Calculate the amount of energy (in kilojoules) needed to heat 346 g of liquid water from –10 °C to 182°C. Assume that the specific heat of water is 4.184 J/g · °C for liquid and that the specific heat of steam is 1.99 J/g · °C.

Respuesta :

Answer : The amount of heat required is, [tex]1.16\times 10^6J[/tex]

Solution :

The process involved in this problem are :

[tex](1):H_2O(s)(-10^oC)\rightarrow H_2O(s)(0^oC)\\\\(2):H_2O(s)(0^oC)\rightarrow H_2O(l)(0^oC)\\\\(3):H_2O(l)(0^oC)\rightarrow H_2O(l)(100^oC)\\\\(4):H_2O(l)(100^oC)\rightarrow H_2O(g)(100^oC)\\\\(5):H_2O(g)(100^oC)\rightarrow H_2O(g)(182^oC)[/tex]

The expression used will be:

[tex]\Delta H=[m\times c_{p,s}\times (T_{final}-T_{initial})]+m\times \Delta H_{fusion}+[m\times c_{p,l}\times (T_{final}-T_{initial})]+m\times \Delta H_{vap}+[m\times c_{p,g}\times (T_{final}-T_{initial})][/tex]

where,

[tex]\Delta H[/tex] = heat required for the reaction

m = mass of ice = 346 g

[tex]c_{p,s}[/tex] = specific heat of solid water or ice = [tex]2.09J/g^oC[/tex]

[tex]c_{p,l}[/tex] = specific heat of liquid water = [tex]4.184J/g^oC[/tex]

[tex]c_{p,g}[/tex] = specific heat of gaseous water = [tex]1.99J/g^oC[/tex]

[tex]\Delta H_{fusion}[/tex] = enthalpy change for fusion = [tex]333J/g[/tex]

[tex]\Delta H_{vap}[/tex] = enthalpy change for vaporization = [tex]2260J/g[/tex]

Now put all the given values in the above expression, we get:

[tex]\Delta H=[346g\times 2.09J/g^oC\times (0-(-80))^oC]+346g\times 333J/g+[346g\times 4.184J/g^oC\times (100-0)^oC]+346g\times 2260J/g+[346g\times 1.99J/g^oC\times (180-100)^oC][/tex]

[tex]\Delta H=1.16\times 10^6J[/tex]

Therefore, the amount of heat required is, [tex]1.16\times 10^6J[/tex]

The total heat required is 1.104 × [tex]10^{6}[/tex] J

In heating mass m = 346g of water from -10°C to 182°C there are the following processes involved:

(i) heating of ice from -10°C to 0°C

  ΔQ(1) = msΔT  

here we consider s is specific heat capacity of ice = 2.108J/gK

ΔT is change in temperature = 0°C-(-10°C) = 10°C

  ⇒ ΔQ(1) = 346×2.108×10= 7293.68 J

(ii) conversion of ice to water at 0°C

  ΔQ(2) = mL

here L is latent heat of fusion = 330 J/g

⇒ ΔQ(2) = 346×330 = 114180 J

(iii) heating  of water from 0°C to 100°C

  ΔQ(3) = msΔT

  here we consider s as the specific heat capacity of water = 4.184 J/g

 ⇒ ΔQ(3) = 346×4.184×100 J = 144766 J

(iv) conversion of water to steam at 100°C

  ΔQ(4) = mL

 here L is latent heat of evaporation = 2260 J/g

⇒ ΔQ(4) = 346×2260 J = 781960 J

(v) heating of steam from 100°C to 182°C

   ΔQ(5)= msΔT

  here we consider s as specific heat capacity of steam = 1.99 J/g

 ⇒ ΔQ(5) = 346×1.99×82 J = 56460.28 J

Now the total heat required to heat water from -10°C to 182°C is:

     ΔQ(1) +  ΔQ(2) +  ΔQ(3) +  ΔQ(4) +  ΔQ(5) =  ΔQ

   ⇒  ΔQ = 1104660 J

   ΔQ = 1.104 × [tex]10^{6}[/tex] J is the total heat required to heat water from -10°C to 182°C.

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