Chemical Equations and Reactions

Signs of Chemical Reactions 1. Evolution of heat and light 2. Production of Gas 3. Formation of a Precipitate

1. Evolution of Heat and Light When matter changes it can Release energy in the form of Heat and/or Light Example: A burning match, the combination of natural gas and oxygen in the air to produce a flame on the burner or the combi- nation of sugar and sulfuric acid as in the picture

2. Production of a Gas When 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 produced Adding vinegar to baking soda, carbon dioxide gas is produced.

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).

4. Color Change A change in color is often a sign of a chemical reaction as in the example of the yellow precipitate

Characteristics of Chemical Equations 1. The equation must represent facts. All the reactants and products must be identified 2. 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 Equations Chemical equations summarize the changes in a reactions

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 BALANCED 1. Word Equation Characteristics Only descriptive The reactants and products are represented by words Does not give the whole story Example: Hydrogen + Oxygen ⇨ water 2. Formula Equation Characteristics Use of symbols and/or formulas to represent reactants and products example: CH4(g) + O2(g) ⇨ CO2 (g)+ H2O(g) The g in parenthesis tells you that it is a gas However the equation is not balanced. Balanced Unbalanced

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 method By 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. http://funbasedlearning.com/chemistry/chemBalance r/ques5.htm

Balancing Equations CH4(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 equation Balance 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, 252-254 in the book (online) my.hrw.com and http://www.nclark.net/ChemicalReactions

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 H2 mol H2 3. 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 products For example, H2 + Cl2 2HCl Tells you that there are: 1 molecule of H2 :1molecule of Cl2:2 molecules of HCl

Types of Chemical Reactions Decomposition reactions – AX -> A + X Synthesis or composition reaction A+X -> AX Combustion reactions – Substance reacts with oxygen, release large amounts of energy in the form of heat and light Double replacement reactions AX+BY -> AY + BX Single replacement reactions A + BX -> AX + B Combustion http://www.pbs.org/wgbh/nova/fireworks/fire.html

Type of Reaction Definition  Equation Synthesis A + B AB Decomposition AB A + B Single Replacement AB + C AC + B Double Replacement AB + CD AC + BD Two or more elements or compounds combine to make a more complex substance Compounds break down into simpler substances Occurs when one element replaces another one in a compound Occurs when different atoms in two different compounds trade places A = Red B = Blue C = Green D = Yellow

____ 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 ____ Cl2 + NaBr → NaCl + Br2 ____ H2 + N2 → NH3

____ 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 ____ Cl2 + NaBr → NaCl + Br2 ____ H2 + N2 → NH3