chapter8_kmjr

Chapter 8 – Chemical Equations and Reactions

Chemical equation- represents, with symbols and formulas, the identities and relative molecular or molar amounts of the reactants and products in a chemical reaction.

To know a chemical reaction has taken place requires evidence that one or more substances have undergone a change in identity. 1. Evolution of energy as heat and light 2. Production of a gas 3. Formation of a precipitate – a precipitate is a solid that is produced as a result of a chemical reaction in solution and that separates from the solution. 4. Color Change
 * Indications of a Chemical Reaction:**

To help to read and write chemical equations correctly- 1. The equation must represent known facts – all reactants and products must be identified. 2. The equation must contain the correct formulas for the reactants and products - knowledge of the common oxidation states of the elements and of methods of writing formulas. 3. The law of conservation of mass must be satisfied - The same number of atoms of each element must appear on each side of a correct chemical equation. To balance numbers of atoms add coefficients – ( a small whole number that appears in front of a formula in a chemical equation) where necessary.
 * Characteristics of chemical equations:**
 * (NH4)2Cr2O7 (s) ---N2 (g) + Cr2O3 (s) + 4H20 (g)**
 * There are 4 moles of water are produced for each mole of nitrogen and chromium (III) oxide that is produced.**

Word Equation: an equation in which the reactants and products in a chemical reaction are represented. - A word equation is only descriptive it does not give the whole story because it does not give quantities of reactants used or products. - When Methane burns in air, it combines with oxygen to produce CO2 and water vapor.
 * Word and Formula Equations**

Next you replace the names of the reactants and products with appropriate symbols and formulas. Methane is a molecular compound composed of one carbon atom and four hydrogen atoms. -- CH4 – Oxygen exists in nature as a diatomic molecule so it is represented as--O2­-- Carbon Dioxide and water are CO2 and H2O. Formula equation- represents the reactants and products of a chemical reaction by their symbols or formulas.
 * Methane + Oxygen** **à Carbon Dioxide + Water**
 * CH4 + O2** **à CO2(g) + H20 (g) (not balanced)**

The (g) means it is gaseous state. A formula equation meets two of the three requirements for a correct chemical equation. - Shows the facts and the correct symbols and formulas for the reactants and products -Take the law of conservation of mass must be taken into account. To do that you need to **Balance the Equation.**


 * CH4 + O2** **à C02 + 2H2O**

symbols.

Quantitive information revealed by a chemical equation
 * Significance of a Chemical Equation**
 * 1.** The coefficients of a chemical reaction indicate relative, not absolute mounts of reactants and products.
 * H2 + Cl2** **à 2HCl**
 * 1 molecule H2 : 1 molecule Cl2 : 2 molecules HCl**


 * -The ratio always has to be the smallest possible amount-**

2. The relative masses of the reactants and products of a chemical reaction can be determined form the reactions coefficients. 3. The reverse action for a chemical equation has the same relative amounts of substances as the forward reaction.

1. Identify the names of the reactants and the products and write a word equation. Water à hydrogen + oxygen
 * Balancing Chemical Equations**

2. Write a formula equation by substituting correct formulas for the names of the reactants and the products

H2O à H2 + O2

3. Balance the formula equation according to the law of conservation of mass. -Balance the different types of atoms one at a time -First balance the atoms of elements that are combined and that appear only once on each side of the equation. -Balance polyatomic ions that appear on both sides of the equation as single units -Balance H atoms and O atoms after atoms of all other elements have been balanced

2H2O à 2H2 + O2

4. Count atoms to be sure that the equation is balanced. 2H2O à 2H2 + O2 (4H+20) = (4H) + (2O)

The most common mistakes of balancing the equations are -writing incorrect chemical formulas for reactants or products -trying to balance an equation by changing subscripts


 * Types of Chemical Reactions**


 * Synthesis Reactions**
 * -**Also known as a composition reaction, two or more substances combine to form a new compound.
 * A +X** **à AX**

-Almost all metals react with oxygen to form oxides
 * Reactions of Elements with Oxygen and Sulfur**


 * 2Mg + O2** **à 2MgO**


 * -**Nonmetals also undergo synthesis reactions with oxygen to form oxides. Sulfur, for example, reacts with oxygen to form sulfur dioxide.


 * S8 + 8O2** **à 8SO2**

-Hydrogen reacts with oxygen to form dihydrogen monoxide or water


 * 2H2 + O2** **à 2H2O**


 * Reactions of Metals with Halogens**

-Most metals react with the halogens to create either ionic or covalent compounds. -Group 2 metals react with halogens to form ionic compounds with the formula MX2-
 * M- Metals X- Halogens**

-Active metals are highly reactive metals. -Oxides of active metals react with water to produce metal hydroxides. For example Calcium oxide reacts with water to form calcium hydroxide
 * Synthesis Reactions with Oxides**


 * CaO + H2O** **à Ca(OH)2**

-Many oxides of nonmetals react with water to produce oxyacids -Certain metal oxides and nonmetal oxides react with each other in synthesis reactions to form salts.


 * CaO + SO2** **à CaSO3**

-Decomposition reaction is a single compound undergoes a reaction that produces two or more simpler substances
 * Decomposition Reactions**
 * AX** **à A + X**

-AX is a compound. A and X can be Elements or Compounds.


 * Decomposition of Binary Compounds**

The simplest kind of decomposition reaction is the decomposition of a binary compound into its elemets.

ELECTRICITY 2H2O -- 2H2 +O2

The decomposition of a substance by an electric current is called –**Electrolysis** -When a metal carbonate is heated, it breaks down to produce a a metal oxide and carbon dioxide gas. - All metal hydroxides except those containing Group 1 metals decompose when heated to yield metal oxides and water. -Certain acids decompose into nonmetal oxides and water. Carbonic acid is unstable and decomposes readily at room temp. to produce Carbon dioxide and water. H2CO3(aq) à CO2 + H2O
 * Decomposition of Metal Carbonates**
 * Decomposition of Metal Hydroxides**
 * Decomposition of Metal Chlorates**
 * -**When a metal chlorate is heated, it decomposes to produce a metal chloride and oxygen.
 * Decomposition of Acids**

-Also known as a replacement reaction one element replaves a similar element in a compound. A + BX à AX + B OR Y+BX à BY + X
 * Single-Displacement Reactions**

-A,B,X, and Y are elements. AX, BX, and BY are compounds

-Aluminum is more active than lead. When solid aluminum is plced in aqueous lead (II) nitrate, Pb (NO3)2(aq), the aluminum replaves the lead. Solid lead and aqueous aluminum nitrate are formed.
 * Displacement of a metal in a compound by another Metal**
 * 2Al + 3 Pb(NO3)2(aq)** **à 3Pb + 2Al(NO3)3(aq)**

-The most – active metals, such as those in Group 2, react vigorously with water to produce metal hydroxide and hydrogen -The less-active metals, such as iron, react with steam to form a metal oxide and hydrogen gas.
 * Displacement of Hydrogen in Water by a Metal**

-The more-active metals react with certain acidic solutions, such as hydrochloric acid and dilute sulfuric acid, replacing the hydrogen in the acid. The reaction products are a metal compound (a salt) and hydrogen gas.
 * Displacement of Hydrogen in an Acid by a Metal**

-In another type of single displacement reaction, one halogen replaces another halogen in a compound. Flourine is the most active halogen. As such, it can replace any of the other halogens in their compounds. Each halogen is less active than the one above it in the periodic table.
 * Displacement of Halogens**

The ions of two compounds exchange plaves in an aqueous solution to form two new compounds.
 * Double Displacement Reactions**
 * AX + BY** **à AY +BY**

A, X, B and Y in the reactants represent ions. AY and BX represent ionic or molecular compounds.

-The formation of a precipitate occurs when the cations of one reactant combine with the anions of another reactant to form an insoluble or slightly soluble compound.
 * Formation of a Precipitate**


 * Formation of a Gas**
 * -**In some double displacement reactions one of the products is an insoluble gas that bubbles out of the mixture.


 * Formation of Water**
 * -**In some double-displacement reactions, a very stable molecular compound, such as water, is one of the products.

-A substance combines with oxygen, releasing a large amount of energy in the form of light and heat
 * Combustion Reactions**

-An activity series is a list of elements organized according to the ease with which the elements undergo certain chemical reactions. -For metals greater activity means a greater ease of loss of electrons, to form positive ions. -For nonmetals greater activity means a greater activity means a greater ease of gain of elections to form negative ions.
 * Activity Series of the Elements**