Question

One gram of water (1cm^3) becomes 1671cm^3 of steam when boiled at a constant pressure of 1atm(1.013×10^5pa). The heat of vaporization at this pressure is Lv= 2256J/g. Compute:

1. The work done by the water when it vaporizes?

2. It increases internal energy?

Answers

GPT-4o mini
To compute the work done by the water when it vaporizes and the increase in internal energy, we can use the following thermodynamic principles.

### 1. Work Done by the Water When It Vaporizes

The work done during the vaporization can be computed using the formula for work done in a constant pressure process:

\[
W = P \Delta V
\]

where \( P \) is the pressure and \( \Delta V \) is the change in volume.

- **Pressure, \( P \)**: 1 atm = \( 1.013 \times 10^5 \, \text{Pa} \)
- **Initial Volume of Water, \( V_i \)**: \( 1 \, \text{cm}^3 = 1 \times 10^{-6} \, \text{m}^3 \)
- **Final Volume of Steam, \( V_f \)**: \( 1671 \, \text{cm}^3 = 1671 \times 10^{-6} \, \text{m}^3 \)

Now calculate the change in volume, \( \Delta V \):

\[
\Delta V = V_f - V_i = (1671 \times 10^{-6} \, \text{m}^3) - (1 \times 10^{-6} \, \text{m}^3) = 1670 \times 10^{-6} \, \text{m}^3
\]

Now substitute these into the formula for work:

\[
W = P \Delta V = (1.013 \times 10^5 \, \text{Pa}) \times (1670 \times 10^{-6} \, \text{m}^3)
\]

Calculating that gives:

\[
W = 1.013 \times 10^5 \times 1670 \times 10^{-6} = 169.1711 \, \text{J}
\]

So, the work done by the water when it vaporizes is approximately:

\[
W \approx 169.17 \, \text{J}
\]

### 2. Increase in Internal Energy

The total energy change (increase in internal energy) during the phase change can be computed using the heat of vaporization. The heat added for vaporization, \( Q \), can be expressed as:

\[
Q = m L_v
\]

where:
- \( m \) is the mass of the water, \( m = 1 \, \text{g} \) (or \( 0.001 \, \text{kg} \))
- \( L_v \) (heat of vaporization) = \( 2256 \, \text{J/g} \) or \( 2.256 \times 10^6 \, \text{J/kg} \)

So the heat added is:

\[
Q = 1 \, \text{g} \times 2256 \, \text{J/g} = 2256 \, \text{J}
\]

Using the first law of thermodynamics, the increase in internal energy \( \Delta U \) for a closed system can be calculated as:

\[
\Delta U = Q - W
\]

Substituting the values obtained:

\[
\Delta U = 2256 \, \text{J} - 169.17 \, \text{J} \approx 2086.83 \, \text{J}
\]

So, the increase in internal energy is approximately:

\[
\Delta U \approx 2086.83 \, \text{J}
\]

### Summary

1. The work done by the water when it vaporizes is approximately \( 169.17 \, \text{J} \).
2. The increase in internal energy is approximately \( 2086.83 \, \text{J} \).