Review for Exam II

What follows is a step through review of concepts and problems for Chapters 4 and 5. Be sure to TRY the problems on your own before looking at the answers or you are wasting your time.

Concept 1: The balanced chemical reaction

In order to balance reactions, the same number of each element must appear on both sides of the reaction arrow.

Balance the following reactions:

1)       ____ N₂ + ____ H₂ à ____ NH₃

2)       ____ KClO₃ à ____ KCl + ____ O₂

3)       ____ NaCl + ____ F₂ à ____ NaF + ____ Cl₂

4)       ____ H₂ + ____ O₂ à ____ H₂O

5)       ____ Pb(OH)₂ + ____ HCl à ____ H₂O + ____ PbCl₂

6)       ____ AlBr₃ + ____ K₂SO₄ à ____ KBr + ____ Al₂(SO₄)₃

7)       ____ CH₄ + ____ O₂ à ____ CO₂ + ____ H₂O

8)       ____ C₃H₈ + ____ O₂ à ____ CO₂ + ____ H₂O

9)       ____ C₈H₁₈ + ____ O₂ à ____ CO₂ + ____ H₂O

10)     ____ FeCl₃ + ____ NaOH à ____ Fe(OH)₃ + ____NaCl

11)     ____ P + ____O₂ à ____P₂O₅

12)     ____ Na + ____ H₂O à ____ NaOH + ­­­____H₂

13)     ____ Ag₂O à ____ Ag + ____O₂

14)     ____ S_8­ + ____O₂ à ____ SO₃

15)     ____ CO₂ + ____ H₂O à ____ C₆H₁₂O₆ + ____O₂

16)     ____ K + ____ MgBr à ____ KBr + ____ Mg

17)     ____ HCl + ____ CaCO₃ à ____ CaCl₂ + ____H₂O + ____ CO₂

18)     ____ HNO₃ + ____ NaHCO₃ à ____ NaNO₃ + ____ H₂O + ____ CO₂

19)     ____ H₂O + ____ O₂ à ____ H₂O₂

20)     ____ NaBr + ____ CaF₂ à ____ NaF + ____ CaBr₂

21)     ____ H₂SO₄ + ____ NaNO₂ à ____ HNO₂ + ____ Na₂SO­₄

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Concept 2: Stoichiometry

Once you have a handle on balancing equations you need to know how to use them. When you read a reaction, it gives you information about how the reactants and products are related to one another.

For Example: H₂SO₄(aq) + 2KOH àK₂SO₄(aq) + 2H₂O(l)

You would read this as one mole of sulfuric acid reacts with two moles of potassium hydroxide yielding one mole of potassium sulfate and two moles of water.

The ratios of reactant to reactant and reactant to product are essentially conversion factors. So if you are given moles of product or reactant you can use these ratios to make predictions.

For example: How many moles of potassium hydroxide are needed to react with 12 moles of sulfuric acid?

12 mol H₂SO₄ x [2 mol KOH/1 mol H₂SO₄ ]= 24 mol KOH

Or

How many moles of sullfuric acid and potassium hydroxide are needed to make 5 moles of water?

5 mol H₂O x [ 2 mol KOH/2 mol H₂O] = 5 mol KOH

5 mol H₂O x [ 1 mol H₂SO₄/2 mol H₂O] = 2.5 mol H₂SO₄

Note that all of the relationships use moles. In order to use the "conversion factors" given in a chemical reaction you must place the amounts of product or reactant into moles first.

Here are some practice problems on calculating molecular weights and converting from grams to moles and moles to grams and using these values to make predictions about product and reactant amounts: PRACTICE

Limiting and Excess Reagents are just a continuation of the use of a balanced reaction to make predictions. There are only 5 questions that can be asked in this type of question:

1) Given an amount of reactant how much product can be made. (This amount is often refered to as the "Theoretical Yield") The reactant that makes the least amount of product or lowest theoretical yield is the limiting reactant. The other reactant will be left over when the reaction is complete so it is the excess reagent.

2) Given an amount of reactant how much of another reactant is needed. You can use the amount of limiting reagent to calculate how much of the excess reagent is used up. The difference between what is used of the excess reagent and what you are given is the amount left over.

3) Given an amount of product how much reactant was used or needed to make it.

4) Given an amount of product what is the percent yield. This problem type requires the completion of problem type 1).

5) A neutralization reaction problem. This problem type requires the completion of problem type 2).

Here is a practice tutorial on limiting and excess reagent problems: PRACTICE

More PRACTICE

Molarity

Molarity (M) is the most common unit of concentration for solutions. It is moles of solute per liter of solution. M = mol/L.

A solute is a chemical molecule dissolved in solution. The solvent is the liquid the solute is dissolved in. The solvent is most commonly water. The solution that has water as its solvent is called an aqueous solution.

Here are some practice problems using solutions rather than grams: PRACTICE

Electrolytes

An electrolyte is any substance containing free ions that behaves as an electrically conductive medium. Because they generally consist of ions in solution, electrolytes are also known as ionic solutions, but molten electrolytes and solid electrolytes are also possible.

Electrolytes are substances which, when dissolved in water, break up into cations (plus-charged ions) and anions (minus-charged ions). We say they ionize. Strong electrolytes ionize completely (100%), while weak electrolytes ionize only partially (usually on the order of 1–10%). That is, the principal species in solution for strong electrolytes are ions, while the principal specie in solution for weak electrolytes is the un-ionized compound itself.

Strong electrolytes fall into three categories: strong acids, strong bases, and salts. (Salts are sometimes also called ionic compounds, but really strong bases are ionic compounds as well.) The weak electrolytes include weak acids and weak bases.

Classifying Electrolytes

Examples of strong and weak electrolytes are given below:

Non-Electrolytes

Precipitates

A precipitate forms when a reaction between two or more reactants results in the production of an insoluble salt. Precipitates are shown as solids in balanced chemical reactions. In the solution itself they fall to the bottom of the container as the density of the insoluble salt is normally greater than the solvent around it.

The production of a precipitate is one of the reasons a reaction will run in the forward direction (as written) until completion. In order to know if a precipitate will form or not you must know which salts are soluble and which are not.

Solubility Rules

1. Salts containing alkali metal cations or ammonium are soluble.
2. Salts containing nitrate, acetate chlorate or perchlorate anions are soluble.
3. Salts containing chloride, bromide or iodide are soluble. (Except for Cu⁺, Ag⁺, Hg₂⁺² and Pb⁺²)
4. Salts containing sulfate or sulfite are soluble. (Except Ca⁺², Ba⁺², Sr⁺² and Pb⁺²)
5. Oxide and hydroxide salts are insoluble. (Except with group IA cations and ammonium.)
6. Sulfide and hydrogensulfide sals are insoluble. (Except with group IA cations and ammonium.)
7. Carbonate and chromate salts are insuluble. (Except with group IA and ammonium cations and ammonium.)

Acids and Bases

Strong acids are acids that are completely or nearly 100% ionized in their solutions. Here are some common strong acids:

Ionization of a strong acid HA can be represented by:

The pH and pOH Scales