Chemistry Ch 8 - Chemical Reactions

Reactions & Equations When you take substances and rearrange their atoms to form new substances you have created a chemical reaction! Chemical reactions are another name for chemical changes. These occur all around us

What are some examples of chemical reactions? How can you tell a chemical reaction has occurred?

Chemical Equations Chemical Equations are used by chemists to communicate to other chemists what is happening during a chemical reaction.

Chemical Equation Chemical equations are used to represent chemical reactions. They show: Reactants The starting substances Products The new substances formed (s)(l)(g)(aq) Tells you the state of matter or if a water solution Coefficients Tell you the lowest whole-number ratio of amounts of substances in the equation.

Reactant 1 + Reactant 2 + etc.  Product 1 + Product 2 + etc. Plus sign separate each substance The arrow is used to separate the reactants from the product. Sometimes above the arrow there is information telling us what substances were used to help the chemical reaction run. These substances are called catalysts

Balancing Equations

Balancing Chemical Equations Balance the formula equation by the use of coefficients to balance the number of each type of atom on the reactant and product sides of the equation.

Balancing Tips: 1. Count the atoms of the elements in the reactants 2. Count the atoms of the elements in the products 3. Change the coefficients to make the number of atoms of each element equal on BOTH sides of the equation. ( NEVER change the subscript in a chemical formula to balance an equation). 4. Make sure the coefficients are in their lowest possible ratio. 5. Check your work!

Example Hydrogen gas reacts with oxygen gas to produce water. H 2 (g) + O 2 (g)  H 2 O (l) 2 H 2 (g) + O 2 (g)  2 H 2 O (l)

Example w/ Polyatomic Ions Solutions of lithium phosphate and calcium chloride react to make a lithium chloride solution with solid calcium phosphate. Li 3 PO 4 (aq) + CaCl 2 (aq)  LiCl (aq) + Ca 3 (PO 4 ) 2 (s) 2 Li 3 PO 4 (aq) + 3 CaCl 2 (aq)  6 LiCl (aq) + Ca 3 (PO 4 ) 2 (s)

Example Solid iron(III) oxide reacts with carbon monoxide gas to produce solid iron (I) oxide and carbon dioxide gas. Fe 2 O 3 (s) + CO (g)  Fe 2 O (s) + CO 2 (g) Fe 2 O 3 (s) + 2CO (g)  Fe 2 O (s) + 2CO 2 (g)

One Last Example Iron metal reacts with chlorine gas to form a solid iron(III) chloride. Fe(s) + Cl 2 (g)  FeCl 3 (s) 2Fe (s) + 3Cl 2 (g)  2FeCl 3 (s)

Reactions follow: THE LAW OF CONSERVATION OF MATTER

The amount of atoms of an element MUST be equal on both sides of the equation. That is why we balance equations, to account for all the matter used in the reaction

Types of Reactions Combination (Synthesis) reaction Decomposition reaction Single-replacement reaction Double-replacement reaction Combustion reaction Neutralization reaction involve acids & bases

You will be able to identify the different types of reactions. You will recognize them by their reactants & products For some you will be able to predict whether a reaction will happen at all.

Combination Reaction What does combination mean? In this reaction two or more simpler compounds or elements form one product. Also called a synthesis reaction General Equation: A + B  AB

Decomposition Reaction What does Decomposition mean? In this reaction: One reactant would break down into two or more simpler substances. General Equation: AB  A + B

Single-replacement Reaction An element and a compound react together and a new element and new compound are formed Sometimes called Displacement Reaction General Equation: A + BX  AX + B General Equation: A + BX  AX + B Examples: Na + KCl  K + NaCl F LiCl  2 LiF + Cl 2 Watch the next two slides

Na + KCl  K + NaCl Na K Cl

F LiCl  2 LiF + Cl 2 F Li Cl F Li Cl Li

More Single Replacement Reactions Metals replace metals (and hydrogen) Examples: 2Al + 3CuSO 4  Al 2 (SO 4 ) 3 + 3Cu Zn + H 2 SO 4  ZnSO4 + H 2 2 Na + 2 H 2 O  2 NaOH + H 2

We can tell whether a reaction will happen using the Activity Series of Metals Some metals are more reactive than others The MORE active metals replace less active metals There is a list on page 217 – Table 8.2 You will be given this list on quizzes/tests

Single Replacement A metal higher on the list replaces one lower on the list. So if the element by itself is higher, the reaction happens, if element by itself is lower, it doesn’t If there is no reaction we write NR after the arrow

Single Replacement Note the * H can be replaced in acids by metals higher than it on the list Only the first 4 (Li - Na) react with water.

Activity Series Examples: Will a reaction occur for the following? Pb + KCl  Al + H 2 O  What does it mean that Ag is on the bottom of the list?

Single Replacement Continued Nonmetals can replace other nonmetals Limited to F 2, Cl 2, Br 2, I 2 (the Halogens) The order of activity follows the periodic table. Higher replaces lower. Will a reaction occur for the following? F 2 + KCl  I 2 + LiBr  Br 2 + KCl 

Double-replacement Reactions Two compounds react together to form two new compounds. Use the charges on the ions to help in determining the new substances formed. These are sometimes called double- displacement reactions General Equation: AX + BY  AY + BX

Double Replacement Reactants must be two ionic compounds or acids. Usually in aqueous solution NaOH (aq) + FeCl 3 (aq)  The positive ions change place. Na + OH - + Fe 3+ Cl -  Fe 3+ OH - + Na + Cl - 3 NaOH + FeCl 3  Fe(OH) NaCl Watch the next slide

3NaOH + FeCl 3  Fe(OH) 3 + 3NaCl Na + O 2- H+H+ Na + O 2- H+H+ Na + O 2- H+H+ Fe 3+ Cl -

Double Replacement Tips The reaction one of the products will do the following: A. doesn’t dissolve in water and forms a solid called a precipitate B. is a gas that bubbles out. C. is a covalent compound (usually water). Polyatomic ions don’t change from side to side

Combustion Reactions: A hydrocarbon compound is reacted with oxygen gas A hydrocarbon? What’s a hydrocarbon? If the combustion is complete, the products will be CO 2 and H 2 O. If the combustion is incomplete, the products will be CO and H 2 O or just C and H 2 O. O 2 will ALWAYS be the second reactant

Examples Complete combustion of C 4 H 10

Examples Complete combustion of C 6 H 12 O 6

Question for you: How do you identify acids & bases?

Common Acids Acids that you must know (memorize): HCl = hydrochloric acid HNO 3 = nitric acid H 2 CO 3 = carbonic acid H 2 SO 4 = sulfuric acid H 3 PO 4 = phosphoric acid

Common Bases How to know the bases: Look for hydroxide polyatomic ion (OH - ) Name the base from the name of the metal and then hydroxide. EX: NaOH potassium hydroxide Mg(OH) 2

Neutralization Reactions A reaction in which an acid and a base react in aqueous solution to produce a salt and water. A salt is an ionic compound made up of a cation from a base and an anion from an acid. These reactions are double-replacement reactions. General Equation: General Equation: Acid (aq) + Base (aq)  Salt (aq) + Water (l) EX: Mg(OH) 2 (aq) + 2 HCl (aq)  MgCl 2 (aq) + 2 H 2 O (l)

More Neutralization Examples H 2 SO NaOH  Na 2 SO H 2 O Ba(OH) HCl  BaCl H 2 O Notice – Reactants are always an acid and a base and one of the products is always water