2 Signs of Chemical Reactions 1. Evolution of heat and light2. Production of Gas3. Formation of a Precipitate
3 1. Evolution of Heat and Light When matter changes it canRelease energy in the form ofHeat and/orLightExample: A burning match,the combination of naturalgas and oxygen in the airto produce a flameon the burner or the combi-nation of sugar and sulfuricacid as in the picture
4 2. Production of a GasWhen gas is produced when two substances are combined is often evidence of a chemical reaction.For example, putting zinc metal in hydrochloric acid and hydrogen gas is producedAdding vinegar to baking soda, carbon dioxide gas is produced.
5 3. Formation of a Precipitate When a solid appears when two liquid solutions are mixed, it is called a precipitate. The appearance of a solid is evidence that a new substance that does not dissolve in water has formed. It is a sign that a chemical reaction has occurred.For example, the mixing of water solutions of ammonium sulfide and cadmium nitrate results in the appearance of a yellow precipitate (cadmium sulfide).
6 4. Color ChangeA change in color is often a sign of a chemical reaction as in the example of the yellow precipitate
7 Characteristics of Chemical Equations 1. The equation must represent facts. All the reactants and products must be identified2. The equation must contain the correct formula. Most elements are represented in their monoatomic form (C, N, Fe). Two exceptions, P4 and S8. Some others that are exist mostly as diatomic molecules are represented as H2, O2.3. The law of conservation of mass must be followed. The same number of atoms must appear on the left and on the right of the equation. To make the numbers equal, we use coefficients or small whole numbers that appear in front of a formula.Characteristics of Chemical EquationsChemical equations summarize the changes in a reactions
8 Three Types of Equations 3. Chemical Equation Characteristics:Like a formula equation but it is balanced. The number of atoms of each kind on the right is equal to the number of each atom on the left.CH4(g) + O2(g) ⇨ CO2 (g)+ 2H2O(g)The equation above follows the law of conservation of mass and is BALANCED1. Word Equation CharacteristicsOnly descriptiveThe reactants and products are represented by wordsDoes not give the whole storyExample:Hydrogen + Oxygen ⇨ water2. Formula Equation CharacteristicsUse of symbols and/or formulas to represent reactants and productsexample:CH4(g) + O2(g) ⇨ CO2 (g)+ H2O(g)The g in parenthesis tells you that it is a gasHowever the equation is not balanced.BalancedUnbalanced
9 Balancing Equations CH4(g) + O2(g) ⇨ CO2 (g)+ H2O(g) This equation above is not balanced.The number of each atom of the left is NOT equal to the number of the same atom on the right. For example, there are 4 H on the left and 2 H on the right.To balance the equation we use the trial and error methodBy changing the coefficients (the number in front of the formula)The resulting number of atoms are counted on both sides of the equation. When the numbers are equal then it is a balanced equation. r/ques5.htm
10 Balancing EquationsCH4(g) + O2(g) ⇨ CO2 (g)+ H2O(g)Balance the different types of atoms one at a time.First balance the atoms of elements that are combined and appear only once on each side of the equationBalance polyatomic ions that appear on both sides of the equation as single units.Balance the H and the O atoms after atoms of all other elements have been balanced.Let’s look at examples p. 251, in the book (online) my.hrw.com and
11 Why is a Chemical Equation Important? 2. The relative masses of the reactants and products of a chemical reaction can be calculated from the reaction’s coefficients.1 mol of H2 x 2.02g H2 = 2.02g H2mol H23. The reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction.. The reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction.1. The coefficients of a balanced chemical reaction indicates relative amounts of reactants and productsFor example,H2 + Cl HClTells you that there are:1 molecule of H2 :1molecule of Cl2:2 molecules of HCl
12 Types of Chemical Reactions Decomposition reactions –AX -> A + XSynthesis or composition reactionA+X -> AXCombustion reactions – Substance reacts with oxygen, release large amounts of energy in the form of heat and lightDouble replacement reactionsAX+BY -> AY + BXSingle replacement reactionsA + BX -> AX + BCombustion
13 Type of Reaction Definition EquationSynthesisA + B ABDecompositionAB A + BSingle ReplacementAB + C AC + BDouble ReplacementAB + CD AC + BDTwo or more elements or compounds combine to make a more complex substanceCompounds break downinto simpler substancesOccurs when one element replaces another one in a compoundOccurs when different atoms in two different compounds trade placesA = Red B = Blue C = Green D = Yellow
14 ____ P + O2 → P4O10 ____ Mg + O2 → MgO 2. Use colored pencils to circle the common atoms or compounds in each equation to help you determine the type of reaction it illustrates. Use the code below to classify each reaction.S = Synthesis D = Decomposition SR = Single Replacement DR = Double Replacement____ P O2 → P4O10 ____ Mg O2 → MgO____ HgO → Hg O2 ____ Al2O3 → Al O2____ Cl NaBr → NaCl Br2 ____ H N2 → NH3
15 ____ P + O2 → P4O10 ____ Mg + O2 → MgO 2. Use colored pencils to circle the common atoms or compounds in each equation to help you determine the type of reaction it illustrates. Use the code below to classify each reaction.S = Synthesis D = Decomposition SR = Single Replacement DR = Double Replacement____ P O2 → P4O10 ____ Mg O2 → MgO____ HgO → Hg O2 ____ Al2O3 → Al O2____ Cl NaBr → NaCl Br2 ____ H N2 → NH3