7/27/2019 A Balanced Chemical Equation Shows the Molar Amounts of Reactants That Will React Together to Produce Molar

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Abalanced chemical equationshows the molar amounts ofreactantsthat will reacttogether to produce molar

amounts ofproducts. In the realworld, reactants are rarelybrought together with the exact

amount needed. One reactantwill be completely used upbefore the others. The reactant

used up first is known as the

limiting reactant. The otherreactants are partially consumedwhere the remaining amount is

considered "in excess". Thisexample problem demonstratesa method to determine the

limiting reactant of a chemical

reaction. Excess and

LimitingReagents

Chemical reaction equationsgive the

ideal stoichiometric relationshipamong reactants and products.

However, the reactants for areaction in an experiment are not

necessarily a stoichiometric

mixture. In a chemical reaction,

reactants that are not use up when

the reaction is finished are called

excess reagents. The reagent that

is completely used up or reacted is

called the limiting reagent,

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7/27/2019 A Balanced Chemical Equation Shows the Molar Amounts of Reactants That Will React Together to Produce Molar

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because its quantity limit theamount of products formed.

Let us consider the reaction

between sodium and chlorine. The

reaction can be represented by the

equation:

2 Na + Cl2 = 2 NaCl,

It represents a reaction of a metal

and a diatomic gas chlorine. This

balanced reaction equationindicates that two Na atoms would

react with two Cl atoms or one Cl2

molecule. Thus, if you have 6 Na

atoms, 3 Cl2 molecules will be

required. If there is an excess

number of Cl2 molecules, they

will remain unreacted. We can

also state that 6 moles of sodium

will require 3 moles of Cl2 gas. If

there are more than 3 moles of Cl2

gas, some will remain as an excess

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reagent, and the sodium is alimiting reagent. It limits the

amount of the product that can be

formed.

Chemical reactions with

stoichiometric amounts of

reactants has no limiting or excess

reagents

Calculate the number of

moles of CO2 formed inthe combustion of ethane

C2H6 in a process when

35.0 mol of O2 is

consumed.Hint...

The reaction is2 C2H6 + 7 O2 = 4

CO2 + 6 H2O4 mol CO2

35.0 mol O2 ----------

= 20.0 mol CO2

7 mol O2

Discussion...

A balanced equation for

the reaction is a basic

requirement for identify

the limiting reagent even if

amounts of reactants areknown.

Example 2

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Two moles of Mg andfive moles of O2 are

placed in a reaction

vessel, and then the Mg is

ignited according to the

reaction

Mg + O2 = MgO.

Balance this equation

and identify the limiting

reagent in this

experiment.

Hint...The balanced reaction is,

2 Mg + O2 = 2

MgO

Thus, two moles of Mg

require only ONE mole of

O2. Four moles of oxygenwill remain unreacted.

Oxygen is the excess

reagent, and Mg is the

limiting reagent.