Question

How much heat is required?

The Melting point of H₂O(s) is Oc, the boiling point of H₂O (l) is 100c, and H₂O has the following thermodynamic data:

C_m(H₂O(l)) = 75.4 J/mol*C deltH_fus = 6.01 kJ/mol

C_m(H₂O(g)) = 33.6 J/mol*C deltH_vap = 40.67 kJ/mol

How much heat is required to convert 25.0g of H₂O(s) at 0c to H₂O(g) at 115c?

Answer 1

To convert water from solid to gas, it is necessary to apply a certain amount of energy to:

1) Increase the temperature from 0 ° C to 115 ° C

2) Overcoming intermolecular forces to occur state changes, solid-liquid (fusion) and liquid-gas (vaporization)

Mathematical expressions for these calculations are:

A) Energy required to increase the temperature:

   n=mol ; T:temperature; Cm:molar heat capacity

B)Energy required for phases changes

   H:Enthalpy

.....Exercise Solution...

1) Energy required for temperature variation: 2 steps, variation 0-100°C and variation 100-115°C

1.1) 0°C to 100°C (Liquid State)

Using ecuation A

n is water moles, PM H₂O= 18g/mol

then, n=1,39 mol, replacing n in A

__________________________________________Q1= 10480,6J

1.2) 100°C to 115°C (Gaseous State)

Using ecuation A

n is water moles, PM H₂O= 18g/mol

then, n=1,39 mol, replacing n again

__________________________________________Q2=700,56 J

2) Energy required for fusion

Using ecuation B

______________________________________________Q3=8,35.10³J

3) Energy required for vaporization

Using ecuation B

______________________________________________Q4=56,53.10³J

Heat Required= Q1+Q2+Q3+Q4=(10480,6 + 700,56 + 8,35.10³ + 56,53.10³) = 76061,16J

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