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Chapter 9 – Chemical Reactions

Chapter 9 – Chemical Reactions. Law of Conservation of Energy. Energy cannot be created or destroyed but can change from one form to another Forms of energy = light, heat, electricity, chemical, kinetic, potential, nuclear. H 2 + O 2  H 2 O 2.

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Chapter 9 – Chemical Reactions

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  1. Chapter 9 – Chemical Reactions

  2. Law of Conservation of Energy • Energy cannot be created or destroyed but can change from one form to another Forms of energy = light, heat, electricity, chemical, kinetic, potential, nuclear

  3. H2 + O2 H2O2 40g + 60g  100g Law of Conservation of Mass (or Matter) • Matter is neither created or destroyed during a chemical reaction • In chemical reactions, the number atoms stays the same no matter how they are arranged….so their total mass stays the same

  4. Classifying Chemical Reactions Synthesis – when two or more substances combine to make a more complex compound +  2 SO2 + O2 2 SO3 SO2 O2 SO3

  5. Classifying Chemical Reactions Decomposition – breaks down compounds into simpler products H2O2 H2 + O2 H2  + O2 H2 O2

  6. Classifying Chemical Reactions Replacement – reaction in which one element replaces another in a compound, or where two elements in different compounds trade places +  + 2 Fe2O3 + 3 C  4 Fe + 3 CO2 Fe2 O3 C Fe CO2

  7. What are Chemical Formulas? Chemical Formula – a way to show the ratio of elements in the compound using chemical symbols and their subscripts H2O – shows 2 Hydrogen and 1 Oxygen atoms

  8. Importance of Accuracy • Examine Figure 6 on page 265: • Why is it important to make sure you read the formulas correctly? • Read the Math Practice box: • Count the number of atoms in the example • Count the number of atoms in each of the formulas and record in your notes.

  9. Writing Formulas for Ionic Compounds What is the formula for writing the names of ionic compounds? Positive Ion 1st, Negative Ion 2nd, Negative ion changes last part of name to -ide

  10. Writing Formulas for Covalent Compounds Read page 263 and answer questions: • How do you write names for covalent compounds? • If the 1st element has only 1 atom, then no prefix • If the 1st element has more than 1 atom, use a prefix • The second atom always has a prefix even if it has 1 atom. • The second element changes name to -ide

  11. Writing Names of Covalent Compounds • What do the prefixes represent? • Copy the prefix chart into notes • What is the formula for: carbon tetrachloride sulfur hexaflouride dihydrogen monoxide octasilicondecoxide • CCl4 • SF6 • H2O • Si8O10

  12. Write the names for these compounds • C2Cl4 • SO4 • N10O2 • I2O5 • P3Br2 • Dicarbon tetrachloride • Sulfur tetroxide • Decanitrogen dioxide • Diodine pentoxide • Triphosphorus dibromide

  13. What causes chemical bonds in molecules to break? • Molecules bump into each other with enough energy to break them • Different substances are combined in a solution • The temperature of a solution is lowered • A solid dissolves into a liquid 2. How do substances form during a chemical reaction? • Two or more substances combine • Chemical bonds break; atoms rearrange and new bonds form • A solid substance is formed in a solution • Chemical bonds in molecules add atoms to make more molecules

  14. 3. Nitrogen Dioxide is a covalent compound. How many oxygen atoms are in the compound? • Three b. one • Two d. None 4. What type of reaction takes place when chlorophyll breaks down into new substances? a. A physical reaction b. a nuclear reaction c. A chemical reaction d. an unbalanced reaction

  15. 5. How can you be sure that a chemical reaction has taken place? • A solid dissolves b. water turns to steam c. A new substance forms d. a mixture is made 6. When chemical bonds break, what do atoms do? • Change into other atoms b. rearrange c. Become energy d. disappear 7. Which statement is true about the law of conservation of energy? • Energy can be created • energy cannot be transferred c. Energy cannot be destroyed d. energy cannot change forms

  16. What are chemical equations? Chemical Equations – use chemical formulas and other symbols to summarize a reaction 4Fe + 3O2 2Fe2O3

  17. Parts of the Equations: • Reactants • Products • Yield Sign • Coefficients • Subscripts Coefficient – tells how many molecules or atoms in a compound Subscript – tells the number of atoms of each element in a compound 2 Fe2O3 + 3 C  4 Fe + 3 CO2 Reactants Yield Products

  18. Parts of Chemical Equations • REACTANTS= the substances that undergo change in chemical reactions(on the left side of the arrow) • PRODUCTS= the new substances that form in chemical reactions(on the right side of the arrow)

  19. Parts of Chemical Equations • COEIFFICIENT= the number in front of the atom or molecule that represents the entire molecule • SUBSCRIPT= the number that follows the element and only represents the atom it follows. • YEILD SIGN= the arrow, shows the chemical reaction has happened

  20. Coefficients and Subscripts • Are there the same number of atoms on both sides of this equation?: H2 + O2 H2O Could you balance it by writing this: H2 + O2 H2O2 How could you actually balance the equation? 2H2 + O2 2H2O

  21. Differences Between Subscripts and Coefficients • Subscripts only apply to the element to the left of the subscript. • Coefficients apply to the whole molecule and multiply by the subscripts (2H2O = 4H and 2O) • If you change the subscript, you change the molecule H2O is not H2O2 • If you add a coefficient, you do not change the molecule, just the number of molecules (2H2O is 2 water molecules)

  22. H2 + O2 H2O2 40g + 60g  100g Conservation of Matter • Matter is neither created or destroyed during a chemical reaction • The total mass of reactants must equal the total mass of the products • In chemical reactions, the number atoms stays the same no matter how they are arranged….so their total mass stays the same

  23. How many atoms? • 1. 2H3O • 2. 3F + O • 3. 4N3F • 4. 2HCl2

  24. Balancing Chemical Equations • The atoms of the compounds or molecules in the reactants must always be equal to the atoms of the compounds or molecules in the products Steps: 1. Write equation 2. Count atoms 3. Use coefficients 4. Count to check __ SO2 + __ O2 __ SO3 __ SO2 + __ O2 __ SO3 1 Sulfur 4 Oxygen __ SO2 + __ O2 __ SO3 1 Sulfur 1 Sulfur 4 Oxygen 3 Oxygen __SO2 + __ O2 2 SO3 2 Sulfur 6 Oxygen 2 SO2 + __ O2 2 SO3 2 Sulfur 2 Sulfur 6 Oxygen 6 Oxygen

  25. Open book to page 266

  26. Are these reactions balanced? • Na + Cl2 NaCl 2. Cu + Br2  CuBr2 3. K + O2  K2O

  27. Balance these equations • CH4 + O2 CO2 + H2O • N2 + H2  NH3 • AgNO3 + MgBr2  Mg (NO3)2 • CS2 + O2  CO2 + SO2

  28. Practice • Complete worksheet to practice balancing chemical equations.

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