Phase Changes

[906023369]

What formula do you use for phase changes?

[Wesley J the BioChem Kid]

Depends on what you're trying to solve for. But typically during a phase change you use the equation delta G = delta H - T delta S with the idea that delta G is equal to zero during a phase change and thus T delta S = delta H.

[805737147]

What formula do you use for phase changes?

During the actual phase change, we usually use q = mCAT to determine the heat change. When there is a plateau, we find the change in enthalpy.

[Ojas Sharma 2G]

In addition to the other responses, most questions give you a value for heat per mol and you can use the information from the problem to determine how much heat is necessary to complete that phase change.

[Andrew Greenberg 2D]

If you were to look at the heating curve graph, the flat horizontal segments use the enthalpy of fusion and vaporization and the vertical segments use the q=n*c*deltat, c being the heat capacity of whatever state the substance is in.

[Luke Smith]

What formula do you use for phase changes?

I would say that the majority of the time for phase changes we use the formula q=nC(delta)T. With this formula you could potentially solve for either heat, moles of the substance, the specific heat capacity, or the temperature change depending on what the problem is asking for.

[Kush Brahmbhatt 2K]

Typically for a phase change, the equation you would want to use is q = mCdeltaT as this equation would represent the change in enthalpy given a change in temperature. The result would tell us the amount of energy needed to execute the phase change.

[Akilah Beatty 2k]

For phase changes we use q=mcdelta T, and add it with the q that is given with the particular phase change. For instance for fusion, we would do q+mdeltaHfus.

[005999765]

We normally use q = mCdeltaT for phase changes.

[Itxhel Jimenez 1B]

Hey, for phase changes it depends, but usually we use q=mct

[Morganne Malloy 1l]

q = mCdeltaT works for almost all of the problems but it does depend

[Brandon_G_1H]

Hello! So say we have we go from ice(-5 degrees Celsius) to a liquid (let’s say at 10 degrees Celsius) we would use the equation q=(m)(C)(deltaT) to go from -5 degrees Celsius to the phase change. For the phase change we use q=(n)(delta Hfus). We use delta Hfus because fusion is going from solid to liquid state. Then to go from the phase change to 10 degrees Celsius we would use the equation q=(m)(C)(deltaT). Then we just add all of the values together.
Hope this helps!

[Brandy_Lopez_1C]

Hi,

The formula you would use for phase changes is q=mcat. The reason for this is that you can rearrange the formula to solve for any of the components such as q(heat), m(mass), c(specific heat capacity), or t(temperature). This formula can be helpful when given questions that ask about the required heat energy in different phase changes such as solid to liquid.

Hope this helps!

[705586182]

Phase change is a physical process in which a substance goes from one phase to another. In lecture, Dr.Lavelle stated there are three physical or phase changes:

Vaporization: liquid -> vapor (endothermic)
enthalpy of vaporization,

Melting (fusion): solid -> liquid (endothermic)

Sublimation: solid -> vapor (endothermic)

The two main phase transition equations we use are for vaporization and for fusion.
vaporization: q= q=m(mass) * delta H (for vaporization)
fusion: q=m(mass) * delta H (for fusion)

For example, if a question asks how much energy is required to melt ice, then you would have to use the equation: q=m(mass) * delta H (for fusion).

For other cases, such as calculating the energy required to make a change in temperature in solid, liquid, or gas, you would use: q= n(moles or mass, it depends) C * delta T
which C stands for specific heat capacity, and delta T stands for change in temperature.

[Bradley Bohall 3L]

To find the enthalpy of a phase change, all you would do is subtract the enthalpy of reactants from the enthalpy of products. We can only do this because enthalpy is a state function.

For example, the enthalpy of vaporization would = enthalpy of vapor(product) - enthalpy of liquid(reactant)

[Ojas Sharma 2G]

for the actual phase changes, you would use the enthalpy of fusion or vaporization and for changes in temperature within the same phase you would use q = m c deltaT

[jadensy1L]

Hi! If you reference a heating curve graph, and plateaus refer to the enthalpy of fusion and vaporization whereas the sloped segments of the graph use the equation q=n*c*dealtat, where c is the heat capacity of the substance in a certain state.

[105736938 AADL]

Hi! So for the phase change itself we use the equation q=n*deltaH. The delta H you use depends on the phase change. So from solid to liquid, you use deltaH fusion, for liquid to gas you use deltaH vaporization. We use this equation because there is no change in temperature during the phase change since all the energy is being used to break the bonds. Outside of the phase change you use the equation q=mcdeltaT. I hope this helps!!

(extra credit 3/4)

[Moselle Stieler 3A]

Just checking my understanding:

The formula for phase changes would q=mcat, with q being the heat, m being mass, c heat capacity, and t for temperature. Rearranging the equation to solve for any of these variables.

[405718358]

I see a lot of people mention q=mCAT but other mention q = mCdeltaT . i personally would say that using the q=mCdeltaT would work bets because i use this equation for some of the problems that are in the homework and it has been working. unless using the q=mCAT would be easier but i have not tried it yet.

[705801855]

Hi there! So q=mcat, with q being the heat, m being mass, c heat capacity, and t for temperature is used for the increasing portion of the gas (when the substance is either completely solid, completely liquid, or completely gas). S = n delta H (fusion or vaporization) is used for the horizontal portions of the graph, where the substance is solid and liquid, or both liquid and gas. Hope this helps!