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# What is a chemical reaction?

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What is a chemical reaction?
When reactants are converted to products, bonds holding the atoms together are broken and new bonds are formed. Reactants Products Recall that atoms themselves are neither created nor destroyed; they are merely rearranged.

What is a word equation? Chemical reactions can be described using words. Either “Iron reacts with oxygen to produce iron (III) oxide (rust)” Or Iron + Oxygen iron (III) oxide

How do we represent chemical equations?
Easier to use chemical shorthand to represent chemical relationships Reactants on the left of the arrow and products to the right of the arrow Fe (s) + O2(g) Fe2O3(s) Where (s)= solid (l) = liquid (g) = gas (aq)= aqueous solution (dissolved in water) Heat or catalyst added to a reaction would appear above the arrow. See Table 8.1

Balancing Chemical Equations
H2(g) O2 (g) H2O(l) Recall that matter is neither created or destroyed, so what is wrong with this picture? It’s not balanced, we lost an oxygen, this is not possible so we balance the equation with coefficients, whole numbers that are placed in front representing the relative quantities of each compound. 2 H2(g) O2 (g) H2O(l) Now both sides are equal.

Rules for Balancing Equations
Determine the correct formulas for all reactants and products in the reaction. Use parentheses when necessary. Count the number of atoms of each element in the reactants and products. For simplicity, a polyatomic ion appearing unchanged on both sides can be counted as a single unit. Balance the elements one at a time using coefficients. if no coefficient is written, it is assumed to be one. You cannot change the subscripts to balance the equation. Check each atom or polyatomic ion to make sure it is balanced. Finally make sure all the coefficients are in lowest possible ratios.

Example

Flow Chart for Balancing Chemical Equations

Types of chemical equations
Section 8.2

Plan of the Day POD—for extra credit Questions on lab? Homework?
Types of Reactions Powerpoint

Plan of the day Major Reaction Types Get into a 5 groups
Make a quick poster (big, bold, letters) These will be your notes for Reaction Types On your poster: Define your reaction Show an example How can you tell a reaction is your type of reaction Show the skeleton reaction!

Plan of the day Finish your notes in the lab while I get set up.
Finish demos Work on our learning goal: Be able to identify types of reactions! Finish Worksheet from yesterday Please Bring in your Textbooks--Tomorrow!

Plan of the day Business Thus far in the Reaction Unit? What is left?
Test corrections, retakes. Lab tomorrow—wear appropriate clothing! Thus far in the Reaction Unit? Balancing Equations√ Identifying the 5 types of reactions √ What is left? Predicting Products Using solubility rules to predict double replacement reactions Net-Ionic Equations

Combination Reactions
Two or more substances combine to form a single substance, or… Two or more products combine to form a single product. Example: Mg(s) + O2 (g) MgO (s)

Predicting Combination Reactions
4 Possibilities: Metal + Non-Metal -- Ionic Compound K + Cl2  KCl Non-metal + Non-Metal  (more than one product possible) S + O2  SO2 or SO3 Non-metal oxide + H2O  acid SO2 + H2O  H2SO4 Metallic oxide + H2O  metal hydroxide CaO + H2O  Ca (OH)2

Decomposition Reactions
A decomposition reaction is one in which a single compound breaks down into two or more elements or new compounds. Decomposition reactions often require an energy source, such as heat, light, or electricity, to occur. Hard to predict! usually a gas or element! Example: heat CaCO3(s) CaO(s) CO2(g)

Combustion Reactions In a combustion reaction, oxygen combines with a substance and releases energy in the form of heat and light. Heated hydrogen reacts with oxygen to produce heat and water in a combustion reaction. This is also a combination reaction.

Predicting Combustion
Element + Oxygen  oxides Mg + O2  MgO Hydrocarbons (or alcohols) + Oxygen  CO2 + H2O CH4 + O2  CO2 + H2O

Single Replacement Reactions
A reaction in which the atoms of one element replace the atoms of another element in a compound is called a single replacement reaction. A + BX → AX + B Cu + AgNO3  Ag + CuNO3

Single Replacement Reactions
Predicting Products — if the activity of the metal is higher than the metal cation in solution there will be a reaction, otherwise there is not a reaction. What is happening? The more reactive metal will form a compound with the anion, and the other metal will be pulled out of solution and become a solid metal again. Ex) Mg (s) + Zn(NO3)2 (aq) Mg(NO3)2 (aq)+ Zn(s)

Reactivity's A metal will not always replace a metal in a compound dissolved in water because of differing reactivity's. An activity series can be used to predict if reactions will occur.

Double Replacement Double replacement reactions occur when ions exchange between two compounds. A precipitate, gas, or molecule will form. This figure shows a generic double replacement equation.

Predicting Double Replacement
The solid product produced during a chemical reaction in a solution is called a precipitate. All double replacement reactions produce either water, a precipitate, or a gas.

Double Replacements Continued
This table shows the steps to write double replacement reactions.

Summary of Reactions This table summarizes different ways to predict the products of a chemical reaction.

Pop Quiz 1) Ca(OH)2 + Al2(SO4)3  CaSO4 + Al(OH)3
2) Mg + Fe2O3  Fe + MgO     3) C2H4 + O2  CO2 + H2O  4) PbSO4  PbSO3 + O2

Solutions Net Ionic Equations

Plan of the day Finals Revisit course expectations
Check your grades to make sure they are accurate. Homework for Chapter 8: (odd) due Friday!

What is a solution? An aqueous solution contains one or more dissolved substances (called solutes) in water. The solvent is the most plentiful substance in a solution.

Aqueous Solutions Water is always the solvent in an aqueous solution.
There are many possible solutes—sugar and alcohol are molecular compounds that exist as molecules in aqueous solutions. Compounds that produce hydrogen ions in aqueous solutions are acids.

Ionic Compounds in Solution
Ionic compounds can also be solutes in aqueous solutions. When ionic compounds dissolve in water, their ions separate in a process called dissociation.

Aqueous Reactions Continued
When two solutions that contain ions as solutes are combined, the ions might react. If they react, it is always a double replacement reaction. Three products can form: precipitates, water, or gases.

Complete Ionic Equations
Aqueous solutions of sodium hydroxide and copper(II) chloride react to form the precipitate copper(II) hydroxide. 2NaOH(aq) + CuCl2(aq) → 2NaCl(aq) + Cu(OH)2(s) Ionic equations that show all of the particles in a solution as they actually exist are called complete ionic equations. 2Na+(aq) + 2OH–(aq) + Cu2+ (aq)+ 2Cl–(aq) → 2Na+(aq) + 2Cl–(aq) + Cu(OH)2(s)

Ions that do not participate in a reaction are called spectator ions and are not usually written in ionic equations. Formulas that include only the particles that participate in reactions are called net ionic equations. 2OH–(aq) + Cu2+(aq) → Cu(OH)2(s)

Predicting Precipitates
Using solubility rules for ionic equations we can predict whether a precipitate is formed and thus predicting what the products are. If the solubility rules predict insolubility we have a reaction, a precipitate! Example: Na2SO4 (aq) + BaNO3 (s) ??? Assume double replacement Look at the products Any insoluble?

Example NaSO4 + BaNO3 ??? Assume double replacement
Look at the products Any insoluble? Na2SO4 + Ba(NO3) NaNO3 + Ba(SO4) Write the complete ionic equation: 2Na+ (aq) + SO42- (aq) + Ba2+ (aq) + 2NO3-(aq) Na+ (aq) + 2NO3- (aq) + Ba(SO4)(s) Eliminate all of the ions that did not change from the reactant side to the product side. Ba2+ (aq) + SO42- (aq) Ba(SO4)(s) (net ionic eqn)

Plan of the day Problems of the day Questions on homework.
Lab Write Up.

Plan of the day Lab write up! Examples?

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