Chemistry- Thermochemistry

Question

Exactly 25 mL of water at 25 °C is added to a hot frying pan made completely out of iron that weighs 1.5 kg. If the 25 mL of added water is converted to steam at 100 °C determine the temperature change of the frying pan (in K). The molar heat capacity of iron is 25.19 J mol^-1 °C^-1

Answer 1

You have to look up the latent heat of vaporization or Enthalpy of vaporization of water and know the c_w= 4.184 J/gK. The heat that enters the water leaves the pan, so...

n_wΔH_vap + m_wc_w(100 - 25) = -n_Fwc_FeΔT (solve for ΔT) (n is moles, m is mass)

The water is heated to its boiling point and vaporized. You have to make sure that you are canceling units appropriately: 25 ml water = 25 g = 25/18 moles. 1.5 kg Fe = 1500/55.8 moles.

Answer 2

Actually, Brett L.,

If it's Chemistry you want, did you include a term for how much rust you made in the frying pan? You will need to consider, is it just Fe₂O₃ , or are there some waters of hydration on that as well. I suspect the reaction will be exothermic (driven by both enthalpy and entropic release of H₂ gas), and therefore the rest of the pan (ROP) will chill less than expected (in the limit, all the water is used in the rusting reaction, which obviates the need for vaporizing the water at all).

If that seems too complex, then just figure the heat balance, liquid water heated to 100C, vaporized at 100C; = opposite of: iron pan cooled over some .delta.K range. A piece of cake (...er, don't try to vaporize the cake batter.....)

--Cheers, --Mr. d.