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July 30, 2014

Homework Help: chemistry - energy and change rev (help)

Posted by Babelii on Wednesday, March 23, 2011 at 7:46am.

the below 2questions i could not answer while revising for this chapter pls help me out a little.

58. When a vehicle is parked in the sunlight on a hot summer day, the temperature inside can approach 55C. One company has patented a non-CFC propelled aerosol that can be sprayed inside a vehicle to reduce the temperature to 25C within seconds. The spray contains a mixture of two liquids: 10% ethanol, C2H5OH, and 90% water by mass.
(a) Use thermochemical equations, and the corresponding enthalpy changes, to explain how the spray works.
(b) 1.0 g of the aerosol is sprayed into a hot vehicle. How much heat (in kJ) can be absorbed due to vaporization of the aerosol?
Note: ΔHvap of water = 44.0 kJ/mol and ΔHvap of ethanol = 38.56 kJ/mol



59. Consider methane, CH4, and hydrogen, H2, as possible fuel sources.
(a) Write the chemical equation for the complete combustion of each fuel. Then find the enthalpy of combustion, ΔHcomb, of each fuel. Express your answers in kJ/mol and kJ/g. Assume that water vapour, rather than liquid water, is formed in both reactions.
(b) Which is the more energetic fuel, per unit mass?
(c) Consider a fixed mass of each fuel. Which fuel would allow you to drive a greater distance? Explain briefly.

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