rewildtv.comical Equations Equations in ~ the Atomic and Macroscopic range Balancing rewildtv.comical Equations
Mole Ratios Predicting Mass produced or consumed in a rewildtv.comical Reaction Limiting Reagents

Because atoms are neither created nor destroyed in a rewildtv.comicalreaction, the complete mass of products in a reaction should be thesame as the full mass that the reactants.

You are watching: Why must chemical reactions be balanced

rewildtv.comical Equations

rewildtv.comical reaction are defined by rewildtv.comical equations.

Example: The reaction in between hydrogen and oxygen to formwater is represented by the complying with equation.

2 H2 + O2" width="17" height="9"sgi_fullpath="/disk2/rewildtv.comistry/">2 H2O

It is often advantageous to indicate whether the reaction orproducts are solids, liquids, or gases by writing an s, l,or g in parentheses after the symbol for the reactants orproducts, as presented in the complying with equations.

2 H2(g) + O2(g)" width="17"height="9"sgi_fullpath="/disk2/rewildtv.comistry/">2 H2O(g)

2 Al(s) + Fe2O3(s)" width="17"height="9"sgi_fullpath="/disk2/rewildtv.comistry/">Al2O3(s) + 2 Fe(l)

Because so plenty of reactions happen when services of twosubstances liquified in water are mixed, a distinct symbol, aq,is offered to describe these aqueous solutions.

The process in i beg your pardon a sample dissolves in water will beindicated by equations such together the following.

C12H22O11(s) C12H22O11(aq)

A rewildtv.comical equation is a statement of what deserve to happen,not necessarily what will certainly happen. The adhering to equation,for example, does not guarantee that hydrogen will certainly react withoxygen to kind water.

2 H2(g) + O2(g)" width="17"height="9"sgi_fullpath="/disk2/rewildtv.comistry/">2 H2O(g)

It is feasible to fill a balloon with a mixture the hydrogenand oxygen and also find the no reaction occurs until you touch theballoon v a flame. The balanced equation for this reactiondescribes the relationship in between the amounts of hydrogen andoxygen consumed and the water formed if or as soon as this reaction isinitiated.

Equations at the Atomicand Macroscopic Scale

rewildtv.comical equations deserve to be provided to represent what wake up oneither the atomic or macroscopic scale.

2 H2(g) + O2(g)" width="17"height="9"sgi_fullpath="/disk2/rewildtv.comistry/">2 H2O(g)

This equation deserve to be read in either of the adhering to ways.

If, or when, hydrogen reacts through oxygen, two molecules of hydrogen and also one molecule the oxygen space consumed because that every 2 molecules of water produced. If, or when, hydrogen reacts through oxygen, 2 moles of hydrogen and also one mole of oxygen are consumed because that every 2 moles of water produced.

rewildtv.comical equations must be well balanced -- they must have actually the samenumber of atom of each element on both political parties of the equation. Asa result, the fixed of the reactants should be equal to the fixed ofthe products of the reaction. ~ above the atom scale, the followingequation is well balanced because the complete mass of the reactants isequal come the massive of the products.

2 H2(g) + O2(g) " width="17" height="9" sgi_fullpath="/disk2/rewildtv.comistry/"> 2 H2O(g)
2 x 2 amu + 32 amu 2 x 18 amu
36 amu 36 amu

On the macroscopic scale, it is balanced because the fixed oftwo moles of hydrogen and also one mole that oxygen is same to the massof 2 moles that water.

2 H2(g) + O2(g) " width="17" height="9" sgi_fullpath="/disk2/rewildtv.comistry/"> 2 H2O(g)
2 x 2 g + 32 g 2 x 18 g
36 g 36 g

Balancing rewildtv.comicalEquations

There is no succession of rules that have the right to be followed blindly toget a balanced rewildtv.comical equation. Manipulate the coefficientswritten in prior of the recipe of the reactants and also productsuntil the number of atoms that each element on both political parties of theequation are the same.

Interactive accuse on balancing equations from

requires Macromedia Shockwave


It is typically a an excellent idea to tackle the easiest part of aproblem first.

Example: consider what happens once propane (C3H8)burns in wait to form CO2 and also H2O. The firstthing come look for once balancing equations are relationshipsbetween the 2 sides of one equation.

_____ C3H8 + _____ O2" width="17" height="9"sgi_fullpath="/disk2/rewildtv.comistry/">_____ CO2 + _____ H2O

It is easier to balance the carbon and also hydrogen atom in theequation than the oxygen atoms in this reaction because all ofthe carbon atoms in propane end up in CO2 and all ofthe hydrogen atoms end up in H2O, however some of theoxygen atoms end up in every compound. This way that over there is noway to predict the number of O2 molecules that areconsumed in this reaction till you recognize how numerous CO2and H2O molecules room produced.

We can start by noting that there are three carbon atoms ineach C3H8 molecule. Thus, three CO2molecules are created for every C3H8molecule consumed.

1 C3H8 + _____ O2" width="17" height="9"sgi_fullpath="/disk2/rewildtv.comistry/">3 CO2 + _____ H2O

If there are eight hydrogen atom in each C3H8molecule, there have to be eight hydrogen atoms, or four H2Omolecules, on the appropriate side that the equation.

1 C3H8 + _____ O2" width="17" height="9"sgi_fullpath="/disk2/rewildtv.comistry/">3 CO2 + 4 H2O

Now that the carbon and also hydrogen atoms space balanced, we cantry to balance the oxygen atoms. Over there are six oxygen atoms inthree CO2 molecules and four oxygen atoms in four H2Omolecules. Come balance the 10 oxygen atoms in the assets of thisreaction us need five O2 molecules amongst thereactants.

1 C3H8 + 5 O2" width="17" height="9"sgi_fullpath="/disk2/rewildtv.comistry/">3 CO2 + 4 H2O

There are now three carbon atoms, eighthydrogen atoms, and also 10 oxygen atom on every side of the equation.The well balanced equation because that this reaction is therefore written asfollows.

C3H8(g) + 5 O2(g)" width="17"height="9"sgi_fullpath="/disk2/rewildtv.comistry/">3 CO2(g) + 4 H2O(g)

Practice trouble 6:

Write a well balanced equation because that the reaction that occurs once ammonia burns in wait to kind nitrogen oxide and water.

_____ NH3 + _____ O2 _____ NO + _____ H2O

Click here to inspect your answer come Practice problem 6

Click here to check out a solution to Practice difficulty 6

Practice balancing equations indigenous

Mole Ratios

A well balanced rewildtv.comical equation enables us come predict what happenswhen the reaction bring away place. A mole ratioconverts mole of one compound in a balanced rewildtv.comical equationinto mole of an additional compound.

Example: The fireworks that brighten the sky each 4th ofJuly are based upon the reaction in between magnesium and oxygen toform magnesium oxide.

2 Mg(s) + O2(g) " width="17" height="9"sgi_fullpath="/disk2/rewildtv.comistry/">2 MgO(s)

The balanced equation for this reaction have the right to be provided toconstruct 2 unit factors that describe therelationship between the lot of magnesium and oxygen consumedin this reaction.


By concentrating on the systems of this problem, us can select thecorrect mole ratio to convert moles of magnesium right into anequivalent variety of moles the oxygen. Assume, for example, thatwe want to calculation the number of moles the oxygen required to burn0.40 moles of magnesium metal.

Practice trouble 7:

A can of butane lighter fluid contains 1.20 moles of butane (C4H10). Calculate the number of moles of carbon dioxide given off as soon as this butane is burned.

Click here to examine your answer come Practice difficulty 7

Click here to watch a systems to Practice difficulty 7

Predicting the fixed ofReactants consumed or assets Given turn off in a rewildtv.comical Reaction

Balanced rewildtv.comical equations can be offered to suspect therelationship in between the quantities of the reactants consumed andthe amounts of the products formed in a rewildtv.comical reaction.

Example: to predict the lot of oxygen that should be inhaledto digest 10.0 grams of street we have the right to assume the the street in ourdiet concerns us as C12H22O11molecules and also that our bodies burn this sugar according come thefollowing equation.

C12H22O11(s)+ 12 O2(g) " width="17" height="9"sgi_fullpath="/disk2/rewildtv.comistry/">12 CO2(g) + 11 H2O(l)

Perhaps the best method to start is come ask: "What room wetrying to find?" and also then summary the vital pieces ofinformation in the problem.

Goal: find out how numerous grams the O2 areconsumed once 10.0 grams that sugar are burned.

Fact: We start with 10.0 grams the sugar.

Fact: Sugar has actually the formula C12H22O11.

Fact: The well balanced equation because that this reaction have the right to bewritten as follows.

C12H22O11 + 12O2"width="17" height="9"sgi_fullpath="/disk2/rewildtv.comistry/">12 CO2 + 11 H2O

Because we understand the molecular weight of sugar, we can convertthe recognized mass the the sugar right into the variety of moles that sugar.


We now have actually a balanced rewildtv.comical equation, and we understand thenumber of moles of street in the sample. As a action towardthe goal of the trouble we might calculate the variety of molesof oxygen consumed in the reaction. The equation for thisreaction says that 12 moles of O2 are consumed forevery mole of sugar in this reaction. Us can therefore calculatethe number of moles the oxygen needed to burn 0.02921 mole ofsugar together follows.


We now have the crucial information to acquire to the goal ofour calculation. We recognize the quantity of O2 consumed inthis reaction in units of moles and also we can calculate the mass of0.3505 mole of O2 native the molecular weight ofoxygen.


According come this calculation, it takes11.2 grams that O2 come burn 10.0 grams of sugar.

Practice problem 8:

Calculate the amount of ammonia and also oxygen necessary to prepare 3.00 grams that nitrogen oxide through the complying with reaction.

4 NH3(g) + 5 O2(g)4 NO(g) + 6 H2O(g)

Click below to inspect your answer come Practice trouble 8

Click below to watch a solution to Practice difficulty 8

Limiting Reagents

It take away 1.70 grams of ammonia and 4.00 grams of oxygen tomake 3.00 grams of nitrogen oxide by the following reaction.

4 NH3(g) + 5 O2(g)" width="17"height="9"sgi_fullpath="/disk2/rewildtv.comistry/">4 NO(g) + 6 H2O(g)

What would occur to the quantity of NO created in thisreaction if we kept the quantity of O2 the exact same (4.00 g)but increases the quantity of NH3 that was presentinitially? The productivity of the reaction would stay the same. Nomatter exactly how much NH3 we include to the system, no an ext NOis produced since the reaction runs the end of O2 beforeall of the NH3 is consumed. When this happens, thereaction should stop. No matter just how much NH3 is added tothe system, we can"t get an ext than 3.00 grams that NO from 4.00grams the oxygen.

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When over there isn"t enough O2 come consume every the NH3in the reaction, the lot of O2 borders the amount ofNO that have the right to be produced. Oxygen is because of this the limitingreagent in this reaction. Due to the fact that there is more NH3than we need, that is the excess reagent.

The principle of limiting reagent is important due to the fact that rewildtv.comistsfrequently run reactions in which only a minimal amount the one ofthe reaction is present.

The crucial to limiting reagent troubles is the adhering to sequenceof steps.

recognize that you have a limiting reagent trouble or in ~ least take into consideration the possibility that there might be a limiting amount of one of the reactants.
assume that one of the reactants is the limiting reagent.
see if friend have sufficient of the various other reactant come consume the material you have actually assumed to it is in the limiting reactant.
If you do, your original assumption was correct.
If you don"t, i think that an additional reagent is the limiting reagent and also test this assumption.

Practice trouble 9:

How much magnesium oxide (MgO) deserve to be formed when 10.0 grams that magnesium reacts with 10.0 grams the O2?