This reaction might be scaled up to make a larger quantity of product. For example, to make three times as much:

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1 The Limiting Reactant Problem When reactants are mixed, they are often combined in proportion: the number of moles of each reactant corresponds to the combining ratio predicted by the balanced chemical equation. For example, ammonia, an important starting material for the production of fertilizers, is prepared according to the following equation: N (g + H (g NH (g or 1 mole N reacts with mol H to produce mole NH 8.0 g N reacts with 6.0 g H to produce.0 g NH This reaction might be scaled up to make a larger quantity of product. For example, to make three times as much: or 8.0 g N reacts with 6.0 g H to produce.0 g NH 8.0 g N reacts with 18.0 g H to produce 10.0 g NH However, many real-life reactions involve the presence of one reactant in excess. A common example is the oxidation (combustion of a fuel, as in a furnace or automobile engine, which requires an excess of oxygen. In effect, the extent of the reaction is limited by the availability of the reactant that is in shortest supply. The limiting reactant is, therefore, the reactant that is in the shortest supply, based upon mole (not mass basis, and based upon the coefficients of the balanced equation. The limiting reactant limits and determines the maximum amount of product that may be obtained from a reaction for specified quant5ities of reactants. To find out which reactant is limiting, you can determine the amount (in moles of product which would form if each reactant were the limiting reactant, and selected the lesser quantity. Let s first consider a non-chemical example. You wish to make turkey sandwiches. Each sandwich will have two pieces of bread and three thin slices of turkey. You determine you have twelve pieces of bread and fifteen slices of turkey. Before you continue reading, think about that scenario. How many sandwiches could you make? If you determined that you can make five sandwiches, you are correct. There are a few ways to determine the answer, but the most common way is to determine the number of sandwiches you can make for each ingredient and see which will make the fewest. Twelve pieces of bread, at two pieces per sandwich, will make six sandwiches. Fifteen slices of turkey, at three slices per sandwich, will make five sandwiches. The fewest is five sandwiches and therefore the turkey iss the limiting reactant. Now consider the following chemical reaction.

2 A 50.6-g sample of Mg(OH reacts with 5.0 g of HCl as follows: What mass of MgCl, in grams, can be produced? Mg(OH (aq + HCl(aq MgCl (aq + H O(l Is this a limiting reactant problem? One way to determine this is to write what is known about any component of the reaction below that component: Mg(OH (aq + HCl(aq MgCl (aq + H O(l 50.6 g 5.0 g? g Notice that the quantity of both reactants is known. This is the indication that it is a limiting reactant problem. There is less HCl present, but this does not automatically mean that the HCl will be consumed before all the Mg(OH is exhausted. In other words, it is not necessarily the limiting reactant. To determine the mass of MgCl that will be produced, determine the moles of MgCl which will be produced for each reactant. Then, determine which quantity of MgCl is the smallest. This will determine the maximum amount of product that can form (in moles and will determine the limiting reactant. Based upon the Mg(OH quantity: 1mol Mg(OH 1mol MgCl 50.6 g Mg(OH Based upon the HCl quantity: mol MgCl 58. g Mg(OH 1mol Mg(OH 1mol HCl 6.5 g HCl 1mol MgCl mol HCl 5.0 g HCl mol MgCl At this point, a comparison can be made. The lesser amount of MgCl produced is mol MgCl. Therefore, the HCl is the limiting reactant. The problem can be finished based upon this value: 95. g MgCl mol MgCl 58.6 g MgCl 1mol MgCl

3 LIMITING REACTANT EXAMPLE PROBLEM 1: A 5.0-g sample of the base magnesium hydroxide, Mg(OH, is mixed with 5.0 g of phosphoric acid, H PO. A neutralization reaction takes place, which is represented by the balanced equation Mg(OH (s + H PO (aq Mg (PO (s + 6H O(l How many grams of Mg (PO are produced? Solution Strategy: The problem is a limiting reactant problem. This is identified by the fact that measured quantities of both reactants are given. Determine the moles of Mg (PO based first on the given quantity of Mg(OH and then for the given quantity of H PO. Finish the problem based upon the smaller quantity produced. Solution Step 1: Determine the moles of Mg (PO formed based upon the reactant Mg(OH. 1mol Mg(OH 1mol Mg (PO 5.0 g Mg(OH 0.1 mol Mg (PO 58. g Mg(OH mol Mg(OH Step : Determine the moles of Mg (PO formed based upon the reactant H PO. 1mol H PO 1mol Mg (PO 5.0 g H PO mol Mg (PO 98.0 g HPO mol HPO Step : Compare the amounts of Mg (PO produced, select the smaller quantity. The limiting reactant is Mg(OH because it produces the lesser amount of product. Step : Finish the problem based upon the limiting reactant quantity. That is, determine the gram amount of Mg (PO produced. 6.9 g Mg (PO 0.1 mol Mg (PO 7.6 g Mg (PO 1mol Mg (PO Does the answer make sense? Since 5.0-g quantity of Mg(OH is the limiting reactant and since the product Mg (PO has a greater molar mass than the reactants, it is expected that the mass of the product would be greater than 5.0 g but less than the mass of the two reactants combined (5.0 g g = 70 g. The answer falls within that range.

4 LIMITING REACTANT EXAMPLE PROBLEM : A mixture of 0.0 g of Fe O and 15.0 g Al is prepared. The mixture is heated, and a vigorous reaction occurs according to the balanced equation How many grams of iron are produced? Fe O (s + Al(s Al O (s + Fe(l Solution Strategy: The problem is a limiting reactant problem. This is identified by the fact that measured quantities of both reactants are given. Determine the moles of iron produced based first on the given quantity of Fe O and then for the given quantity of Al. Finish the problem based upon the smaller quantity produced. Solution Step 1: Determine the moles of Fe formed based upon the reactant Fe O. 1mol FeO mol Fe 0.0 g FeO g Fe O 1mol Fe O Step : Determine the moles of Fe formed based upon the reactant Al. 1mol Al mol Fe 15.0 g Al 7.0 g Al mol Al 0.501mol Fe mol Fe Step : Compare the amounts of iron produced, select the smaller quantity. The limiting reactant is Fe O because it produces the lesser amount of product. Step : Finish the problem based upon the limiting reactant quantity. That is, determine the gram amount of iron produced g Fe 0.501mol Fe 8.0 g Fe 1mol Fe Does the answer make sense? Since a 0.0-g quantity of Fe O is the limiting reactant and since the product Fe is this reactant minus the mass of the oxygen, it is expected that the product would be somewhat less than 0.0 grams. Practice Problem 1: Given the reaction represented by the balanced equation Sn(s + HF(aq SnF (s + H (g Calculate the number of grams of SnF preoduced by mixing g Sn with 60.0 g HF.

5 Practice Problem : Given the reaction represented by the balanced equation CH (g + Cl (g HCl(g + CHCl (g Calculate the number of grams of CHCl produced by mixing 105 g Cl with.0 g CH. Answer to Practice Problem 1: 1.0 g SnF Answer to Practice Problem : 58.9 g CHCl