Chapter 8 – Chemical Equations & Reactions Chemistry

Chapter 8 – Section I I. A chemical reaction is the process by which one or more substances are changed into one or more different substances The original substances are the reactants (they do the reacting). The resulting substances are the products (they are the things that are produced). According to the Law of Conservation of Mass the total mass of the reactants must equal the total mass of the products in a chemical reaction

A. Indications of a Chemical Reaction 1. The production of energy in the form of heat and light Examples: A gas stove burning The decomposition of ammonium dichromate 2. The production of a gas Examples: Carbon dioxide & vinegar forming bubbles An alkaseltzer tablet in water 3. The formation of a precipitate (a solid that forms during a chemical reaction and separates from the solution is a precipitate) Examples: Ammonium sulfide & cadminum nitrate Potassium iodide & lead nitrate

4. A color change B. Characteristics of Chemical Equations 1. Equation must represent all known facts 2. The equation must contain the correct formulas for reactants and products 3. The Law of Conservation of Mass must be followed so the equation will be balanced a. The total mass of the reactants and products must be equal b. A whole number placed in front of symbols or formulas in an equation to balance it is a coefficient

C. Word and Formula Equations 1. Write or state a word equation a. This is qualitative or descriptive only b. A + sign is read as “and” c. An → is read as “yields” 2. Write a formula equation a. Gives symbols and formulas of the reactants and products b. Abbreviations are used to show the state of the reactants and products (g) gas (l) liquid (s) solid

3. Balance the equation a. The equation must show the Law of Conservation of Mass b. Coefficients are placed in front of symbols and formulas in the equation to balance it c. Subscripts can never be changed, since this would change the identity of the compound

D. Additional Symbols Used in Chemical Equations 1. These are symbols used in equations to better explain the reaction

(equation symbols continued…) 2. A catalyst is a substance that speeds up the rate of the reaction, but is not changed itself during the reaction

3. An inhibitor is a substance that slows a reaction’s rate or prevents the reaction from occurring 4. A chemical reaction in which the products reform the original reactants is called a reversible reaction

1 molecule H2 : 1 molecule Cl2 : 2 molecules HCl E. Significance of a Chemical Equation 1. The coefficients of a chemical reaction indicate relative (not absolute) amount of reactants and products 2. Relative masses can be determined from the reaction’s coefficients H2(g) + Cl2(g) 2HCl(g) 1 molecule H2 : 1 molecule Cl2 : 2 molecules HCl

F. Balancing chemical Equations 3. The reverse reaction has the same relative amounts of the substances as the forward reaction F. Balancing chemical Equations 1. Identify the names of the reactants and products by writing or stating a word equation Hydrogen and chlorine yield hydrochloric acid 2. Write a formula equation 3. Balance the equation H2(g) + Cl2(g) → HCl(aq) H2(g) + Cl2(g) → 2HCl(aq)

4. Count the atoms on both sides of the yield sign to make sure the equation is balanced 4H + 2O → 4H + 2O 5. If the atoms do not add up to be the same on both sides of the →, then adjust the coefficients only in order to make the atoms balance 2H2O(l) → 2H2(g) + O2(g)

Chapter 8 – Section II There are five basic types of chemical reactions The type of chemical reaction is determined by the way the atoms rearrange themselves during the chemical reaction A. Synthesis Reactions 1. In a synthesis (or composition) reaction two or more substances combine to form a new compound A + X  AX

2. Some common synthesis reactions are elements combining with oxygen to form oxides and metals combining with halogens to form salts B. Decomposition Reactions 1. When a single compound undergoes a reaction that produces two or more simpler substances, this is a decomposition reaction AB  A + B 2. When decomposition occurs by the u se of an electric current, this is called electrolysis H2O electricity H2 + O2

C. Single Displacement (Replacement) Reactions 1. When one element replaces a similar element in a compound, this is a single displacement reaction A + BX  AX + B 2. Some common reactions are one metal replacing another metal in a compound, a metal replacing hydrogen in a compound, or one halogen replacing another halogen in a compound

D. Double Displacement (Replacement) Reactions 1. When the ions in two different compounds exchange places and form two new compounds, this is a double displacement reaction AX + BY  AY + BX 2. During double displacement reactions, it is common for one of the products to be a precipitate, or a gas, or water

E. Combustion Reactions 1. When a substance combines with oxygen to release large amounts of energy in the form of heat and light, this reaction is called combustion 2. This is what occurs during the burning of any fuel or during any explosion

3. When hydrogen undergoes combustion, water will be produced 4. Most fuels will produce carbon dioxide and water when they undergo combustion 5. You can usually expect carbon dioxide and water to be products when a compound undergoes combustion

Chapter 8 – Section III III. Activity Series of the Elements A. The ability of an element to react is called the element’s activity 1. The more readily it reacts, the greater its activity is 2. Those that react more readily will be at the top of the list B. A list of elements organized according to the ease with which the elements undergo certain chemical reactions is known as the activity series 1. The order of these elements is used in determining single displacement reactions

2. A metal in the series can replace any element below it in the list, but cannot replace an element above it 3. On the periodic table a halogen can replace any halogen below it on in the group, so the halogens are listed this way in the activity series 4. According to periodic trends, this holds true for most nonmetals