Chemical Reactions Chapter 7 MAKE INTO TWO TESTS!!!!!!!!!!!

What type of change is happening in the picture? When charcoal burns, it changes into other substances while producing heat and light. Burning is a chemical change Describing Reactions 7.1

One way to describe a change of state is to describe what is present before and after the change. In a chemical reaction, the substances that undergo change are called reactants. The new substances formed as a result of that change are called products. In the picture, the reactants are the carbon in the charcoal, and the oxygen in the air. The product is CO2 gas. Chemical Equations

Using Equations to Represent Reactions Reactants  Products To describe burning of charcoal: Carbon + Oxygen  Carbon Dioxide C + O2  CO2 A chemical equation is a representation of a chemical reaction in which the reactants and products are expressed as formulas Using Equations to Represent Reactions

What happens to the products in a chemical reaction? During a chemical reaction, the mass of the products is always equal to the mass of the reactants. This is called the Law of Conservation of Mass, that mass is neither created nor destroyed. This law was established by Antoine Lavoisier Conservation of Mass

Equation reads “one carbon atom reacts with one molecule of oxygen and forms one molecule of carbon dioxide”. If you have 6 C atoms, they will react with 6 O2 molecules to form 6 CO2 molecules The equation has the same number of atoms on each side of the equation Conservation of Mass

Some chemical reactions are powerful enough to propel a space craft. Rocket fuels contain a compound called hydrazine, N2H4 When hydrazine burns in the presence of oxygen, the reaction produces nitrogen, water vapor, and heat. You can describe this reaction by writing a chemical equation: N2H4 + O2  N2 + H2O Balancing Equations

Balancing Equations N2H4 + O2  N2 + H2O If we count the atoms on both sides, we will see that the # of atoms on the left are not equal to the # of atoms on the ride side of the equation. This equation is NOT balanced In order to show that mass is conserved during a reaction, a chemical equation must be balanced. You balance a chemical equation by changing the coefficients (# in front of the formula) Balancing Equations

Steps to Balancing Equations No coefficient- is an understood “1” First, count the number of atoms of each element on each side of equation Starting with metals, change coefficients in front of formulas until balanced. Try Na + H2O  NaOH + H2 HCl + CaCO3  CaCl2 + CO2 + H2O Steps to Balancing Equations

Write a balanced equation for the reaction between copper and oxygen to produce copper (II) oxide, CuO. Steps: Write equation with reactants on the left and products on the right Cu + O2  CuO Balance atoms 2Cu + O2  2CuO MATH PRACTICE

Math Practice H2O2  H2O + O2 Mg + HCl  H2 + MgCl2 Balance the following chemical equations: H2O2  H2O + O2 Mg + HCl  H2 + MgCl2 Math Practice

Counting Chemicals How many shoes do you have? Shoes are counted in pairs. How many eggs are in a dozen? Bottle rockets in a gross? Pair, dozen, gross are all UNITs we use to count. How do chemists count particles? DEMO Counting Chemicals

Counting Moles Chemists need to be able to count atoms or molecules. These units are too small to be counted, so chemists have another way to count them. Chemists use a unit called the mole (No, not that kind of mole!) A mole is an amount that contains 6.02 x 1023 particles of that substance Aka “Avogadro’s #’ Particles: atoms, molecules or ions Ex: 1 mole of Fe (Iron) contains 6.02 x 1023 atoms of iron. Counting Moles

Molar Mass Does a dozen eggs weigh the same as a dozen oranges? Demo A mole of carbon has a different mass than a mole of sulfur The mass of one mole of substance is called a molar mass The molar mass for chemicals is the same as the atomic mass. Ex: Carbon’s mass is 12 amu or 12 grams per one mole of carbon 12 grams C or 1 mole C 1 mole C 12 grams C Molar Mass

“Molar mass” is the same as “formula mass” For a compound, add the atomic masses of its components atoms EX: CO2 1 atom of Carbon x 12 g + 2 atoms of Oxygen x 16 g 44 grams of CO2 per one mole of CO2 You can use this molar mass to convert moles of a substance to mass and vice versa. 44.0 g CO2 or 1 mol CO2 1 mol CO2 44.0 g CO2 “Molar mass” is the same as “formula mass” Mole Mass Conversions

Mole-Mass Conversions Suppose you have 55 g of CO2. You can use the g/mol conversion factor to calculate how many moles: 55.0 g CO2 1 mol CO2 1.25 mol CO2 44g CO2 You can also convert from moles back to grams 2 mol CO2 44g CO2 88.0 g CO2 1 mol CO2 Mole-Mass Conversions

Types of Reactions 7.2

Classifying Reactions How do you classify matter? (solid, liquid, gas – remember?) Chemical reactions are also classified into different types: Synthesis Decomposition Single-replacement Double-replacement Combustion Classifying Reactions

Synthesis & Decomposition Synthesis reactions are reactions in which 2 or more substances react to form a single substance A + B  AB (like a MARRIAGE) Ex: 2 Na + Cl2  2 NaCl Video Decomposition reactions are reaction in which one substance is broken down into two or more simpler substances (opposite of synthesis) AB A + B (like a DIVORCE) Ex: 2H2O  2 H2 + O2 Synthesis & Decomposition

Here is another example of a synthesis reaction

Another view of a decomposition reaction

Single and Double Replacment Single Replacement is a reaction where 1 element takes the place of another element in a compound A + BC  B + AC (like a LOVE TRIANGLE) Ex: Cu +2Ag(NO3)  2Ag + Cu(NO3)2 Video Double Replacement two different compounds exchange positive ions and form 2 new compounds AB + CD  AD + CB (like DO-Si-DO and change partners) Ex: Pb(NO3) + 2KI  PbI2 + 2 KNO3 With precipitation Single and Double Replacment

Single Replacement Reactions

Combustion reaction one in which a substance reacts rapidly with oxygen, producing heat and light Ex: CH4 + 2O2  CO2 + 2 H2O Always react with oxygen and usually produces CO2 , gas and water Products in combustion are ALWAYS carbon dioxide and water. (although incomplete burning does cause some by-products like carbon monoxide) Many reactions can be classified by more than one type. Combustion

Quick Lab Identifying a type of Reaction pg 203

Oxidation-Reduction (Redox) Oxidation-reduction reaction: a reaction where electrons are transferred from one reactant to another, aka redox Ex: Calcium reacts with oxygen to produce calcium oxide 2Ca + O2  2CaO The product (CaO is composed of ions where the reactants were neutral atoms) When calcium reacts with oxygen, each neutral atom loses electrons to form Ca+2 ions Ca  Ca2+ + 2e- When an element loses electrons during a chemical reaction it is called oxidation The calcium lost electrons so it was oxidized Oxidation Can occur without oxygen Oxidation-Reduction (Redox)

Oxidation-Reduction (Redox) Reduction is the process where an element gains electrons Each neutral oxygen atom gains two electrons becoming O2- ion O + 2e-  O2- The oxygen gained electron and has been reduced Oxidation and reduction always occur together If one element loses electrons another element HAS to gain them OIL RIG Oxidation Is Loss Reduction Gain Oxidation-Reduction (Redox)

State the type, classify and balance the following reactions: _Pb(NO3)2 + _HCl  _PbCl2 + _HNO3 _C2H6 + _O2  _CO2 + _HOH _Ca + _HCl  _CaCl2 + _H2 _Hg + O2  HgO _SO2 + _O2  _SO3 DR Combustion SR Redox Synthesis Synthesis Do you see a Redox reaction? Mixed Practice

Energy Changes in Reactions 7.3

Chemical Bonds and Energy Where does the heat come from when you light a propane grill? C3H8 + 5O2  3CO2 +4H2O + Heat This equation shows that the heat released in the reaction came from the reactants. Chemical energy is the energy stored in the chemical bonds of a substance energy changes in chemical reactions are determined by changes that occur in the chemical bonding Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products Chemical Bonds and Energy

Breaking & Forming Bonds Breaking chemical bonds requires energy. Where could this energy come from when using a propane grill? Grills have a lighter which produces a spark, giving enough energy to break the bonds and start the reaction The formation of chemical bonds releases energy. (resulting in heat and light that you see) Breaking & Forming Bonds

Endothermic and Exothermic Reactions Physical changes can release or absorb energy Exothermic: releases Endothermic: absorbing During a chemical reaction energy is either released or absorbed A chemical reaction that releases energy to its surroundings is called an exothermic reaction A chemical reaction that absorbs energy from its surroundings is an endothermic reaction Endothermic and Exothermic Reactions

What point on each graph represents the highest energy? As you go from left to right in each graph, what happens to the reactants? They react to form the products What point on each graph represents the highest energy? The energy is highest at each curve’s peak What do the double-headed arrows represent? The difference in chemical energy between the reactants and products Which type of reaction has products with a greater amount of energy that the reactants? endothermic

Reaction rates 7.4

A reaction rate is the rate at which reactants change into products over time Rate just means a change over time, like distance over time= speed Reaction rates tell you how fast a reaction is going How fast reactants are consumed, products are formed or energy released/absorbed. Factors that affect reaction rates are: Temperature Surface Area Concentration Stirring Catalysts Reaction Rates

Temperature and Surface Area How does temperature affect reaction rates? Increasing temp causes particles to move faster and collide, # of collisions increases then rate increases Decreasing the temperature will decrease the reaction rate Surface area is the amount of area exposed An increase in the surface area increases the exposure of reactants to one another allowing more collisions And therefore allowing an increase in the reaction rate (newspapers) Temperature and Surface Area

Stirring and Concentration Stirring increases the reaction rate by increasing the number of collisions between the particles of the reactants. (washing machine vs. soaking) Concentration is the number of particles in a given volume. (ex sugar in tea) The more particles of reactant, the higher the reaction rate Gas concentration changes with pressure (less room) The greater the pressure of the gas, the greater it’s concentration and the faster it’s reaction rate Stirring and Concentration

They can be used to speed up or slow down reactions A catalyst is a substance that affects the reaction rate without being used up in the reaction. They can be used to speed up or slow down reactions Catalysts Graph shows how a catalyst can lower the amount of energy needed to cause a reaction