High School Chemistry : Balancing Chemical Equations

Study concepts, example questions & explanations for High School Chemistry

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Example Questions

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Example Question #1 : Solubility And Equilibrium

What is the formula for the dissociation of iron (II) phosphate?

Possible Answers:

Correct answer:

Explanation:

Iron (II) has a positive two charge: .

Phosphate has a negative three charge: .

The initial compound must be constructed to cancel these charges. The dissociation is: .

Example Question #1 : Balancing Chemical Equations

Calcium hydroxide is treated with hydrochloric acid to produce water and calcium chloride. Write a balanced chemical reaction that describes this process.

Possible Answers:

Correct answer:

Explanation:

Calcium is in the second group of the periodic table, and is therefore going to have a  oxidation number. Hydroxide ions have a charge. Calcium hydroxide will have the formula .

Chloride ions have a charge and hydrogen ions have a charge. The formula for hydrochloric acid is .

On the products side, water has the formula and calcium chloride has the formula .

Now that we know all of the formulas, we can write our reaction:

In order to balance the chloride atoms, we need to add coefficients.

Example Question #1 : Balancing Chemical Equations

What is the coefficient for oxygen gas when the following equation is balanced?

Possible Answers:

Correct answer:

Explanation:

The balanced reaction for the combustion of pentane is:

When balanced, oxygen gas has a coefficient of eight.

To balance the equation, it is easiest to leave oxygen and hydrogen for last. This means we should start with carbon.

Now that carbon is balanced, we can look at hydrogen.

Finally, we can balance the oxygen.

The final reaction uses five carbon atoms, twelve hydrogen atoms, and sixteen oxygen atoms per side.

Example Question #2 : Balancing Chemical Equations

Given the unbalanced reaction below, how many moles of solid iron can be made from ten moles of iron oxide?

Possible Answers:

Correct answer:

Explanation:

The balanced chemical equation is:

The mole ratio of iron oxide to solid iron is 1:2. You can set up the following proportion to solve:

Example Question #1 : Balancing And Predicting Reactions

What is the chemical formula of the salt formed when a chemist mixes solvated Potassium and Arsenic ions in solution?

Possible Answers:

K_2As

KAs_2

KAs_3

KAs

K_3As

Correct answer:

K_3As

Explanation:

Potassium is a Group I element, so to get to a filled valence shell, it will lost one electron, yielding K^+.

Arsenic is a Group 5 element, so it needs to gain three electrons to obtain a filled valence shell, yielding As^{-3}.

In order to balance out the charges, the resultant salt will be K_3As.

Example Question #44 : Chemical Reactions

What is the net ionic equation for the ion exchange reaction between ferrous sulfate and calcium iodide? Assume all compounds are soluble.

Possible Answers:

None of the available answers

Correct answer:

Explanation:

First, we must know what ferrous sulfate is. Ferrous refers to , and sulfate has the formula . When we combine the two together we get .

Calcium is a divatent cation and iodide is a monovalent anion, so their salt is . The ion exchange reaction is then:

Example Question #4 : Balancing Chemical Equations

Select the net ionic equation from this molecular reaction:

Possible Answers:

None of the other choices

Correct answer:

Explanation:

The net ionic equation is derived by removing all spectator ions from the total ionic equation (in which all ions are listed). To put it another way, the net ionic equation involves only the ions that participate in a reaction which, in this case, is the precipitation of barium sulfate.

Begin by writing all aqueous compounds in their dissociated (ionic) forms.

Cancel out any ions that appear in equal quantities on both sides of the equation. In this case, we can cancel the nitrate and potassium ions.

This is our net ionic equation.

Example Question #5 : Balancing Chemical Equations

What is the balanced chemical equation for the combustion of butane ?

Possible Answers:

Correct answer:

Explanation:

Combustion is the chemical reaction of a hydrocarbon with molecular oxygen, and it always produces carbon dioxide and water. Knowing the reactants and products, the unbalanced equation must be: 

We start by balancing the hydrogens. Since there are 10 on the left and only 2 on the right, we put a coefficient of 5 on water.

Similarly, we balance carbons by putting a 4 on the carbon dioxide.

To find the number of oxygens on the right, we multiply the 4 coefficient by the 2 subscript on O (which gets us 8 oxygens) and then add the 5 oxygens from the 5 water molecules to get a total of 13. The needed coefficient for  on the left would then have to be 13/2.

Because fractional coefficients are not allowed, we mutiply every coefficient by 2 to find our final reaction:

Example Question #3 : Fundamental Reaction Concepts

Determine whether or not solid aluminum reacts with aqueous zinc chloride. If it does, determine the balanced equation for the reaction.

Possible Answers:

No reaction occurs

Correct answer:

Explanation:

When we check the activity series, it is fairly easy to see that aluminum metal is more reactive than zinc metal. So, in this case, the two metals undergo a redox reaction, where the aqueous  is reduced to solid , and the solid  is oxidized to aqueous . These charges are the common oxidation states for zinc and aluminum and should be memorized.

Because  is the new species, it bonds with 3  ions. The unbalanced equation is:

We note that there are 2 chlorine atoms on the left and 3 chlorine atoms on the right. To balance, we use a 3 coefficient on the left and a 2 coefficient on the right. This gives a total of 6 chlorine atoms on eahc side.

However, now we have also increased the amounts of zinc and aluminum. We copy the necessary coefficients to balance those—2 for aluminum on the left, 3 for zinc on the right—and we are done:

Example Question #2 : Balancing Equations

_Fe2O3 + _HCl ⇌ _FeCl3 + _H2O

The Following question will be based on the unbalanced reaction above

For the reaction above to be balanced what coefficient should be in front of the compound HCl?

Possible Answers:

2

3

6

1

4

Correct answer:

6

Explanation:

Balancing reactions is best acheived by using a stepwise approach.  It is useful to work through each atom making sure it is present in a balanced fashion on both sides of the equation.  Starting with Fe, Fe2Ois the only molecule with Fe present on the left side of the equation and FeCl3 is the only molecule with Fe present on the right side of the equation.  Thus the molar ratio of Fe2Oto FeCl3 must be 1:2.  Moving on to O, the O on the left side of the equation exists as Fe2O3 and H2O on the right.  The molar ratio of Fe2Oto H2O is therefore 1:3.  Cl exists in HCl on the left and FeClon the right. Thus the molar ratio of HCl to FeCl3 must be 3:1. To ensure that these molar ratios are maintaned the balanced formula is then determined to be Fe2O3 + 6HCl -> 2FeCl3 + 3H2O

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