Chapter 21 Chemical Equations Section 1 Chemical Change

What is a chemical reaction?

A. Describing chemical reactions Chemical Reactions- change of one or more substances converted into new substances Reactants - substances that combine or change Products- new substances that are produced produce Reactants  Products

B. Conservation of Mass Conservation of Mass-a law which states that, in a chemical reaction, mass is not created or destroyed; it stays the same Antione Lavoisier – Father of Modern Chemistry Found mass of products equaled the mass of the reactants

C. Writing Equations Chemical equation - uses chemical formulas and symbols to describe a chemical reaction and the products it produces Chemical formula - expresses the relationship between elements in the compounds and molecules they make up Symbols used to show physical state of reactants; (s) solids (aq) aqueous (g) gas (l) liquid

C. Writing Equations Coefficients – whole numbers in front of the symbol, which represent the number of units of each substance in a reaction **Located before the symbols Subscripts – small lowered numbers which represent the number of atoms in a molecule of a particular element *Located after and below the symbols Nomenclature – way to name things scientifically

Discussion Questions What does the law of conservation of mass state? What is the function of coefficients in a chemical equation?

Answers In a chemical reaction, matter is neither created nor destroyed. The total sum of the reactants must equal the sum of the product. Chemist need to know how much of a reactant will produce a certain amount of product

Describe 3 chemical reactions that you have observed (EX: burning marshmallow) There are many things that let you know it’s a chemical reaction: color change, forming a precipitate, production of heat.

Section 2 Chemical Equations

Checking for Balance A balanced chemical reaction- both sides of equation have the same number of atoms of each element Coefficients are used in front of the element or compound to balance. Choosing coefficients - becomes easier with practice; trial and error at first

Writing balanced chemical equations - 4 step process Describe the reaction in words Write the equation using formulas and symbols Check for balance Add coefficients where needed for balance

Discussion Question Why do chemical equations need to be balanced?

Answer Law of conservation of mass says mass will not be created or destroyed

Section 3 Classifying Chemical Reactions

Types of Reactions 1. Synthesis Reaction- two or more substance form a new substance A + B --> AB 2. Decomposition Reaction-One substance breaks down into two or more substances AB ---> A + B 3. Single-displacement Reaction- one element replaces another one in a compound; A + BC ---> AC +B or D + BC --> BD + C 4. Double-displacement Reaction- results if a precipitate, water, or a gas forms when two ionic compounds in a solution are combined; AB + CD --> AD + CB 5. Combustion Reaction-a substance combines with oxygen to produce energy “X” + O2 --> CO2 + H2O

Oxidation-Reduction (Redox) These reactions often involve oxygen Oxidation Loss of electrons Substance becomes “oxidized” Reduction Gain of electrons Substance becomes “reduced”

Discussion Questions 1. What two chemical reactions are “opposites” of each other and why? 2. What are the 5 types of chemical reactions?

Answers Synthesis and decomposition synthesis multiple substances combine to form a new one decomposition a single substance breaks apart into multiple simpler substances Synthesis, decomposition, single-displacement, double-displacement, combustion

Section 4 Chemical Reactions and Energy

A. Chemical Reactions involve energy exchange Exergonic-Breaking chemical bonds release energy Endergonic-Forming chemical bonds requires energy Exothermic-Chemical Reaction where energy is primarily given off in the form of heat Endothermic-Chemical reaction that requires heat energy in order to proceed

B. More energy out A catalyst speeds up a chemical reaction without itself being permanetly changed An inhibitor prevents or slows a chemical reaction or interferes with the catalyst’s action

Discussion Question How are chemical bonds involved in energy exchanges?

Answer Breaking bonds release energy; forming bonds to require energy