Pure Iron Question 2. Iron is a critical material in manufacturing. Assume the i

Question

Pure Iron Question

2. Iron is a critical material in manufacturing. Assume the iron is pure. a. How many atoms in a kilogram of iron? b. Below similarity1185K iron is BCC, above this temperature, it is FCC. What is the volume change in the unit cell when the iron transforms on heating? Do not include thermal expansion terms. Report your answer in percent. c. What are the possible oxidation states of iron? d. What are the four major iron oxide compounds? Comment on the issues with using iron in an outdoor environment. 3. You are making a precision, high stability, electrical resistor of 1kOhm value. a. Would you use carbon or nickel for the resistive element? Why? b. You use this resistor in an electronic circuit designed to work between -40C up to +125C. What is the change in resistance over this temperature range?

Explanation / Answer

2,
a) no. of atoms = Na(avogadro no.)* moles

moles = Weight/MW = 1000/56 = 17.857 moles

so,
no .of atoms = 6.022*10^23 * 17.857 = 1.075*10^25.

b)
if radius of atom be 'r',
so ,
in BCC structure,
a= (4/sqrt3) r
so , volume of unit cell = a^3 = 64/(3*sqrt3) * r^3

in FCC structure,
a= (4/sqrt2) r,
so ,volums of one unit cell = 64/(2*sqrt2)*r^3,

so ,
% volume change = {[64/(2*sqrt2)*r^3] -[64/(3*sqrt3) * r^3] }*100/[64/(3*sqrt3) * r^3]

% volume change = {[64/(2*sqrt2)] -[64/(3*sqrt3)] }*100/[64/(3*sqrt3) ] = 83.7 %

c)possible oxidation states of iron :
Iron exists in a wide range of oxidation states, -2 to +6, although +2 and +3 are the most common.

d)four major oxide compounds are :
Fe O, Fe2O3 , Fe3O4, Fe4O5

In an outdoor environment ,
Iron reacts with the atmospheric oxygen and moisture and forms oxides and hydroxides, which deteriorates the quality and surface of iron.This process is called corrosion. There are various techniques to prevent it viz. painting,galvanizing, electroplaing etc.

3.
a) we should use Nickel -chromium alloy for the resistive element as . Carbon composition resistors are also limited to power ratings of up to 1 watt and carbon composition resistors have a poor resistance tolerance, typically only 5%

b)
resistance change due to temperature change in case of nickel chromium resistor
delta R = Ro* alpha * ( T-To)

alpha for nickel-chromium alloy = 0.4 *10^-3
delta R =1000* 0.4*10^-3*( 125+40)

delta R =66 ohm

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