Some types of hand warmers and warming pads use a slow combustion (oxidation) re

Question

Some types of hand warmers and warming pads use a slow combustion (oxidation) reaction. The same reaction occurs when rust forms in a drainpipe or on your bicycle.

4 Fe(s) + 3 O2 (g) 2 Fe2O3 (s) + heat H = - 1650 kJ

a.  How many moles of Fe are needed to react with 6 moles of O2?

8 moles Fe

b.  How many grams of rust will be produced if 425 g of Iron are used in the reaction?

c.  Is the reaction exothermic or endothermic?

d.  Is entropy increasing or decreasing?

e.  What would be the effect on the equilibrium of removing oxygen to the reaction?  (Would the reaction proceed in the forward or reverse direction?)

f.  How would adding heat shift the reaction? (Would the reaction proceed in the forward or reverse direction?)

Explanation / Answer

(a) From the balanced equation 3 mol of O2 needs 4 mol of Fe

Then, 6 mol of O2 needs 8 mol of Fe

(b)

From the balanced equation, 4 mol (or 4 * 56 g.) of Fe forms 2 mol of rust

Then, 425 g. of iron can form 425 * 2 / (4 * 56) = 3.79 mol

Therefore, mass of rust formed = 3.79 * 160 = 606.4 g.

(c)

Since deltaH = - ve, the reaction is exothermic

(d)

Since there is decrease in number of moles of gas substances during this reaction, the entropy is also decreased.

(e)

Removing of reactants from the equilibrium shifts the equilibrium towards left side i.e reverse direction.

(f)

Addition of heat favors endothermic reaction hence in the present case it favors reverse reaction.

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