Answer :
Final answer:
The energy change when 78.0g of Hg melts at -38.8 degrees Celsius is 4.39 kJ.
Explanation:
When a substance undergoes a phase transition, such as melting, energy is either absorbed or released. In the case of mercury (Hg), the energy change during melting can be calculated using the heat of fusion.
The heat of fusion for mercury is 11.3 kJ/mol. This means that 11.3 kJ of energy is required to melt one mole of mercury at its melting point.
To calculate the energy change when 78.0g of mercury melts at -38.8 degrees Celsius, we need to convert the mass of mercury to moles. The molar mass of mercury is 200.59 g/mol.
First, calculate the number of moles of mercury:
Number of moles = mass / molar mass
Number of moles = 78.0g / 200.59 g/mol = 0.389 mol
Next, calculate the energy change using the heat of fusion:
Energy change = heat of fusion * number of moles
Energy change = 11.3 kJ/mol * 0.389 mol = 4.39 kJ
Therefore, the energy change when 78.0g of mercury melts at -38.8 degrees Celsius is 4.39 kJ.
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