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Chemical Reactions and Mass Conservation

Learn about chemical reactions, reactants, and products. Understand the law of conservation of mass and how to balance chemical equations. Explore the concept of moles and molar mass.

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Chemical Reactions and Mass Conservation

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  1. Chapter 7 Chemical Reactions

  2. 7-1 Describing Reactions • 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 • Reactants  Products • Ex. Carbon + oxygen  carbon dioxide • Word equation

  3. Chemical equations • Ex. C + O2 CO2 • Chemical formula • A chemical equation is a representation of a chemical reaction in which the reactants and products are expressed as formulas

  4. Conservation of Mass • During chemical reactions, the mass of the products is always equal to the mass of the reactants. • Established by Antoine Lavoisier and is known as the law of conservation of mass • The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction

  5. How is mass conserved in chemical change? Activity • Follow the directions on page 191 in your book • Fill out following information • Mass of the bag and the water=____________ • Mass of a square piece of paper with sides 10cm__________________ • Mass of the tablet and the paper together____________ • Mass of just the tablet_______________ • The combined masses of the bag, the water, and the tablet___________ • Mass of the bag and its contents_____________

  6. Activity questions • How do you know whether a chemical change took place?

  7. Activity Question • What happened to the mass of the plastic bag and its contents after the bubbling stopped? • What might this information tell you about a chemical change?

  8. Beaker Breaker • A chemical__________ is a representation of a chemical reaction in which the reactants and products are expressed as formulas • True or False mass can be destroyed during a chemical reaction. • In the equation H2 + O  H2O write the symbols that represent the reactants.

  9. Balancing Equations7-1 Continued • In order to show that mass is conserved during a reaction, a chemical equation must be balanced • You can balance a chemical equation by changing the coefficients, the numbers that appear before the formulas • As you balance equations, you should never change the subscripts in a formula

  10. Steps to balancing equations • Count the number of atoms of each element on each side of the equation • Ex: N2H4 + O2 N2 + H2O • Left side has 2 nitrogen, four hydrogen, and 2 oxygen atoms • The right side has 2 nitrogen, 2 hydrogen, and 1 oxygen atom • The hydrogen and oxygen atoms need to be balanced.

  11. Steps to balancing equations 6. Change one or more coefficients until the equation is balanced _1_N2H4 + _1_O2 _1_N2 + _2_H2O • The equation is now balanced • Each side has two nitrogen, 2 oxygen, and 4 hydrogen atoms

  12. Practice Balancing equations problems • ___Na + ___H2O ____NaOH + ___H2 • ___HCl + ___CaCO3  ____CaCl2 +___CO2 + ____H2O • ____Al + ____Cl2  _____AlCl3 • ___Cu + _____O2  _____CuO • ____H2O2  ____H2O + ____O2

  13. Beaker Breaker • Balance the following equation • _____Fe + _______Cl2 ______FeCl3 • _______K + _______Br2 ______KBr

  14. Beaker Breaker • Balance the following equations • _______K + _______Br2 ______KBr • ______Mg + ____O2 _______MgO

  15. Scientists work with samples large enough for us to SEE and WEIGH on a balance using units of ……. GRAMS.

  16. This Creates a Problem…… A pile of atoms big enough for us to see contains billions of atoms. How do we keep track of that many atoms??????

  17. Development of the Term Mole The word “mole” was introduced about 1896 by Wilhelm Oswald, who derived the term from the Latin word moles meaning a “heap” or “pile”.

  18. Avogadro’s Number Named in honor of Amadeo Avogadro (1776 – 1856) He studied gases and discovered that no matter what the gas, there were the same number of molecules present.

  19. What is a MOLE? It is a number of things….. - Just like a dozen 1 dozen eggs = 12 eggs So, instead of 12 the mole is equal to 602 billion trillion – OR- 6.02 x 1023 602 000 000 000 000 000 000 000

  20. Just How Big is a Mole? 6.02 X1023  Pennies: Would make at least 7 stacks that would reach the moon.

  21. How Big is a mole? Cover the earth to a depth of 200 miles with soft drink cans. Cover the USA to a depth of 9 miles of unpopped popcorn kernels.

  22. The mole is many things…. 1 dozen cookies = 12 cookies 1 mole of cookies = 6.02 x 1023 cookies 1 dozen cars = 12 cars 1 mole of cars = 6.02 x 1023 cars 1 dozen Al atoms = 12 Al atoms 1 mole of Al atoms = 6.02 x 1023 atoms NOTE: the mole is abbreviated mol (gee, that’s a lot quicker to write……)

  23. Everybody Dance Now!!!!!!! Moleshake My moleshake brings all the moles to the yard, And they’re like, “Man, Chemistry’s hard” Yeah right, yeah Chemistry’s hard, I can teach you, and I won’t even charge.

  24. Molar Mass • Mass of 1 mole of a pure substance • Numerically equal to the atomic mass but expressed in GRAMS • The atomic mass of carbon is 12.0amu, so the molar mass of carbon is 12.0grams

  25. What is the molar mass of the following: • Potassium ? • 39.098 g (or 39.098 g/mol) Nickel ? • 58.693 g (or 58.693 g/mol)

  26. What is the atomic mass of the following: • Potassium ? • 39.098 amu Nickel ? • 58.693 amu

  27. Practice • What is the molar mass of the following: • KCl • C2H6 • CaCO3 • H2O

  28. Homework/More Practice • What is the molar mass of the following: • HCl • NaCl • CaO

  29. Homework/More Practice • Balance the following ___Na + ____H2O ___H2 + ___NaOH ___C2H6 + ____O2  ___CO2 + ___H2O

  30. Beaker Breaker • Calculate the molar mass of the following: • MgCl2 • CaBr2

  31. Dude!!! Atoms are REALLY small!!! We can’t work with individual atoms or mass individual atoms. BECAUSE…….. We can’t see things that small!

  32. Gram- Mole Conversions

  33. How many grams are needed to have 0.852 moles gold? • 0.852 mol Au 196.97 g Au 1 1 mol Au = 168 g Au

  34. Convert 2.50moles of KClO3 to grams

  35. Calculate how many grams are in 0.700moles of H2O2?

  36. How many moles are in 55 g of lead? • 55 g Pb 1 mol Pb 1 207.2 g Pb • = 0.26 moles Pb

  37. How many moles are in 86.1 g sodium? • 86.1 g ? mole Na = 1 ? g • 86.1 g 1 mole Na = 1 22.99g • 3.74 moles Na

  38. How many moles are in 22 grams of copper metal?

  39. 7-2 Types of Reactions • General types of chemical reactions • Synthesis • Decomposition • Single-replacement • Double-replacement • Combustion

  40. Single Replacement • Single-replacement reaction is a reaction in which one element takes place of another element in a compound. • A + BC  B + AC

  41. Teacher Demonstration • See handout • Cu + 2AgNO32Ag + Cu(NO3)2 • Copper replaces the silver nitrate to form copper(II) nitrate • The products is silver, which you can see. The other product is copper(II) nitrate, gives the solution its blue color.

  42. 7-2 Continued Reactions as Electron Transfer • The discovery of subatomic particles enabled scientists to classify certain chemical reactions as transfers of electrons between atoms. • A reaction in which electrons are transferred from one reactant to another is called an oxidation-reduction reaction, redox reaction

  43. Oxidation • Synthesis reactions, in which a metal combines with oxygen, traditionally have been classified as oxidations. • 2Ca +O2  2CaO • Ca  Ca+2 + 2e- • Calcium loses two electrons • A reactant is oxidized if it loses electrons

  44. Reduction • As calcium atoms lose electrons during the synthesis of calcium oxide, the oxygen gains electrons • O + 2e-  O2- • The process in which an element gains electrons during a chemical reaction is called reduction • A reactant is said to be reduced if it gains electrons. • Oxidation and reduction always occur together.

  45. Beaker Breaker • Identify the following reactions as either synthesis, decomposition, single replacement, double replacement, or combustion: 2C2H6 + 7O2 4CO2 + 6H2O Ca + 2HCl  CaCl2 + H2

  46. Beaker Breaker Cont. • Write a paragraph explaining why the formation of water can be classified as a synthesis or combustion.

  47. 7-3 Energy Changes in ReactionsChemical Bonds and Energy • Heat produced by a propane(C3H8) grill is a form of energy • Balance equation: • C3H8 + ____O2___CO2 +___H2O • Heat will be added to the right side of the equation

  48. Propane Combustionusing models • C3H8 + 5O2 3CO2 + 4H2O • Open to page 207 follow Figure 17

  49. Chemical Energy • Chemical energy is the energy stored in the chemical bonds of a substance. • Using the models to make C3H8 (propane) • How many C-H bonds_________ • How many C-C bonds_________

  50. Chemical Bonds • Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products. • Each propane molecule reacts with 5 oxygen molecules. • In order for the reaction to occur, the 8 C-H single bonds, 2 C-C single bonds and 5 O=O double bonds must be broken. • Breaking bonds require energy • Propane grills require a igniter to provide enough energy to break the bonds

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