The Haber-Bosch process is a very important industrial process. In the Haber-Bosch process, hydrogen gas reacts with nitrogen gas to produce ammonia according to the equation 3H2(g)+N2(g)→2NH3(g) The ammonia produced in the Haber-Bosch process has a wide range of uses, from fertilizer to pharmaceuticals. However, the production of ammonia is difficult, resulting in lower yields than those predicted from the chemical equation. 1.15 g H2 is allowed to react with 9.93 g N2, producing 1.12 g NH3. Part A What is the theoretical yield in grams for this reaction under the given conditions? Express your answer to three significant figures and include the appropriate units.

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Answer:

6.46 grams (rounded to 3 sig. fig.)

Explanation:

The Theoretical Yield is calculated from the amount of reactants that are available. The value of the theoretical yield is independent of the actual yield.

This reaction involves two reactants:

  • [tex]\rm H_2[/tex] and
  • [tex]\rm N_2[/tex].

This question stated the mass of both. Start by determining which of the two is the limiting reactant.

Relative atomic mass data from a modern periodic table:

  • H: 1.008;
  • N: 14.007.

(1)

Assume that [tex]\rm H_2[/tex] is the limiting reactant. How many moles of [tex]\rm NH_3[/tex] will be produced?

[tex]M(\mathrm{H_2}) = 2\times 1.008 = \rm 2.016\; g\cdot mol^{-1}[/tex].

[tex]\begin{aligned} n(\mathrm{H_2}) &= \frac{m}{M}\\ &= \rm \frac{1.15\; g}{2.016\; g\cdot mol^{-1}}\\&\approx \rm 0.570437\; mol \end{aligned}[/tex].

Look up the coefficient ratio between [tex]\rm H_2[/tex] and [tex]\rm NH_3[/tex] in the balanced equation:

[tex]\displaystyle \frac{n(\mathrm{NH_3})}{n(\mathrm{H_2})} = \frac{2}{3}[/tex].

[tex]\begin{aligned} \frac{n(\mathrm{NH_3})}{n(\mathrm{H_2})}\cdot {n(\mathrm{H_2})} &= \frac{2}{3}\times \rm 0.570437\; mol \\&\approx \rm 0.380291\; mol\end{aligned}[/tex].

In other words, if [tex]\rm H_2[/tex] is the limiting reactant, approximately [tex]\rm 0.380291\; mol[/tex] of [tex]\rm NH_3[/tex] will be produced.

(2)

Similarly, assume that [tex]\rm N_2[/tex] is the limiting reactant. How many moles of [tex]\rm NH_3[/tex] will be produced?

[tex]M(\mathrm{N_2}) = 2\times 14.007 = \rm 28.014\; g\cdot mol^{-1}[/tex].

[tex]\begin{aligned} n(\mathrm{N_2}) &= \frac{m}{M}\\ &= \rm \frac{9.93\; g}{28.014\; g\cdot mol^{-1}}\\&\approx \rm 0.354466\; mol \end{aligned}[/tex].

Look up the coefficient ratio between [tex]\rm N_2[/tex] and [tex]\rm NH_3[/tex] in the balanced equation:

[tex]\displaystyle \frac{n(\mathrm{NH_3})}{n(\mathrm{N_2})} = 2[/tex].

[tex]\begin{aligned} \frac{n(\mathrm{NH_3})}{n(\mathrm{N_2})}\cdot {n(\mathrm{N_2})} &= 2\times 0.354466\rm \; mol \\&\approx \rm 0.708931\; mol\end{aligned}[/tex].

In other words, if [tex]\rm N_2[/tex] is the limiting reactant, approximately [tex]\rm 0.708931\; mol[/tex] of [tex]\rm NH_3[/tex] will be produced.

In theory, only around [tex]\rm 0.570437\; mol[/tex] of [tex]\rm NH_3[/tex] will be produced. The reaction will run out of [tex]\rm H_2[/tex] before all [tex]\rm N_2[/tex] are consumed.

The mass of that much [tex]\rm NH_3[/tex] is approximately 6.46 grams.

The theoretical yield of the reaction is 6.51 g of ammonia.

The equation of the reaction is; 3H2(g)+N2(g)→2NH3(g)

Number of moles of H2 = 1.15 g/2 g/mol = 0.575 moles

Number of moles of N2 = 9.93 g/28 g/mol = 0.355 mole

We need to find the limiting reactant which is the reactant that yields the least amount of product.

Given that;

3 moles of H2 yields 2 moles of ammonia

0.575 moles of H2 yields 0.575 moles × 2 moles/3 moles

= 0.383 moles of ammonia

Also;

1 mole of N2 yields 2 moles of ammonia

0.355 mole of N2 yields 0.355 mole× 2 moles/ 1 mole of N2

= 0.71 moles of ammonia

The limiting reactant is hydrogen.

Theoretical yield of ammonia =  0.383 moles of ammonia × 17 g/mol = 6.51 g of ammonia

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