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Hydrogen cyanide, HCN, can be made by a two-step process. First, ammonia is reacted with O2 to...

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Hydrogen cyanide, HCN, can be made by a two-step process. First, ammonia is reacted with O2 to give nitric oxide, NO. 4 NH3 + 5O_2 (g) ? 4 NO(g)+ 6H2O(g) RXN 1 Then nitric oxide is reacted with methane, CH4. 2 NO(g) + 2 CH4(g) ? 2 HCN(g) + 2H20(g) + H2(g) RXN2 When 24.2 g of ammonia and 25.1 g of methane are used, how many grams of hydrogen cyanide can be produced?

Limiting Reactant

A chemical reaction is a process by which reactants are transformed into products. As the process moves forward the amount of reactants is being used up where the amounts of the products are increasing. One rule of thumb in chemistry is that for a chemical reaction to occur, all reactants must be available. Once one of the reactants becomes unavailable the chemical reaction ceases to exist. In the same way when a car runs out of gas it will not move forward even though the engine is in working order and the car is mechanically sound. In chemistry we call the reactant that runs out first the limiting reactant because it limits the amount of product that is made.

Answer and Explanation: 1

The first thing we must determine to answer this problem is the number of moles of nitrogen monoxide (NO) that is made in the first reaction from the mass of ammonia that is given the problem through stoichiometry:

{eq}24.2\;g\;NH_3\;\times \dfrac{1.0\;mol\;NH_3}{17\;g\;NH_3}\;\times \dfrac{4\;mol\;NO}{4\;mol\;NH_3}\;=\;1.42\;mol\;NO {/eq}

Next we will turn the mass of methane given in the problem into moles so we can compare the reactants of the second reaction and find the limiting reactant. To make this comparison we will divide each by their coefficient and choose the one in lesser amount as the limiting reactant:

{eq}25.1\;g\;CH_4\;\times \dfrac{1.0\;mol\;CH_4}{16\;g\;CH_4}\;=\;\dfrac{1.57\;mol\;CH_4}{2}\;=\;0.784 {/eq}

{eq}\dfrac{1.42\;mol\;NO}{2}\;=\;0.710 {/eq}

NO is the limiting reactant.We will finish by using the amount of moles of limiting reactant and convert into the mass of hydrogen cyanide:

{eq}1.42\;mol\;NO\;\times \dfrac{2\;mol\;HCN}{2\;mol\;NO}\;\times \dfrac{27\;g\;HCN}{1.0\;mol\;HCN}\;=\;38.3\;g\;HCN {/eq}


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Limiting Reactant: Definition, Formula & Examples

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Chapter 9 / Lesson 7

Learn the definition of a limiting reactant, the formula, and how to determine a limiting reactant. See examples of limiting reactants and their problems.


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