Ch.3 - Chemical Reactions and Reaction Stoichiometry

Brown - Chemistry: The Central Science 14th Edition

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Brown 14th Edition

Ch.3 - Chemical Reactions and Reaction Stoichiometry

Problem 75b

Chapter 3, Problem 75b

Consider the mixture of ethanol, C2H5OH, and O2 shown in the accompanying diagram. (b) Which reactant is the limiting reactant?

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Identify the balanced chemical equation for the combustion of ethanol. The equation is: C2H5OH + 3 O2 → 2 CO2 + 3 H2O.

Determine the molar ratio between ethanol and oxygen from the balanced equation. For every 1 mole of ethanol, 3 moles of oxygen are required.

Calculate the moles of each reactant present in the mixture using their given quantities and molar masses. Ethanol has a molar mass of 46.07 g/mol, and oxygen has a molar mass of 32.00 g/mol for O2.

Compare the actual mole ratio of ethanol to oxygen in the mixture to the stoichiometric mole ratio from the balanced equation.

The limiting reactant is the one that is present in less than the required stoichiometric amount compared to the other reactant. Determine which reactant runs out first based on the calculated mole ratios.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Limiting Reactant

The limiting reactant is the substance that is completely consumed first in a chemical reaction, thus determining the maximum amount of product that can be formed. In a reaction, if one reactant is present in a lesser amount than required by the stoichiometric ratios, it limits the extent of the reaction, leading to incomplete conversion of other reactants.

Stoichiometry

Stoichiometry is the calculation of reactants and products in chemical reactions based on the balanced chemical equation. It involves using mole ratios derived from the coefficients of the balanced equation to determine how much of each reactant is needed or how much product can be formed, which is essential for identifying the limiting reactant.

Molar Ratios

Molar ratios are the ratios of the amounts of reactants and products in a balanced chemical equation, expressed in moles. These ratios are crucial for determining how much of each reactant is required for a reaction and for identifying which reactant will be consumed first, thus establishing the limiting reactant in the process.

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Related Practice

Textbook Question

Consider the mixture of propane, C3H8, and O2 shown here. (c) How many molecules of CO2, H2O, C3H8, and O2 will be present if the reaction goes to completion?

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Textbook Question

(a) Define the terms limiting reactant and excess reactant.

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Textbook Question

Consider the mixture of propane, C3H8, and O2 shown here. (a) Write a balanced equation for the combustion reaction that occurs between propane and oxygen.

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Textbook Question

Consider the mixture of ethanol, C2H5OH, and O2 shown in the accompanying diagram. (c) How many molecules of CO2, H2O, C2H5OH, and O2 will be present if the reaction goes to completion?

(c) Why should you base your choice of which compound is the limiting reactant on its number of initial moles, not on its initial mass in grams?

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