CHAPTER 6 Chemical Reactions and Equations

Warm-up Who can recall what the 4 pictures were on the intro slide for this chapter? Fire Test tube with a blue solid substance Two flasks (one with a blue liquid and one with a yellow liquid) Smoke trail What do all of these things have to do with chapter 6? They all show signs of a chemical change

Indications of a Chemical Change Chemical Change The change of one or more substances into new substances by the rearrangement of atoms. Change in energy Formation of a Precipitate (two liquids combine and a solid is produced) Formation of a gas Change in color

Parts of a Chemical Equation Reactants Physical States (solid, liquid, gas or aqueous) Coefficients Products

Writing Equations Word Equations Using the names of chemicals to describe chemical reactions Chemical Equations Using chemical formulas and numbers to describe chemical reactions Hopefully you noticed that this equation defies the Law of Conservation of Matter. It is not balanced. Once you master writing equations, you will also learn how to balance equations.

Practice Problems Use your ion tables Solid copper + fluorine gas = solid copper (II) fluoride Re-write the following chemical equation in words Re-write the following word equation as a chemical equation aqueous calcium chloride +Aqueous lithium chromate aqueous lithium chloride Solid calcium chromate + Cu (s) +F 2(g) CuF 2 (s)

Energy When chemical reactions take place, energy is either absorbed or released in the process An endothermic reaction An exothermic reaction

Warm up Watch the following video to complete the warm up Write the following chemical equation for the reaction that occurred in the video: Aluminum + iodine = aluminum iodide You should be able to answer the following questions: What are the reactants? What are the products? What was the indication that a chemical change took place? Was this an endotherm or exothermic reaction?

Physical States of Elements Metals o Metals are always represented by their symbols from the periodic table, with no subscripts o All metals, with the exception of mercury are solid and will have an (s) following their symbols in an equation Cu (s) Non-metals o Are often diatomic (Br,I,N,Cl,H,O,F) so they will have a subscript of 2. o Most of these diatomic elements will be a gas except iodine (s) and bromine (l) Cl 2(g)

Physical States of Ionic Compounds Ionic Compounds o Their physical state will be o (aq) = aqueous if they can dissolve in water (soluble) o (s) = solid if they can’t (insoluble) Use this table to determine the solubility of ionic compounds:

More Practice Problems What would be the physical states of the following substances if written in a chemical equation? PbI 2 __________KNO 3 ____________ KI __________ Pb(NO 3 ) 2 ____________ Write the following equation with physical states: Lead (II) nitrate combines with potassium iodide to form potassium nitrate and lead (II) iodide. (s) (aq) Pb(NO 3 ) 2(aq) + KI (aq) KNO 3(aq) + PbI 2(s)

Warm-up Watch the following video to see what happens Write the chemical equation for the reaction: (Use a solubility table to predict the physical states of each compound) potassium iodide + lead (II) nitrate = potassium nitrate and lead (II) iodide Did you write the correct physical state for each compound in the reaction?

Balancing Chemical Equations Why? To demonstrate the Law of Conservation of Mass which states: In a chemical reaction mass is neither created or destroyed Antoine Lavoisier What must be conserved? The number and type of atoms 2 sodium atom + 2 chlorine atoms = 2 sodium atoms + 2 chlorine atoms

How to Balance a Chemical Equation o Make sure that both sides of an equation have an equal number of each type of atom represented. o Balance equations by adding coefficients. o Never mess with subscripts Na Cl Mg Br

Strategies for Balancing Chemical Equations 1. The even/odd combination Make the subscript of one, the coefficient of the other and vice versa. Iron (III) oxideIron + Oxygen Practice :

Strategies Continued 2. Balance groups as a whole if the elements of the group are only represented in the group on each side of the equation. Mg OH Al Practice:

Strategies Continued 3. Balancing Hydrocarbon Combustion Reactions The first step is to add a coefficient to the hydrocarbon so that the total number of hydrogens in the molecule is divisible by four. Then, balance the product side of the equation before finishing up with oxygen. CHOCHO Practice:

Strategies Continued 4. Splitting Water When hydroxide shows up on one side of an equation and water is on the other side, split water into H(OH) to balance out hydroxide. Practice:

Things to Keep in Mind Again, do not mess with subscripts In the final solution, the coefficients have to be in the lowest whole number ratio. If at first you don’t succeed, try, try again! Most of the solutions to these problems come from trial and error.

Comprehensive Practice Problem  Write the equation  Note the physical state of each substance  Balance the equation Potassium chromate combines with lead (II) nitrate to form potassium nitrate and lead (II) chromate

Warm-up

Types of Chemical Reactions 5 General Types Synthesis Decomposition Single displacement Double displacement Combustion You will need to know how to identify these reactions

Combustion C x H y + O 2 CO 2 + H 2 0 Synthesis A + B AB (Two elements combine to form a compound) Decomposition AB A + B (One compound splits to form two compounds) Types of Chemical Reactions – 5 General Formulas Always the products of these combustion reactions

Reactions Cont. Single Replacement (Single Displacement) A + BC AC + B ( Metals replace metals and non-metals replace non-metals ) Double Replacement (Double Displacement) AB + XY AY + XB (Compounds switch partners)

Practice Problems Identify the type of reaction Combustion DecompositionSingle replacememt SynthesisDouble Replacement

Warm-up What type of reaction does the picture represent? What are the reactants in this reaction? Which one will run out first?

Equilibrium Many reactions can change direction. These are called reversible reactions. When no net change occurs in the amount of reactants and products, a system is said to be in equilibrium. A system in which opposite actions are taking place at the same rate, it is said to be in dynamic equilibrium. Products and reactants are forming at the same rate. Reactants are never fully used up because they are constantly being formed from products.

Reaction Rate ew=detail&FORM=VIRE7 ew=detail&FORM=VIRE7 How fast do reactions reach equilibrium? The amount of energy the particles must have when they collide is called the activation energy of that reaction. The speed of reaction measures how quickly the reactants disappear or how quickly the products appear. The amount of substance changed per unit time is the rate of reaction.

Factors that Affect the Rate of a Reaction  Temperature affects the rate of reaction also.  The higher the temperature, the faster the reaction; the lower the temperature, the slower the reaction.  The amount of substance present in a certain volume is called the concentration of the substance. Raising the concentration of a reactant will speed up a reaction because there are more particles per volume When one reactant is used up, the reaction will stop The reactant that is used up first is called the limiting reactant

Factors that Affect the Rate of a Reaction Presence of a catalyst speeds up the rate of a reaction without being permanently changed of used up itself. A catalyst speeds up the reaction by lowering the activation energy. Biological catalysts are called enzymes. Reactions that have undesirable products sometimes have to be slowed down. A substance that slows down a reaction is called an inhibitor.