Chemical Reactions

Introduction What is a chemical reaction? Any process in which one set of substances is converted into a new set of substances Any process in which a chemical change occurs

Parts of a Chemical Reaction Reactants – the substances at the beginning of the reaction Products – the substances created because of the reaction

What has to happen for a chemical reaction to occur? Chemical reactions typically do not occur at random “Spontaneous” reactions are very rare Most chemical reactions require some specific conditions in order to occur. These include:

1) Adding heat

2) Adding water or another liquid solution

3) Breaking reactants into smaller pieces

4) Mechanical stirring or mixing of reactants

Chemical Equations Represent chemical reactions Reactants are on one side of the equation, products on the other An arrow shows the direction of the reaction – it always points at the products and away from the reactants. Reactants  Products -3 types: word, skeleton and balanced equations

1. Word Equations Only element and formula names are used. No symbols, chemical formulas or numbers Ex: Write the following as a word equation: Hydrogen reacts with oxygen to produce water. Hydrogen + Oxygen  Water

Hydrogen and oxygen are reactants, which are typically written on the left side of the equation. Water is a product and is written on the right side of the equation.

Try it out! Please try out the “Word Equations” sheet in your worksheet booklet.

2. Skeleton Equations Symbols replace words in the word equation Not very useful in itself – but is a step toward writing finished, balanced equations Ex: hydrogen + oxygen  water H2 + O2  H2O

Remember diatomic elements (elements that, by themselves, are found in pairs of atoms.) There are 7 of them to remember: H2, O2, I2, Br2, Cl2, F2, N2 *Two other elements to be aware of: sulfur (by itself, written as S8) and phosphorus (by itself, written as P4)

Please work on the “Word Equations” sheet, converting your word equations to skeleton equations.

3. Balanced Equations The next type of equation is the balanced equation To understand why we balance equations, we must look at the Law of Conservation of Mass.

Law of Conservation of Mass The total mass of ALL reactants before a chemical reaction must be equal to the total mass of ALL products after a reaction. Ex: Mg + O2  MgO *Not balanced! We have one Mg and two O on reactants side, and one Mg and one O on products side. **We must balance the equation to show that mass has been conserved!

Balancing Equations – Rules to Follow: 1. Make sure that the elements and compound in the equation are written correctly. 2. Place a number (coefficient) IN FRONT of the compound or element to adjust the number of atoms. 3. Balance all other elements first, and leave O’s and H’s until the end. 2 Mg + O2  2 MgO

You may deal with them by: Some equations may be more troublesome to balance (usually involve diatomic gases) You may deal with them by: Using a common multiple Using a fraction Ex: Al + H3PO4  H2 + AlPO4 (NOT BALANCED) Using a common multiple: 2 Al + 2 H3PO4  3 H2 + 2 AlPO4

Using a fraction: 1 Al + 1 H3PO4  3/2 H2 + 1 AlPO4 **Can’t use fractions on single atomic elements. Diatomic elements can have fractions because they are pairs of atoms. diatomic

Something else to note: You may notice letter subscripts following substances in equations. These subscripts describe the state of the substance in the reaction. (s) – solid (l) – liquid (g) – gas (aq) – aqueous – dissolved in water

Please work on the “Balancing Equations Practice” sheet (on the back of the Word Equations worksheet). Go back to the “Word Equations” sheet and balance the equations you’ve written!

Types of Chemical Reactions There are six main types of chemical reactions: 1. Double replacement (also called double displacement) 2. Single replacement (also called single displacement) 3. Synthesis (formation) 4. Decomposition 5. Combustion 6. Acid-Base (Neutralization)

Reaction Type 1 – Double Replacement Involves two ionic compounds (does not happen with covalent compounds!) Involves an exchange of cations (positive ions) The cation of one compound trades places with the cation of another compound to form two new compounds

Double Replacement - Characteristics occur in solution when the compounds are in an aqueous state (water is involved) reactants are either aqueous to begin with, or solid compounds dissolved in water to form an aqueous solution of ions in order to drive the reaction, one of the products must be removed from the aqueous solution this can occur by:

Double Replacement - Characteristics 1) Formation of a precipitate (an insoluble solid forms in the solution) ex.: AgNO3 (aq) + NaCl (aq) = NaNO3 (aq) + AgCl (s) Solid silver chloride is formed as precipitate and comes out of the solution

Double Replacement - Characteristics 2) Formation of a gas ex.: FeS (aq) + 2HCl (aq) = H2S (g) + FeCl2 (aq) Hydrogen sulfide gas is formed and comes out of the solution

Double Replacement - Characteristics 3) Formation of water ex.: NaOH (aq) + HCl (aq)  HOH (l) + NaCl (aq) Water (HOH) is formed and comes out of the solution

Identifying a Double Replacement Reaction Double replacement reactions always have two ionic compounds on the reactants side and two ionic compounds on the products side of an equation Two compounds react to form two new compounds AB + CD  CB + AD A & C are cations, B & D are anions A & C switch places

Reaction Type 2 – Single Replacement Reaction

Single Replacement - Characteristics Atom (s) of a lone element replace the atom (s) of an element in a compound Cations replace cations (happens with metals) Anions replace anions (happens with non-metals)

Single Replacement - Characteristics Metal Cation Replacement: For the reaction: A + BC  B + AC A and B are cations A “replaces” B in the compound

Single Replacement - Characteristics Non-metal Anion Replacement: For the reaction: D + EF  ED + F D & F are anions D “replaces” F in the compound Non metal replacements usually involve halogens

Restrictions on Single Replacement Reactions Activity Series: A characteristic of metals and halogens referring to their reactivity

Restrictions on Single Replacement Reactions A single replacement reaction WILL NOT OCCUR if the reactivity of the pure element reactant is less than that of the compound reactant ex.: Sn (s) + NaNO3 (aq)  no reaction b/c tin is less reactive than sodium

Restrictions on Single Replacement Reactions A single replacement reaction WILL OCCUR if the reactivity of the pure element reactant is greater than that of the compound reactant ex.: Zn (s) + H2SO4 (aq)  ZnSO4 (aq) + H2 (g) Reaction occurs because the reactivity of zinc is higher than hydrogen

Reaction of Silver Nitrate and Copper

Identifying Single Replacement Reactions Single replacement reactions ALWAYS have 1 lone element and 1 compound on the reactants side and the products side Reactants will always be: 1 lone element + 1 compound Products will always be:

Example 1. __Al + __ Pb(NO3)2 

Decomposition, Synthesis/Formation and Combustion Reactions

Reaction Type 3 – Synthesis/Formation Occurs when two or more components come together to form one compound Energy is released when these reactions occur Reactants will always be: ALWAYS more reactants than products! 2 or more components (either elements or compounds) Products will always be a single compound. Ex: A + B  AB

Synthesis/Formation Example Two individual components coming together to form one unit Ex. Single Brad Pitt and single Angelina Jolie join forces to become a couple!

Reaction Type 4 - Decomposition Occurs when a single compound breaks down into two or more components Energy is required for them to occur Reactants will always be 1 compound Always more products than reactants 2 or more components (either compound or elements) ex.: AB  A + B

Decomposition Example: One unit breaks down into its component parts ex. – The marriage of Brad Pitt and Jennifer Aniston breaks into the single Brad Pitt and the single Jennifer Aniston

Did you notice? Decomposition and Synthesis/Formation Reactions are opposite processes!

Skip the section in your notes about special cases.

Reaction Type 5 - Combustion Also known as BURNING Hydrocarbon Compounds composed of only carbon and hydrogen Reactants will always be: Hydrocarbon + oxygen Products will always be: Carbon dioxide + water ex.: C6H6 + O2  CO2 + H2O *NOT BALANCED!

Balancing combustion reactions Can be tricky (get high numbers) As always, balance all other elements before O’s and H’s if possible. 2C6H6 + 15O2  12CO2 + 6H2O

Reaction Type 6: Acid-Base Reactions (Neutralization) An acid reacts with a base to form a salt and water. This is a double replacement reaction but we can think of it as a special type because the products are always going to be a salt and water. This is called a neutralization reaction because the acid and base properties of the reactants are destroyed (neutralized).

In this type of reaction, the H+ from the acid combines with the OH- from the base to form water (HOH, written as H2O). The anion from the acid and the cation from the base form the salt. Ex: hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water. HCl + NaOH  NaCl + H2O

Predicting Solubilities In many chemical reactions, especially double replacement reactions, a new solid (precipitate) may be formed. Since we know how to finish chemical equations (or predict the products of a reaction), we may also predict whether the new products are soluble or insoluble.

Soluble : stays dissolved (aq) Insoluble : forms a precipitate (s) Because solubility refers to the ability of a compound to dissolve, the chemical reactions we’ll be looking at typically consist of solutions reacting.

Refer to your Solubility Rules & Exceptions chart to help you predict the states of the products in the following reaction: Pb(NO3)2 (aq) + 2 KI(aq)  PbI2 ( ) + 2 KNO3 ( )

Solubility

Refer to your Solubility Rules & Exceptions chart to help you predict the states of the products in the following reaction: Pb(NO3)2 (aq) + 2 KI(aq)  PbI2 (s) + 2 KNO3 (aq)

Energy in Chemical Reactions

What is energy? Energy is the ability to do work. There are many forms of energy. All forms of energy can be converted from one form to another.

Types of Energy Electrical energy – toaster, batteries Gravitational energy – water falling from a waterfall Light energy – light from the sun

Elastic energy – bungee cord Mechanical energy – a pencil sharpener taking the wood off of a pencil Sound Energy – a student yelling or screaming after failing a test… hopefully this isn’t you…

Types of Energy Nuclear energy – nuclear reactors Potential energy – stored energy Kinetic energy – energy of motion

And of course … chemical energy! We can think of chemical energy as the potential of a substance to undergo a chemical reaction. It is found in the bonds of molecules.

Energy in Chemical Reactions Energy is always involved in a chemical reaction. Sometimes energy is released or given off as the reaction takes place. Sometimes energy is absorbed. Based on the type of energy change involved, chemical reactions are classified as either exothermic reactions or endothermic reactions.

In either type of reaction, energy is neither created nor destroyed – it merely changes position or form. The energy released or absorbed usually takes the form of heat or visible light.

Exothermic Reactions An exothermic reaction is a chemical reaction where energy is released. Heat comes out of, or is released from, a reacting substance during an exothermic reaction. A reaction that involves burning or combustion is a good example of an exothermic reaction.

Reactants  products + energy In an equation, an exothermic reaction has energy on the side of the products. Reactants  products + energy

Endothermic Reactions An endothermic reaction is a chemical reaction where energy is absorbed from the surroundings. The energy absorbed during an endothermic reaction is usually in the form of heat or light. The most familiar endothermic reactions are those involved in cooking – as you fry an egg, for example, the white begins to become solid. If you were to remove the egg from the heat, the reaction would stop. The egg would remain half cooked until you heated it again.

Reactants + energy  products In an equation, energy is found on the side of the reactants in an endothermic reaction. Reactants + energy  products

Please note: If you are given questions that ask you to identify the type of reaction without specifying endothermic or exothermic, then the question is asking you to classify the reaction according to the 6 main types of reaction only (single replacement, double replacement, synthesis, decomposition, combustion, acid-base.)

Rates of Reaction What affects how fast a reaction occurs? Watch the analogy in the following video to help you understand this concept. We will be doing a lab to explore rates of reaction as well.

Rates of Reaction Videos https://www.youtube.com/watch?v=2OEPFhV16Pg