Chapter_8

=Chemical Equations and Reactions=


 * This chapter discusses the following topics:**


 * Describing Chemical Reactions**
 * //Signs of Chemical Reactions// [[image:picture1.JPG align="right" caption="(Also can be found on page 265)"]]
 * Characteristics of Chemical Equations
 * Symbols Used in Chemical Equations
 * //Balancing Chemical Equations//


 * Types of Chemical Reactions**
 * //Synthesis Reactions//
 * Decomposition Reactions
 * Single-Displacement Reactions
 * Double-Displacement Reactions
 * Combustion Reactions


 * //Activity Series of the Elements

Key Terms//**
 * **Active metals** – highly reactive metals
 * **Activity** – the ability for an element to react
 * **Activity series** – a list of elements organized according to the ease with which the elements undergo certain chemical reactions
 * **Catalyst** – changes the rate of a chemical reaction but can recover unchanged
 * **Chemical equation** – represents the identities and relative molecular or molar amounts of the reactants and products in a chemical reaction, using symbols and formulas
 * **Chemical reaction** – process by which one or more substances are changed into one or more different substances
 * **Coefficient** – a small whole number that appears in front of a formula in a chemical equation
 * **Combustion reaction** – occurs when a substance combines with oxygen, releasing a large amount of energy in the form of light and heat
 * **Decomposition reaction** – occurs when a single compound undergoes a reaction that produces two or more simpler substances
 * **Double-displacement reaction** – occurs when the ions of two compounds exchange places in an aqueous solution to form two new compounds
 * **Electrolysis** – the decomposition of a substance by an electric current
 * **Formula equation** – represents the reactants and products of a chemical reactions by their symbols or formulas
 * **Precipitate** – a solid that is produced as a result of a chemical reaction in solution and that also separates from the solution
 * **Products** – the resulting substances following a chemical reaction
 * **Reactants** – the original substances in a chemical reaction
 * **Reversible reaction** – a chemical reaction in which the products re-form the original reactants
 * **Single-displacement reaction** – occurs when one element replaces a similar element in a compound
 * **Synthesis reaction** – occurs when two or more substances combine to form a new compound


 * Describing Chemical Reactions**

//Signs of Chemical Reactions//
 * //A chemical reaction may have occurred if heat and light is released.//
 * //Another clue that a chemical reaction has happened is the production of gas bubbles in a substance.//
 * //After mixing two solutions, the formation of a precipitate is indicative of the occurence of a chemical reaction.//
 * //When a substance changes color, a chemical reaction may have transpired.//

//Chemical Equations//
 * A chemical equation must identify all reactants and products. It must also include the proper formulas for the reactants and products. Use your knowledge of diatomic molecules (BrINClHOF) and charges of elements to help you with this. Always make sure that the same number of atoms of each element appears on both sides of the equation. Sometimes a coefficient must be used to balance the number of atoms.
 * Word equations are often a useful step in writing chemical equations. Remember that they only use words to name the reactants and products, but do not give the quantities. Use an → between the different sides of the equation. When reading, the arrow represents “react to yield.”
 * Next, write a formula equation by substituting the symbols/formulas in for the names of the reactants and products. We place either (g), (s), or (l) to indicate whether the formula is a gas, solid, or a liquid. Note that this equation still does not give the quantities of the reactants or products.
 * Then, the equation must be balanced by matching the charges and using coefficients. Note: Coefficients show how many molecules of a compound you have.

For instance, the coefficient two in media type="custom" key="409895" width="100" height="21" indicates that you have two water molecules.
 * You **cannot** change subscripts when balancing equations, although it is tempting. Changing subscripts alters the identity of a compound.
 * For example: media type="custom" key="409895" width="100" height="21" is NOT the same as media type="custom" key="409897" width="100" height="14". The first shows that there are two separate water molecules; four hydrogens and two oxygens. The latter, which has four hydrogens and one oxygen, does not even exist as a compound.
 * Always place coefficients in front of chemical formulas. Do not change the subscripts!


 * Rules to follow when balancing equations:**
 * 1) Balance each different atom separately.
 * 2) Balance the atoms of elements that are combined, but only occur once on both sides.
 * 3) Balance polyatomic ions as single units.
 * 4) Balance hydrogen and oxygen atoms last.
 * 5) Double check to make sure that the same amount of atoms of each element appears on both sides.

//Synthesis Reactions A + X → AX//
 * Types of Chemical Reactions**
 * A common type of this reaction occurs when an element combines with oxygen to form an oxide. Most metals can do this. When this type of reaction happens, Group 2 elements take the formula MO and Group 1 elements take the formula media type="custom" key="409909" width="100" height="13".
 * For example: media type="custom" key="409959" width="100" height="15"
 * When a metal has two different charges, it can form two different oxides.
 * For example: media type="custom" key="409957" width="100" height="15"
 * This reaction can also occur between Group 1 and 2 elements with sulfur to produce sulfides. Group 2 elements take the formula MS and Group 1 elements take the formula M2S.
 * Nonmetals can also use synthesis reactions with oxygen to form oxides.
 * For example: media type="custom" key="409975" width="100" height="13"
 * When Group 1 metals react with halogens, ionic compounds are formed with the formula MX (M is the metal; X is the halogen).
 * For example: media type="custom" key="409981" width="100" height="13"
 * Group 2 metals and halogens react to take the formula media type="custom" key="409983" width="100" height="17". Because of its reactivity, fluorine combines with most metals.
 * For example: media type="custom" key="409985" width="100" height="14"
 * When an oxide of an active metal reacts with water, a metal hydroxide is created.
 * For example: media type="custom" key="409991" width="100" height="16"
 * Most nonmetal oxides react with water to produce oxyacids.
 * For example: media type="custom" key="410001" width="100" height="21"
 * When some nonmetal oxide reacts with metal oxides, it forms a salt.
 * For example: media type="custom" key="410005" width="100" height="15"

//Decomposition Reactions AX → A + X//
 * These reactions can be simple when they involve turning binary compounds into their original elements. In most cases, a decomposition reaction requires electricity or heat. When done with electricity, it is called electrolysis. This method is used when water is decomposed into hydrogen and oxygen.
 * For example: media type="custom" key="410019" width="100" height="15"
 * Oxides decompose using heat. Oxygen was actually discovered through this process in the 1700s. Mercury(II) oxide, when heated, forms mercury and oxygen.
 * For example: media type="custom" key="410037" width="100" height="16"
 * When heated, metal carbonates form metal oxide and carbon dioxide gas.
 * For example: media type="custom" key="410043" width="100" height="17"
 * Metal hydroxides, excluding those with metals in Group I, when heated, form a metal oxide and water.
 * For example: media type="custom" key="410061" width="100" height="13"
 * When heated and influenced by a catalyst, metal chlorates decompose to form a metal chloride and oxygen.
 * For example: media type="custom" key="410053" width="100" height="14"
 * Nonmetal oxides and water are produced when some acids decompose.
 * For example: media type="custom" key="410067" width="100" height="17"

//Single-Displacement Reactions//
 * This type of reaction often occurs in aqueous solutions. The formulas are A + BX → AX + B.
 * In a compound, a metal can displace another metal.
 * For example: media type="custom" key="410083" width="100" height="16"
 * A metal can also displace hydrogen in water.
 * For example: media type="custom" key="410085" width="100" height="17"
 * Hydrogen found in an acid can be displaced by a metal as well.
 * For example: media type="custom" key="410089" width="100" height="19"
 * Halogens can displace each other.
 * For example: media type="custom" key="410095" width="100" height="13"

//Double-Displacement Reactions AX + BY → AY + BX//
 * This reaction usually forms a precipitate, a gas, or water.
 * Precipitates arise through the combination of cations and anions during the formation of an insoluble compound.
 * For example: media type="custom" key="410101" width="100" height="19"
 * Gas forms as a product of the reaction. It bubbles out of the mixture.
 * For example: media type="custom" key="410107" width="100" height="16"
 * Water can form as a product as well.
 * For example: media type="custom" key="410113" width="100" height="19"

//Combustion Reactions//


 * Combustion reactions occur when a substance combines with oxygen and releases energy. [[image:chart1.JPG align="right" caption="(Also can be found on page 286)"]]
 * For example: media type="custom" key="410131" width="100" height="18"


 * Activity Series of the Elements**


 * When a metal has a larger activity, it loses electrons more easily. When a nonmetal has a larger activity, it gains electrons more readily.
 * The series is listed from the most-active element to the least-active element.
 * Single-displacement reactions help to decipher the order of an element’s activity.
 * An element can replace any element below it in the series, but no element above it.
 * Group 17 elements, the halogens, are actually listed properly according to their activity on the periodic table.
 * It also helps to interpret if a written chemical equation can actually occur. (See if one element can or cannot replace the other based on activity)
 * For example: media type="custom" key="410137" width="100" height="21"

Works Cited

Holt, Rinehart and Winston, ed. **Modern Chemistry.** Austin: Harcourt Education Company, 2006.