`LIMITING REACTANTS AND PERCENT YIELD In real-life situations the number of moles of reactants available for a chemical reaction isn t always exactly the same as the ratio of moles in the balanced equation. One of the reactants will run out before the other reactant. For instance, when a candle burns in an open room, the wax in the candle will run out long before all the oxygen in the air will be used up. On the other hand, put the candle in a closed jar, and the oxygen will be used up quickly, leaving the wax left over. When one of the reactants is used up in a situation like this, the reaction will stop, and no more product will be made. The reactant that gets used up first, and therefore limits the amount of product that can be made, is called the limiting reactant. The reactant that is left over is called the excess reactant. Limiting reagent is another word for limiting reactant. A limiting reactant problem will provide two given values, one for each reactant, and ask you to find the amount of one product as unknown. To solve these you will use the convert/ratio/convert process for each of the given values, using the same unknown for both. The reactant that produces the least number of moles (or grams) of the unknown will be the limiting reactant. The other reactant is the excess reactant. 18.1 g of ammonia and 90.4 g of copper(II) oxide are used in the reaction 2NH3 18.1 g + 3CuO 90.4 g N2 + 3Cu + 3H2 O. ?gWhich is the limiting reactant? How many grams of nitrogen will be formed?18.1 g NH3x1 mol NH3 17.03 g NH3 1 mol CuO 79.55 g CuOx1 mol N2 2 mol NH3 1 mol N2 3 mol CuOx28.02 g N2 1 mol N2 28.02 g N2 1 mol N2=14.9 g N290.4 g CuOxxx= 10.6 g N2The limiting reactant is copper(II) oxide and it can produce a maximum of 10.6 grams of nitrogen.To solve this problem, first do a convert/ratio/convert for each of the two given values. Use the same unknown for both. Then determine which of the two reactants produced the smallest amount of product. That reactant is the limiting reactant. The other reactant will be left over and is called the excess reactant.Page 1 of 2Remember that the amount of product predicted by stoichiometry is just a theoretical amount. In real life , the amount of product obtained in the lab may or may not be the same as what was predicted due to human and equipment error. This theoretical amount predicted by stoichiometry is called the theoretical yield. Expected yield is another name for theoretical yield. When the reaction is carried out in the laboratory and the product actually collected and measured, the amount obtained is called the actual yield. If all goes well, the actual yield will be very close to the amount predicted by stoichiometry (the theoretical yield). Percent yield is the percent of the theoretical yield that is obtained through laboratory procedures. Percent yield is done to determine how efficient a laboratory process is in producing the desired product. Percent yield is an important calculation in manufacturing situations, since a high percent yield (close to 100%) means more product and therefore more profit for the manufacturer. In reality, percent yield rarely is 100%. There always seems to be something that causes some product to be lost, if nothing more than some of it sticking to the inside of the test tube. If a percent yield over 100% is obtained, it is likely that some error in measurement was made. The formula for percent yield is: percent yield = actual yield X theoretical yield 100In the calculations grams will cancel out, and the unit will be %. Be sure to round off to the correct number of significant figures. In the reaction on the previous page, if 9.87 grams of nitrogen were obtained when the reaction was done in the lab, what is the percent yield? % yield = 9.87 g 10.6 g x 100 = 93.1%Page 2 of 2`

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