A 488.3 gram sample of an unknown substance (MM = 92.41 g/mol) is heated from -23.1 °C to 51.8 °C. (heat capacity of solid = 2.96 J/g・°C; heat capacity of liquid = 1.75 J/g・°C; ∆Hfus = 8.04 kJ/mol; Tfinal = 17.6 °C)
a) How much energy (in kJ) is absorbed/released to heat the solid?
b)How much energy (in kJ) is absorbed/released to melt the solid?
c)How much energy (in kJ) is absorbed/released to heat the liquid?
d) What is the total amount of energy that must be absorbed/released for the entire process?

Respuesta :

Answer:

a) Q₁ = 58.82 KJ

b) Q₂ = 42.48 KJ

c) Q₃ = 29.22 KJ

d) Q = 130.52 KJ  

Explanation:

a)

In order to find the energy absorbed to heat the solid, we will use:

[tex]Q_{1} = mC_{1}\Delta T_{1}[/tex]

where,

Q₁ = Heat absorbed for heating solid = ?

m = mass of solid = 488.3 g = 0.4883 kg

C₁ = Specific Heat Capacity of Solid = 2.96 J/g °C

ΔT₁ =  Change in temperature of Solid = Melting Temperature - Initial Temp.

ΔT₁ = 17.6°C - (-23.1°C) = 40.7°C

Therefore,

[tex]Q_{1} = (488.3\ g)(2.96\ J/g\ ^{0}C)(40.7\ ^{0}C)[/tex]

Q₁ = 58.82 KJ

b)

In order to find the absorbed to melt the solid at 17.6°C, we will use:

[tex]Q_{2} = nH_{fus}[/tex]

where,

Q₂ = Heat absorbed for melting solid = ?

H_fus = Heat of Fusion = 8.04 KJ/mol

n = no. of moles = [tex]\frac{m}{MM} = \frac{488.3\ g}{92.41\ g/mol} = 5.28 mol[/tex]

Therefore,

[tex]Q_{2} = (5.28\ mol)(8.04\ KJ/mol)[/tex]

Q₂ = 42.48 KJ

c)

In order to find the energy absorbed to heat the liquid, we will use:

[tex]Q_{3} = m C_{3}\Delta T_{3}[/tex]

where,

Q₃ = Heat absorbed for heating Liquid = ?

m = mass of solid = 488.3 g = 0.4883 kg

C₃ = Specific Heat Capacity of Liquid = 1.75 J/g °C

ΔT₃ =  Change in temperature of Liquid = Final Temp. - Melting Temp.

ΔT₃ = 51.8°C - 17.6°C = 34.2°C

Therefore,

[tex]Q_{3} = (488.3\ g)(1.75\ J/g\ ^{0}C)(34.2\ ^{0}C)[/tex]

Q₃ = 29.22 KJ

d)

Total amount of energy absorbed during entire process is:

[tex]Q = Q_{1} + Q_{2} + Q_{3}[/tex]

[tex]Q = 58.82\ KJ + 42.48\ KJ + 29.22\ KJ[/tex]

Q = 130.52 KJ

In order to heat a 488.3 g solid, 58.8 kJ are required. To melt the solid, 42.5 kJ are required. To heat the liquid, 29.2 kJ are required. The total amount of energy absorbed is 130.5 kJ.

Initially, a 488.3 g solid at -23.1 °C is heated up to 17.6 °C (melting point). We can calculate the heat required (Q₁) using the following expression.

[tex]Q_1 = c \times m \times \Delta T = \frac{2.96J}{g.\° C } \times 488.3g \times (17.6\° C-(-23.1\° C)) \times \frac{1kJ}{1000J} = 58.8 kJ[/tex]

where,

  • c: heat capacity of the solid
  • m: mass
  • ΔT: change in the temperature

At 17.6 °C, we can calculate the heat (Q₂) required to melt the solid using the following expression.

[tex]Q_2 = \Delta H_{fus} \times \frac{m}{MM} = 8.04 kJ/mol \times \frac{488.3 g}{92.41g/mol} = 42.5kJ[/tex]

where,

  • ∆Hfus: enthalpy of fusion
  • m: mass
  • MM: molar mass

The liquid is heated from 17.6 °C to 51.8 °C. We can calculate the heat required (Q₃) using the following expression.

[tex]Q_3 = c \times m \times \Delta T = \frac{1.75J}{g.\° C } \times 488.3g \times (51.8\° C-17.6\° C)) \times \frac{1kJ}{1000J} = 29.2 kJ[/tex]

  • c: heat capacity of the liquid
  • m: mass
  • ΔT: change in the temperature

The total amount of energy absorbed (Q) is the sum of the energy absorbed in each step.

[tex]Q = Q_1 + Q_2 + Q_3 = 58.8kJ+42.5kJ+29.2kJ= 130.5kJ[/tex]

In order to heat a 488.3 g solid, 58.8 kJ are required. To melt the solid, 42.5 kJ are required. To heat the liquid, 29.2 kJ are required. The total amount of energy absorbed is 130.5 kJ.

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