High School Chemistry : Balancing Reactions

Study concepts, example questions & explanations for High School Chemistry

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Example Questions

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Example Question #1 : Balancing Reactions

What is the coefficient on sulfur dioxide if the following redox reaction is balanced in an acidic solution?

Possible Answers:

Correct answer:

Explanation:

Balancing redox reactions involves the following steps:

1. Divide the reaction into oxidation and reduction half reactions and balance all atoms that are not oxygen and hydrogen:

2. Balance the oxygens by adding water molecules on the opposite side of the reactions:

3. Balance the hydrogens by adding protons to the opposite side of the equation:

4. Add electrons in order to equal the charges on both sides of the equation:

5. In order to make the electron exchange equal in each half step, we must multiply the top half reaction by 3:

 

6. Add up the reactants and products while cancelling out substances on opposite sides of the reactions. For example: we will cancel the 6 water molecules as reactants and be left with only one water molecule as a product. In addition, only 2 protons will be left on the reactant's side after canceling the 12 from the product's side.

In the balanced redox reaction, the coefficient on sulfur dioxide is 3.

Example Question #502 : Ap Chemistry

For the following unbalanced redox reaction, how many electrons are transferred and which chemical species is being oxidized?

Possible Answers:

Two electrons are transferred; P is oxidized

One electron is transferred; P is oxidized

One electron is transferred; Hg is oxidized

Two electrons are transferred; Hg is oxidized

Correct answer:

Two electrons are transferred; Hg is oxidized

Explanation:

To begin, we will need to separate the given reaction into the two half-reactions by identifying changes in oxidation number. In this case, mercury (Hg) and phosphorus (P) show a change in oxidation number. Mercury begins with an oxidation number of zero, and ends with an oxidation number of . Phosphorus begins with an oxidation number of  and ends with an oxidation number of . Note that the oxidation numbers for fluorine and iodine reamain constant at  for each.

Now we can begin to look at the half-reactions.

Balance the atoms.

Now balance the electrons. We know that each mercury atom loses one electron and each fluorine atom gains one electron.

We can see that two electrons are tranferred. To identify the element being oxidized, we must find the element that is losing electrons. In this case, mercury is being oxidized.

Example Question #1 : Balancing Oxidation Reduction Reactions

Oxidation is the __________ of electrons, reduction is the __________ of electrons.

Possible Answers:

gain . . . loss

loss . . . loss

loss . . . gain

None of these; it depends on the reaction.

gain . . . gain

Correct answer:

loss . . . gain

Explanation:

An oxidation-reduction (redox) reaction is a reaction where electrons are transferred between two substances. When an atom is oxidized, it is called the reducing agent, and it loses a number of electrons. Similarly, a reduced atom is called the oxidizing agent, and gains the same number of electrons. A popular mnemonic to help remember is OIL RIG, or Oxidation is Loss of electrons, Reduction is Gain of electrons.

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 Reactions

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 Chemical Equations

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

Possible Answers:

K_3As

KAs_2

KAs

K_2As

KAs_3

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 #1 : Balancing Chemical Equations

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 #1 : Balancing And Predicting Reactions

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.

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