Question

1. Microwave ovens work by irradiating food with microwave radiation. Microwaves are absorbed by polar molecules (mostly water) and physically rotate. The increased rotation is an increase in kinetic energy which increases the temperature of the food.

a.) Assuming that radiation with λ = 15.0cm is used, that all of the microwaves are absorbed and converted to heat, how many photons are necessary to melt a 10.0g ice cube at -5°C, and raise the temperature of the melted ice to 25°C (room temperature).

b.) Which of the following molecules would also absorb microwave radiation: phosphorus pentachloride, SiCl₄, nitrogen triiodide, benzene. Draw Lewis structures to support your answer.

Answer 1

energy required for converting ice at -5 deg.c to ice at 0 deg.c = mass* specific heat of ice* temperature difference =10*2.05*5 = 102.5 joules.  

at 0 deg.c, water needs to be supplied with latent heat for convertin into liquid water. This heat is given by

=334 j/g*10 =3340 joules.

from 0 deg.c to 25 deg.c, water needs to be supplied with sensible heat = 10*4.184*(25-0)=1045.5 joules

total heat energy to be supplied = 102.5+3340+1045.5 joules=4488 joules

energy of microwave of wave length ,15cm = hC/wave length

h= 6.627*10-34 J.s, C= velocity of light = 3*10⁸m/s

energy of one photon = 6.627*10^-34*3*10⁸/(15/100) m=1.325*10-24 joules/photon

number of photons required = 4488/1.325*10^-24 =3,386*10²⁷ photons

Polar molecules can absorb radiation. Accordingly Nitrogen triiodide is polar,

Lewis structure suggest Nitrogen triiodide to be polar.