two...
  2. bruhtation

    B Gas cylinder at constant pressure being exposed to atmosphere, how?

    im learning thermodynamics and currently in a lesson about thermal processes. one process has constant pressure and before diving into equations or any proof the book provides a figure of a gas cylinder. the cylinder has a movable piston/lid on one side. the book then says "...and the piston end...
  3. P

    Calculating Work Done by Gas at Constant Pressure

    A gas with a volume of 8m^3 with a temperature of 400K gets warmed up to 550K with a constant pressure of 200Kpa. How much work has the gas done to the environment? I think i might need to use the ideal gas law for this which is: ( P * V / T = N * K ) Where V is volume, T is temperature and N...
  4. questionmonkey123

    Constant pressure process (piston) on gas violates Newtons 2nd law?

    I often see this set up in thermodynamic problems and need clarification on how Newton's Laws are involved for the piston: Gas inside a piston cylinder (1) is heated expanding the gas and raising the piston (initially at rest) to a height (2) in a constant pressure quasi-equilibrium process...
  5. Krokodrile

    Chemistry Work, temperature and energy in a constant pressure process

    So, the Cp and Cv its very confusing for me. But, i understand what's its happening in this process, so, i use the logic and first i obtain a ecuation for obtain the final temperature ecuaticon: Q=m*C*△T Q=m*C*(T2-T1) T2=(Q+T1)/(m*C) If the process its in constant pressure, i use the Cp valor...
  6. Chestermiller

    Thermodynamics: Constant Pressure Tank Heating Problem

    I found this interesting thermodynamics problem on another site. I thought PF members might find it challenging to attack. I'm not asking for help since I've already solved it. So pease feel free to present your entire solution if you desire. Chet
  7. J

    Why constant pressure over different shapes produces the same force?

    Left and right piston produce same vertical force.On both piston, side force is zero,how this is possible if left piston don't has simetrical shape? Why shape of object is irrelevant when fluid has constant/same pressure around object? How can I prove with integration of pressure over left...
  8. P

    Constant Pressure Specific Heat in terms of Entropy and Enthelpy

    If ##N## is constant (per the partial derivatives definitions/ the subscripts after the derivatives) then ##G## is constant ##H - TS = constant## Taking the derivative of both sides with respect to ##T## while holding ##N,P## constant we get the following with the use of the product rule...
  9. Baibhab Bose

    Final temperature of an adiabatic process between two reservoirs

    *Here, no mention of these reservoirs being a gas, so I'm not sure if I can use the PV=nRT or the P*V^(gamma)=K equation. SO I am left with only the 1st law. If I can write dQ1( going out from object 1)= Cp (indep of T)*(Tf-T1) dQ2(coming into the object 2)= Cp*(T2-Tf)...
  10. jybe

    Constant pressure heat of reaction -> constant volume q

    Homework Statement For the reaction below, the constant pressure heat of reaction is qp = −3256 kJ mol−1 at 25 °C. What is the constant volume heat of reaction, qV , at 25 °C? 16 CO(g) + 33 H2(g) ⟶ C16H34(l) + 16 H2O(l) Enter your answer in kJ mol−1, rounded to the nearest kilojoule...
  11. TheExibo

    How can heat change be measured under constant pressure?

    So enthalpy is the heat content of a system at constant pressure. Enthalpy change is equal to the heat absorbed or evolved by the system at constant pressure. If my understanding is correct, a system whose temperature goes up will return back to that starting temperature if pressure is kept...
  12. C

    Heat released during combustion at constant pressure vs volume

    Greetings! I've been brushing up on some thermodynamics recently and came across a perplexing sentence in my notes and text from undergrad. It says that for a combustion reaction, such as the combustion of heptane: C7H16 (l) + 11O2 (g) ---> 7CO2 (g) + 8H2O (l) That this process carried out at...
  13. S

    When does gas do more work: constant pressure or temperature?

    A quantity of ideal gas undergoes an expansion that doubles its volume. Does the gas do more work on its surroundings if the expansion is at constant pressure or at constant temperature? The answer in the book says W at constant temperature gives a greater value for the given case. But when I...
  14. yecko

    Specific heat at constant pressure formula help

    Homework Statement https://i.imgur.com/f2vSXtq.png Homework Equations https://i.imgur.com/Kjy1Tzh.png The Attempt at a Solution In this question, the pressure is different at different point, in other words it is not constant throughout the system. Why the solution use c(p) (or "enthalpy" h...
  15. chopnhack

    Gas at a constant pressure: using Cv.dT correctly

    Hello, my questions is not so much homework, but a request for a definition. When we use Cv*dT to solve for dU (internal energy) in a constant pressure example, what is the order of the temperatures entered into dT? Is one to assume its final temp minus initial temp? I ask because it leads to...
  16. M

    Drawing constant pressure lines in flow field

    Hi PF! Can someone help me understand how to draw lines of constant pressure in an inviscid flow field, say flow around a cylinder. I am having trouble understanding how to draw these. Any help is greatly appreciated! I ask this question because I am preparing for the Q exam for PhD and one of...
  17. person123

    Curved Tube For Constant Pressure Increase?

    My design concept stems from the fact that the pressure in a tube increases exponentially based on the length of the tube compressed. I would like to design a tube that increases pressure at a more constant rate. I would imagine that a tube curved inward toward the base would achieve this. As...
  18. G

    Mass flow rate through a regulator

    Hi, We have a canister at a set pressure (4500psi) full of compressed air. A single stage pressure regulator is fitted to the canister that reduces the outlet pressure to 850psi. The flow from the regulator is then used in a divergent nozzle to create a thrust force capable of lifting a small...
  19. J

    Heat capacity under constant pressure or volume question

    HOMEWORK POSTED IN WRONG FORUM, SO NO TEMPLATE I have encountered a problem at the university in which there is a thermally isolated container of constant volume in which the number of particles and temperature change with time(the temperature increases). The change in particle number ensures...
  20. I

    Constant pressure specific heats when temperature changes

    Homework Statement Im trying to understand what would be the correct approach for calculating the constant pressure specific heat for an ideal gas undergoing a process where the temperature is changing. The reason I am asking is because the equation used to calculate Cp0 is dependent on the...
  21. henrco

    Ideal Gas compressed at constant pressure

    Hi, Could I please get some guidance on my approach and solution, for this particular problem. Any assistance welcome. 1. Homework Statement An ideal gas is compressed at a constant pressure of 1.3 atm from a volume of 20 L to 12 L. During this process it gives off 3.69 kJ of heat. What is...
  22. F

    What is meant by heat capacity at constant pressure?

    Generally in chemistry and physics, we learned that a material has a heat capacity at constant pressure. However, I am confused at whether this refers to a constant external pressure or a constant system internal pressure? Thanks.
  23. C

    Work of isothermal compression at constant pressure

    The Problem: A 1.80 mol sample of an ideal gas is compressed isothermally from 58.1 L to 15.6 L using a constant external pressure of 2.72 atm . Calculate W: Calculate Q: Calculate ΔU: Attempt at solution: I tried W = P(ΔV) to no effect, W = nRTln[Vf/Vi] to no effect. The online...
  24. S

    Pressure washers constant pressure

    My GF and I got into a petty argument about pressure washers. She believes that the constant back-pressure you feel holding a pressure wand increases the closer you hold it to a wall (for example). Where as I believe that if the wand had a constant psi, with outward pressure, wanting to force...
  25. O

    Fluid Mechanics: Constant Pressure and Incompressible flow

    Assuming a flow can be idealised as incompressible, then can you use the constant pressure assumption ? I just want to get my understanding clear. My problem is the following.Consider a fluid element with volume ##V## and a fixed number of molecules. If the flow is incompressible, then the...
  26. DocZaius

    How Safe Is The Constant Pressure Assumption For Enthelpy?

    I am curious about a statement that is usually tacked onto the end of a derivation showing enthalpy with constant pressure being equal to the heat into the system. First Law of Thermodynamics \Delta U = Q - W_{by} Define Enthalpy and look at its change: H=U+PV \\ \Delta H = \Delta U +...
  27. S

    Specific Heat at constant pressure for photon gas

    Homework Statement A blackbody photon gas is contained within an evacuated cavity (V = 0.01 m^3). Calculate C_p for the photon gas at T = 1000K Homework Equations C_p - C_v = T(\frac{\partial S} {\partial V}) (\frac{\partial V}{\partial T}) C_v = T(\frac{\partial S} {\partial T}) S =...
  28. kq6up

    Enthelpy for Constant Pressure and Constant Volume Systems?

    I understand why entropy (a state function) is very useful in chemistry. Since most chemistry systems are at constant pressure, ##\Delta H=Q_v## from the derivation below: ##H=U+PV## ##dH=dU+dPV+PdV## ##dU=\delta Q-\delta W## ## dU=\delta Q-PdV## ## dH=\delta Q+dPV## ##dPV=0## ## dH=\delta...
  29. MexChemE

    Exothermic reaction in adiabatic reactor

    Homework Statement One mole of gas A, two moles of gas B, and one mole of inert gas I are fed into an adiabatic reactor of variable volume and constant pressure at 25 °C. At this temperature, the reaction yielding liquid R proceeds normally as: \textrm{A} (g) + \textrm{B} (g) \rightarrow...
  30. MexChemE

    Adiabatic expansion against constant pressure

    Homework Statement a) For a certain ideal gas CP = 8.58 cal mol-1 K-1. We have 2 moles of said gas at 293.15 K and 15 atm. Calculate the final volume and temperature when the gas expands adiabatically and reversibly until it reaches a pressure of 5 atm. (Answers: V = 7.45 L; T = 227.15 K) b)...
  31. R

    Thermodynamics Constant Pressure Process

    Hi, I found a derivation for heat transfer in a constant pressure process. It goes as follows: Q + W = u2 - u1 Q = u2 - u1 + p(v2 - v1) Since h = u + pv, Then Q = h2 - h1 The first equation states that the sum of heat and work done is equal to the change in internal energy, I can...
  32. S

    Why does heat addition happen at constant pressure in diesel cycle ?

    Why does heat addition happen at constant pressure in diesel cycle ? what is the basic concept behind it?? please do enlighten me on this topic. thanks in advance
  33. Soumalya

    Constant pressure or isobaric expansion of gas

    It is said that in an isobaric expansion of a gas pressure remains constant throughout the expansion process. Suppose we have a quantity of gas at initial pressure P1 and volume V2 in a piston cylinder arrangement.We heat it slowly such that it expands to obtain a state with pressure P2 and...
  34. K

    Heat Capacity at Constant Pressure

    Homework Statement I have an ideal gas of n=1023 point particles with a constant pressure of P = 3x105 Pa as 200J of heat flows into it. What is the heat capacity at constant pressure, CP? Homework Equations CV = (ΔU + PΔV)/ΔT The Attempt at a Solution Okay... So I have N, P and Q...
  35. N

    Change in internal energy at constant pressure?

    Change in internal energy at constant pressure?? Hello everybody, I am stuck with a concept in thermodynamics. We know that ΔU=CvΔT. But what if we want the change in internal energy at constant pressure? ΔU→Change in internal Cv=heat capacity at constant volume
  36. H

    Adiabatic Expansion: Constant pressure and temperature

    I'm new to the forum, so please be kind. I was reading through my pchem textbook, and I noticed something. We're given the equation: ΔU = q + w For an adiabatic expansion, we're told that q = 0. Fair enough, no heat transfer. But when there is a constant T and change in V, my book...
  37. F

    Non-flow process with steam (Constant Pressure) Part II

    In a non-flow process, 0.6kg of steam at a pressure of 10 bar and a temperature of 235°C is cooled at a constant pressure in a cylinder. After 45 minutes the cylinder contains 0.12 kg of saturated water and 0.48kg of dry saturated steam. Calculate (a)the change in internal energy, the work...
  38. F

    Non-flow process with steam (Constant Pressure)

    Question 1 In a closed system, 0.6kg of wet steam at a pressure of 5 bar and a dryness fraction of 0.2 is heated at constant pressure until the volume occupied is 0.18m3. (A)Calculate the change in internal energy, the work energy and the heat energy transferred during the process. Question 2...
  39. S

    Does a pressure regulator maintain constant pressure drop?

    I wanted to know the function of a pressure regulator.. Am using one to control liquid flow rate. Here is my understanding- A pressure regulator can be connected across a valve. The pressure regulator maintains a constant "pressure drop" across the valve so if i set it to give a drop...
  40. D

    Finding molar specific heat at constant pressure

    Homework Statement Let 25.6 J of heat be added to a particular ideal gas. As a result, its volume changes from 41.0 to 82.0 cm3 while the pressure remains constant at 1 atm(= 101 kPa). a) By how much did the internal energy of the gas change? -- 21.5 J got this part b) What is the molar...
  41. W

    Calculating percentage change in volume at constant pressure.

    Homework Statement The coefficient of the thermal expansion of ethanol is given by: \alpha (per degree Celsius) = 1.0414 x 10^(-3) + 1.5672 x 10^(-6) t + 5.148 X 10^(-8) t^2 Calculate the percentage change in volume when ethanol is heated from 0 to 100 (celsius) at constant pressure...
  42. M

    Gas heated under constant pressure - thermal energy transferred?

    Homework Statement A 1.0 sample of nitrogen gas (diatomic)is heated at constant pressure from 300K to 420K, Cp (nitrogen) = 29.1 Jmol-1°C-1 The thermal energy transferred to the gas is? 1. 8370 J 2. 12,200 J 3. 34.90 J 4. 3590 J 5. 12.20 J Homework Equations KE = 1/2mv2 =...
  43. S

    Constant pressure in a heated syringe

    Homework Statement When a syringe with its end sealed is heated, the piston is pushed back such that the gas pressure in the syringe remains the same. Since the force exerted is dependent on 1) frequency of collision with the walls and 2) the force of the collisions. So comparing before and...
  44. S

    Basic Thermodynamics; Change in U at Constant Pressure

    Homework Statement What is the ΔU° at 25°C for the following reaction at constant pressure: C_{2}H_{2} (g) + 5/2O_{2} (g) \rightarrow 2CO_{2} (g) + H_{2}O (g) ΔH° = -1299.5kJ Homework Equations ΔU = Q ± W PV = nRT W = PΔV The Attempt at a Solution Since we have constant pressure, ΔH° =...
  45. F

    Physics: Expansion at constant pressure

    Homework Statement An unknown number of moles of an ideal monoatomic gas expand reversibly from Vi = 2.30 m3 to Vf = 3.10 m3, at a constant pressure of 1.71 atm and an initial temperature of 300 K. Find the number of moles of gas. Find the final temperature of the gas K. Calculate the...
  46. F

    Physics: Expansion at constant pressure

    An unknown number of moles of an ideal monoatomic gas expand reversibly from Vi = 2.30 m3 to Vf = 3.10 m3, at a constant pressure of 1.71 atm and an initial temperature of 300 K. Find the number of moles of gas. Find the final temperature of the gas K. Calculate the work done by the gas...
  47. D

    Why are liquid surfaces at constant pressure?

    This is probably a stupid question with a simple answer, but please bear with me. Suppose we have a bucket of water that we begin to spin at a constant angular speed. My textbook asked to find the shape of the surface of water, with the hint that the surface would be a surface of constant...
  48. P

    Thermodynamics-why is this constant pressure?

    A piston-cylinder device contains 0.85kg of refrigerant at -10 deg C. The piston that is free to move has a mass of 12kg and diameter of 25cm. the local atmospheric pressure is 88kPa. Now, heat is transferred to the refrigerant-134a until the temperature is 15 deg C. Determine the final...
  49. J

    Gibbs free energy change at constant pressure is zero?

    gibbs free energy change at constant pressure is zero?? IS gibbs free energy change at constant pressure zero? ΔS = q / T. At constant pressure q = ΔH so ΔS = ΔH / T So ΔG = ΔH - ΔH/T . T = 0 I think i am wrong but where? My friend tells me that q involved in enetropy is...
  50. W

    Work done by gas under constant pressure

    Hi, just wanted to ask a simple question: In the following equation: Work done by gas = Pressure x Change in volume does pressure refer to the pressure of the gas (that is doing the work), or does it refer to the external pressure (i.e. pressure of the surrounding gas)? I've heard...