Chemical Equations and Reactions Chapter 8

Fireworks http://jeffmilner.com/index.php/2006/01/

Chemistry of Fireworks http://www.scifun.org/CHEMWEEK/fireworks/fireworks.htm

Objectives I can identify a chemical equation and parts it is composed of.

Chemical Reactions Chemical Equation – represents, with symbols, formulas, the identities and relative amounts of the reactants and products in a chemical reaction. Example: (NH4)2Cr2O7 (s)  N2 (g) + Cr2O3 (s) + 4 H2O (g)

Indication of a Chemical Reaction 1. Producing heat or 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.

Characteristics of a Chemical Equation Basic Characteristics 1. The equation must represent known facts. 2. The equation must contain the correct formulas for the reactants and the products. Reactants are on the left of the equation and react to form the products Products are on the right and are produced during the reaction 3.The law of conservation of mass must be satisfied. The law of conservation of mass states that atoms neither are created nor destroyed

Parts of a chemical equation Look at the following what are they composed of? 1. CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) Subscripts Coefficients Yields State of Matter

Parts of a Chemical Equation Cont. Look at the following: 3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g) 2HgO(s)  2Hg(l) + O2(g) Additional symbols are located on pg 246 Table 8-2

Table 8-2

Word equations Word equations are equations in which the reactants and products in a chemical reaction are represented by words. Methane + oxygen  carbon dioxide + water Formula equation represents the reactants and products of a chemical reaction by their symbols or formulas. CH4 + 2O2  CO2 + 2H2O This must be balanced!

Objective I can balance any chemical equation given.

Significance of Balancing Chemical Equations 1. Coefficients of a chemical reaction indicate relative, not absolute, amounts of reactants and products. (Coefficients are used to balance the side of the reaction that must have the same number of atoms because atoms can not be created nor destroyed, just rearranged.) 2. The relative masses of the reactants and products of a chemical reaction can be determined from the reaction’s coefficients. The masses on both sides of the equation must also be the same to conserve the conservation of mass.

Steps to Balancing Chemical Equations 1. Identify the names of the reactants and products, and write a word equation. 2. Write a formula equation by substituting correct formulas for the names of the reactants and products. 3. Balance the formula equation according to the law of conservation of mass. 4. Count atoms to be sure that the equation is balanced.

Balancing Tips: 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. (some people choose to make atom inventories which I will show in a moment)

Example Identify the names of the reactants and products: Water  Hydrogen + Oxygen Write the formula equation: H2O(l)  H2(g) + O2(g) Balance: Left side --------- Right side 2 H 2 1 0 2 This is an atom inventory, now I need to balance by adding coefficients which will add more atoms of what is needed. 2H2O(l)  2H2(g) + O2(g) Count atoms to be sure the equation is balanced 4H 2O --------- 4H 2O

Example of balancing equations This is another example using an atom inventory.

Practice: Magnesium and hydrochloric acid reacts to produce magnesium chloride and hydrogen Magnesium + hydrochloric acid magnesium chloride + hydrogen Mg + HCl  MgCl2 + H2

Practice: Aqueous nitric acid reacts (HNO3) reacts with solid magnesium hydroxide to produce aqueous magnesium nitrate and water.

Practice: Ammonium Chloride + calcium hydroxide  calcium chloride + ammonia + water

Objective I can Identify any chemical reaction as synthesis, decomposition, single replacement, double replacement, or combustion reactions.

Types of Chemical Reactions Synthesis reactions – also known as a composition reaction, two or more substances combine to form a new compound. A + X  AX Decomposition Reactions – a single compound undergoes a reaction that produces two or more simpler substances. (opposite of synthesis reactions) AX  A + X

Type of Reactions Continued Single-Replacement reaction, also known as a displacement reaction, one element replaces a similar element in a compound. A + BX  AX + B Double-Replacement reaction, the ions of two compounds exchange places in a n aqueous solution to form two new compounds. AX + BY  AY + BX Combustion Reactions – a substance combines with Oxygen, releasing a large amount of energy in the form of light and heat.

Objective I can explain the significance of predicting products. I can predict products of synthesis and decomposition reactions.

Predicting PRODUCTS A VERY IMPORTANT SKILL IN CHEMISTRY IS KNOWING WHAT THE PRODUCTS OF A REACTION WILL BE BEFORE MIXING THE REACTANTS NOT KNOWING THE PRODUCTS OF A REACTION CAN LEAD TO THINDS SUCH AS CHEMICALS BLOWING UP IN YOUR FACE,.

Synthesis Reactions Synthesis reactions – also known as a composition reaction, two or more substances combine to form a new compound. A + X  AX

Decomposition Reaction Decomposition Reactions – a single compound undergoes a reaction that produces two or more simpler substances. (opposite of synthesis reactions) AX  A + X

To Predict Products: 1. Identify the type of reaction. If it is synthesis or decomposition you will be combining or breaking apart of the compounds. Example: Synthesis, Na + Cl  NaCl Example: decomposition, NaCl  Na + Cl You see these are just opposites of each other.

Special Cases 1. Metal oxides react with water to produce metal hydroxides/ metal hydroxides decompose to produce metal oxides and water Example: MgO + H2O  Mg(OH)2 Mg(OH)2  MgO + H2O

Special Cases 2. Metal Chlorides react with oxygen to form metal chlorates/ metal chlorates decompose to form metal chlorides and oxygen Example: MgCl2 + O2  Mg(ClO3)2 Mg(ClO3)2  MgCl2 + O2

Special Cases 3. Metal oxides react with carbon dioxide to produce metal carbonates/metal carbonates decompose to form metal oxides and carbon dioxide Example: MgO + CO2  MgCO3 MgCO3  MgO + CO2

Practice: Write the balanced chemical equation Lithium reacts with the air. Potassium Iodide is heated until it decomposes. Calcium hydroxide is decomposed Sulfur dioxide reacts in water Calcium oxide reacts with sulfur dioxide.

Objective I can Predict the products of single and double replacement reactions.

Single Replacement Single-Replacement reaction, also known as a displacement reaction, one element replaces a similar element in a compound. A + BX  AX + B

Double Replacement Reaction Double-Replacement reaction, the ions of two compounds exchange places in a n aqueous solution to form two new compounds. AX + BY  AY + BX

Predicting Products Identify the type of reaction that you have and if single replacement or double replacement you will swop like elements for like elements. In others words metals can switch with metals and nonmetals can switch with nonmetals.

Practice: Copper reacts with Silver nitrate Zinc dissolves in hydrochloric acid Silver nitrate reacts with sodium chloride Iron (II) sulfide dissolves in hydrochloric acid

Objective I can write the formula equation for any reaction. I can balance any reaction. I can predict the products of any reaction.

Combustion Reactions Combustion reactions- a substance combines with oxygen, releasing a large amount of energy in the form of light and heat. 2H2 + O2  2H2O Almost all combustion reactions are hydrocarbons reacting with oxygen to produce carbon dioxide and water.

Practice: Propane being burned (C3H8) 2C6H14 + 19 O2 

Predicting Products Remember when we combine all of the types together, like the exist in the world around you. You must: 1. first identify the reactants and the type of reaction. 2. Then predict the products using what you have learned. 3. balance the reaction