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Chemical Reactions

Chemical Reactions. Chapter 7. 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.

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Chemical Reactions

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  1. Chemical Reactions Chapter 7

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

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

  12. 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

  13. 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 1023particles 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

  14. Does a dozen eggs weigh the same as a dozen oranges? • Demo • A mole of carbon has a differentmass 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 amuor 12 grams per one mole of carbon 12 grams C or 1 mole C 1 mole C 12 grams C Molar Mass

  15. 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

  16. 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

  17. Types of Reactions 7.2

  18. 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

  19. 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 • Video Synthesis & Decomposition

  20. Here is another example of a synthesis reaction

  21. Another view of a decomposition reaction

  22. 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 Replacementtwo 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 • Video • With precipitation Single and Double Replacment

  23. Single Replacement Reactions

  24. 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

  25. 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)

  26. 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 Is Gain Oxidation-Reduction (Redox)

  27. 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

  28. Energy Changes in Reactions 7.3

  29. 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

  30. 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

  31. 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 releasesenergy to its surroundings is called an exothermic reaction • A chemical reaction that absorbsenergy from its surroundings is an endothermic reaction Endothermic and Exothermic Reactions

  32. 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

  33. Reaction rates 7.4

  34. 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

  35. 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

  36. 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

  37. Catalysts • 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 Graph shows how a catalyst can lower the amount of energy needed to cause a reaction

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