Chapter 8-1 “Describing Chemical Reactions” pp 241-254 Chemistry Chapter 8-1 “Describing Chemical Reactions” pp 241-254

Objectives List three observations that suggest that a chemical reaction has taken place List three requirements for a correctly written chemical equation Write a word equation and a formula equation for a given chemical reaction Balance a formula equation by inspection

What is a Chemical Reaction? chemical reaction- the process by which one of more substances are changed into one or more different substances original substances = reactants final substances = products chemical reactions are represented through chemical equations

Indications of a Chemical Reaction 1) production of heat and light 2) production of a gas 3) formation of a precipitate 4) color change keep in mind, that these phenomena do not always represent a chemical reaction! They may be physical changes!

Characteristics of Chemical Equations Reactants of the reaction are written on the left, products on the right Proper chemical equations must have three criteria met….

Characteristics of Chemical Equations (cont.) 1) The equation must represent known facts all of the reactants and products must be known, not guessed 2) The equation must contain the correct formulas for the reactants and products use Ch 7! Watch for diatomic molecules (H2, O2, N2, F2, etc.- see table 8-1, p243) or S8, P4

Characteristics of Chemical Equations (cont.) 3) The law of conservation of mass must be satisfied Use coefficients (numbers before a formula) to balance the equation

Word Equations word equation- an equation in which the reactants and products in a chemical reaction are represented by words word equations do not indicate amounts of substances, only the identities of substances Ex: methane + oxygen carbon dioxide + water

Formula Equations formula equation: represents the reactants and products of a chemical reaction by their symbols or formulas ex: CH4(g) + O2(g) -----> CO2(g) + H2O(g) (not balanced) note: a formula equation does not show quantities, only the types of atoms or molecules involved to indicate quantities, the equation must be balanced

Additional Symbols Used in Chemical Equations

Ex Problem Write word and balanced chemical equations for the following chemical reaction Hydrogen peroxide in an aqueous solution decomposes to produce oxygen and water

Your Turn Write word and balanced chemical equations for the following reaction Solid copper metal reacts with aqueous silver nitrate to produce solid silver metal and aqueous copper(II) nitrate

Significance of a Chemical Equation A chemical equation is similar to a math equation, it gives you a lot of information 1) The coefficients of a chemical reaction indicate relative (not absolute) amounts of reactants and products usually shows the smallest whole number ratio of amounts between reactants and products

Significance of a Chemical Equation (cont.) 2) The relative masses of the reactants and products of a chemical reaction can be determined from the reactions coefficients since the coefficients give you the ratio of molecules, you can use molar mass to determine how many grams of x will react with so many grams of y to give you z

Significance of a Chemical Equation (cont.) 3) The reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction The equation works both ways! Chemical reactions are useful, but they do not tell you whether the reaction will actually occur! Experimentation is needed for proof

Balancing Chemical Equations A balanced equation just means that the law of conservation of mass is followed i.e.: the same number of atoms of each element is equal on both sides of the equation There are 4 steps to balancing an equation

Balancing Chemical Equations (cont.) 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 the products

Balancing Chemical Equations (cont.) 3) Balance the formula equation according to the law of conservation of mass a) Balance the different types of atoms one at a time b) First balance the atoms of elements that are combined and that appear only once on each side of the equation

Balancing Chemical Equations (cont.) C) Balance polyatomic ions that appear on both sides of the equation as single units D) Balance H and O atoms after atoms of all other elements have been balanced 4) Count atoms to be sure that the equation is balanced

Balancing Chemical Equations (cont.) Most common mistakes: 1) Incorrect formula equations 2) Changing subscripts instead of coefficients 3) Failure to check work! After you get comfortable, you may skip writing word equations (step 1), but never skip step 4!

Problems: Write balanced chemical equations for the following: 1) Solid calcium metal reacts with water to form aqueous calcium hydroxide and hydrogen gas 2) Nitrogen dioxide gas reacts with water to form aqueous nitric acid and nitrogen monoxide gas 3) Solid potassium chlorate decomposes to form solid potassium chlride and oxygen gas

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