Chemical Reactions Review

Below is a solutions guide to the optional review for tomorrow's test on chemical reactions. The test will consist of 28 multiple choice questions and seven short answer problems. The multiple choice part of the test will cover balancing reactions, classifying types of reactions, use of an activity series for predicting if reactions will occur, and use of a solubility table. The short answer problems will be like those on the Chemical Reactions Review that you worked on in class today. A complete VODCast key for the Chemical Reactions Review is available on the class Events Calendar on the date Tuesday, February 21st, 2012. The only type of reaction that you will be held accountable on to know the state of matter will be double replacement reactions. Below the key for the optional review is a brief tutorial on how to balance equations. Come and see me tomorrow morning if you have any questions.

For those of you experiencing a little difficulty on the balancing chemical reactions worksheet, I have some advice for a possible sticking point.

Elements combine in whole number, that we know to be true. Thus, we need to have the smallest whole number ratio of the coefficients used to balance the equation. To balance an equation initially, you may use a fraction, but then you must modify the coefficients to make them whole numbers. Below is an example.

Reaction: C3H6O2 + O2 --> CO2 + H2O

Hint: Wait to balance oxygen last since placing a coefficient in front of elemental oxygen (O2) will not change any other elements.

Initial balancing: C3H6O2 + ?O2 --> 3CO2 + 3H2O

The carbons and hydrogens have been balanced. All that is left are the oxygens. There are a total of 9 oxygens of the product side (right of the arrow). Notice that 2 oxygens exist in the compound on the reactant side. Thus, to balance out the oxygens, an equation can be set up.

2 + x = 9 --> x = 7 --> Seven oxygens are required to balance the reaction, so this dictates that we use a coefficient in front of oxygen (O2) that will produce 7 oxygens. This will cause a coefficient of 3.5 to be used. This is not a whole number, but we can deal with that later.

Almost balanced: C3H6O2 + 3.5O2 --> 3CO2 + 3H2O

Having a fraction as a coefficient is not allowed, thus we must convert 3.5 to a whole number. To accomplish this, we will multiply 3.5 by 2. Just like in algebra, if something is done to one number, we must do the same thing to all of the numbers. Thus, all coefficients will be multiplied by 2. Doing this keeps the equation balanced.

Almost, nearly balanced: 2(C3H6O2 + 3.5O2 --> 3CO2 + 3H2O)

BALANCED!! 2C3H6O2 + 7O2 --> 6CO2 + 6H2O

Make sure that you check all of the elements on each side of the reaction to ensure that is has been balanced correctly.

Key for Word Equations Worksheet

Below is the key for the "Word Equations" worksheet. A common mistake made is forgetting which elements are diatomic elements. The elements hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine all are two atom molecules when isolated alone in nature. The VODCast for the "Chemical Reactions Review" has been posted under the date Friday, February 17th, 2012. The "Chemical Reactions Review" problems are on the back of the Word Equations worksheet. Please do the example problems before class on Tuesday, February 21st, 2012. The problems that are not shown as examples will be completed by you with your work groups in class on Tuesday. After classes are complete on Tuesday, another VODCast with all of the solutions to the review will be posted. The test for the chemical reactions unit will be given on Wednesday, February 22nd, 2012.

The chapters in the book that cover this material are Chapter 6 and Chapter 7. Sections 7.4 and 7.5 will not be covered by Wednesday's test. They will be covered later in the semester.

Quiz Review for Wednesday, February 15th, 2012

Tomorrow (Wednesday, February 15th, 2012) will be a quiz over balancing equations, classifying types of reactions, the use of the activity series chart, and the use of the solubility table. The types of reactions were given in your notes, but I will summarize them again in this blog post. The types of reactions and how they are abreviated are shown below. The key to the types of reactions on the balancing reactions worksheet are given below.

• synthesis reactions (shown as "syn" below) - less complex atoms or molecules become more complex molecules. Remember, a three element compound is more complex than a two element compound.


• decomposition reactions (shown as "decomp" below) - more complex molecules become less complex molecules or atoms. A two element compound is less complex than a three element compound.


• dissociation reactions (shown as "diss" below) - ionic compounds, when placed in water, dissolve in water to go from solid to aqueous. The aqueous products will always be shown as ions with charges.


• combustion reactions (shown as "comb" below) - elements oxygen (O2) must always be a reactant. Each product must contain the element oxygen. **All hydrocarbons (compounds that contain the elements carbon, hydrogen, and sometimes oxygen), when combusted, produce the products carbon dioxide and water. Both CO2 and H2O have oxygen in them.


• double replacement reactions (shown as "D.R." below) - two aqueous ionic compounds switch ions. The metal (positive ion) of one compound possibly combines with the non-metal (negative ion) of another and vice-versa. A new state of matter matter must be produced. This is usually a precipitate (solid), but not always.


• single replacement reactions (shown as "S.R." below) - a single element and an ionic compound are reactants. A general rule is "a metal replaces a metal and a non-metal replaces a non-metal" for single replacement reactions. If the single element is a metal and is more active than the metal in the ionic compound, the more active metal will replace the less active metal in the compound. Example #1: Cu +2AgNO3 --> Cu(NO3)2 + 2Ag OR a more active non-metal will replace a less active non-metal in the ionic compound. Example #2: Cl2 + CaBr2 --> CaCl2 +Br2. A metal or non-metal's reactivity can be determined by where it is on the activity series chart given in class. If a less active element is the element by itself, it will not replace the more active element. This will be classified as a NO REACTION (N.R.)

**Some reactions can have more than one type of classification. This often occurs with synthesis reactions that involve elemental oxygen as a reactant. They can be often be classified as combustion reactions also. Also, some of the reactions in the worksheet do not always fit nicely into a category, so that is indicated by a question mark. You will not be given any questionable reactions on the quiz.

The quiz will require you to do the following:

• Balance equations and classify them just like the work sheet given in class.


• You will also be shown single replacement reactions and asked to predict if a reaction will occur because the single element reactant is more active than its like element in the compound or the reaction will not occur because the single element reactant is less active than its like element in the compound.


• You will be given ionic compounds, and using a solubility table, determine if the compound is soluble in water (aqueous) or not soluble in water (solid).

**You will be able to use your own periodic table and solubility / activity series table on the quiz. The quiz will be going into the "Assessment Category" of your grade.

New VodCast - Writing Double Replacement Rxns

The new VODCast showing how to write double replacement reactions for the Observing Chemical Reactions in Solution Lab is available on the Events Calendar under the date Saturday, February 11th, 2012. Please view it prior to class on Monday to prepare for the lab. Have a good weekend.