Examples of constant-pressure calorimeter in the following topics:
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- A constant-pressure calorimeter measures the change in enthalpy of a reaction at constant pressure.
- A constant-pressure calorimeter measures the change in enthalpy of a reaction occurring in a liquid solution.
- In that case, the gaseous pressure above the solution remains constant, and we say that the reaction is occurring under conditions of constant pressure.
- A simple example of a constant-pressure calorimeter is a coffee-cup calorimeter, which is constructed from two nested Styrofoam cups and a lid with two holes, which allows for the insertion of a thermometer and a stirring rod.
- A styrofoam cup with an inserted thermometer can be used as a calorimeter, in order to measure the change in enthalpy/heat of reaction at constant pressure.
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- Constant-volume calorimeters, such as bomb calorimeters, are used to measure the heat of combustion of a reaction.
- A bomb calorimeter is a type of constant-volume calorimeter used to measure a particular reaction's heat of combustion.
- As such, bomb calorimeters are built to withstand the large pressures produced from the gaseous products in these combustion reactions.
- Since the volume is constant for a bomb calorimeter, there is no pressure-volume work.
- Thus, the total heat given off by the reaction is related to the change in internal energy (ΔU), not the change in enthalpy (ΔH) which is measured under conditions of constant pressure.
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- Gibbs free energy measures the useful work obtainable from a thermodynamic system at a constant temperature and pressure.
- When a system changes from an initial state to a final state, the Gibbs free energy (ΔG) equals the work exchanged by the system with its surroundings, minus the work of the pressure force.
- Gibbs energy (also referred to as ∆G) is also the chemical potential that is minimized when a system reaches equilibrium at constant pressure and temperature.
- As such, it is a convenient criterion of spontaneity for processes with constant pressure and temperature.
- Therefore, Gibbs free energy is most useful for thermochemical processes at constant temperature and pressure.
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- For gas-phase reactions, the equilibrium constant can be expressed in terms of partial pressures, and is given the designation KP.
- For gas-specific reactions, however, we can also express the equilibrium constant in terms of the partial pressures of the gases involved.
- Our equilibrium constant in terms of partial pressures, designated KP, is given as:
- Note that in order for K to be constant, temperature must be constant as well.
- Therefore, the term RT is a constant in the above expression.
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- The enthalpy of reaction measures the heat released/absorbed by a reaction that occurs at constant pressure.
- When you run a chemical reaction in a laboratory, the reaction occurs at constant pressure, because the atmospheric pressure around us is relatively constant.
- We will examine the change in enthalpy for a reaction at constant pressure, in order to see why enthalpy is such a useful concept for chemists.
- Thus, at constant pressure, the change in enthalpy is simply equal to the heat released/absorbed by the reaction.
- An explanation of why enthalpy can be viewed as "heat content" in a constant pressure system.
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- Boyle's Law describes the inverse relationship between the pressure and volume of a fixed amount of gas at a constant temperature.
- Remember that these relations hold true only if the number of molecules (n) and the temperature (T) are both constant.
- What is the final pressure of the gas?
- The moving wall converts the effect of molecular collisions into pressure and acts as a pressure gauge.
- An animation of Boyle's Law, showing the relationship between volume and pressure when mass and temperature are held constant.
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- Charles' and Gay-Lussac's Law states that at constant pressure, temperature and volume are directly proportional.
- A visual expression of Charles' and Gay-Lussac's Law is shown in a graph of the volume of one mole of an ideal gas as a function of its temperature at various constant pressures.
- The plots show that the ratio $\frac{V}{T}$ (and thus $\frac{\Delta V}{\Delta T}$) is a constant at any given pressure.
- This model contains gas molecules on the left side and a barrier that moves when the volume of gas expands or contracts, keeping the pressure constant.
- A visual expression of the law of Charles and Gay-Lussac; specifically, a chart of the volume of one mole of an ideal gas as a function of its temperature at various constant pressures.
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- Osmotic pressure is the pressure needed to nullify the effects of osmosis and is directly influenced by the amount of solute in the system.
- Osmotic pressure is the pressure that needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane.
- Osmotic pressure can also be explained as the pressure necessary to nullify osmosis.
- The osmotic pressure is the pressure required to achieve osmotic equilibrium.
- Here, i is the van 't Hoff factor, M is the molarity of the solution, R is the gas constant, and T is the absolute temperature in Kelvin.
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- Imagine a container where the pressure is
increased.
- where P is the pressure, V is the volume, R is the universal gas constant, and T is the absolute temperature.
- Isotherm (plots of pressure versus volume at constant temperature) can be produced using the van der Waals model.
- The constants a and b have positive values and are specific to each gas.
- The term involving the constant a corrects for intermolecular attraction.
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- Increasing pressure will increase the solubility of a gas in a solvent.
- Typically, a gas will increase in solubility with an increase in pressure.
- In this equation, C is the concentration of the gas in solution, which is a measure of its solubility, k is a proportionality constant that has been experimentally determined, and Pgas is the partial pressure of the gas above the solution.
- The proportionality constant needs to be experimentally determined because the increase in solubility will depend on which kind of gas is being dissolved.
- Henry's law does not apply to gases at extremely high pressures.