Entropy change when converting water to steam

In summary, the conversation discusses the conversion of 20g of water from 30C to steam at 250C under constant atmospheric pressure. The heat capacity of liquid water and the heat of vaporization at 100C are provided. The molar heat capacity of water vapor at constant pressure is also given. The task is to find the entropy change of the water. The solution involves breaking down the problem into three stages and using the equations for entropy change and heat capacity to calculate the entropy change in each stage. The heat of vaporization plays a role in one of the stages.
  • #1
doombanana
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Homework Statement



At constant atmospheric pressure, 20g of water at 30C is converted into steam at 250C. Assume the heat capacity of liquid water is constant at 4.2 J/gK and the heat of vaporization at 100C is 2260 J/g. The molar heat capacity of water vapor at constant pressure is given by

[itex]\frac{c_p}{R} = a + bT + CT^2[/itex]

where [itex] a= 3.634,{ } b= 1.195*10^{-3} K^{-1},{ } c=1.350*10^{-7} K^{-2}[/itex]

Find the entropy change of the water.

Homework Equations



[itex]\Delta S = \frac{dQ}{T}[/itex]
[itex] dQ = \int c_p dT[/itex]

The Attempt at a Solution


My first thought was just to plug in the given equation for C_p into (1) and integrating, but I'm not sure how the heat of vaporization comes into play.
 
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  • #2
There are 3 different 'stages' to this problem. You need to think of what they are, then find the entropy change that happens in each stage.
 

FAQ: Entropy change when converting water to steam

What is entropy?

Entropy is a thermodynamic property that measures the disorder or randomness of a system. In simple terms, it is a measure of how much energy is spread out or dispersed in a system.

How does entropy change when water is converted to steam?

When water is converted to steam, its entropy increases. This is because the molecules in steam are more disordered and have a higher energy state compared to the molecules in liquid water.

Why does the entropy of water increase when it is converted to steam?

As water is heated and converted to steam, the molecules gain more energy and are able to move and vibrate more freely. This leads to a higher degree of disorder and an increase in entropy.

How does the entropy change during the conversion process?

The entropy change during the conversion process can be calculated using the formula ΔS = Q/T, where ΔS is the change in entropy, Q is the heat added to the system, and T is the temperature at which the heat is added.

Can the entropy change during the conversion of water to steam be reversed?

No, the increase in entropy during the conversion process is irreversible. This is because the increase in entropy represents a permanent change in the system and cannot be reversed without the input of additional energy.

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