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Understanding Chemical Reactions: Processes, Clues, and Balancing Equations

This chapter explores the fundamentals of chemical reactions, including the rearrangement of atoms, the characteristics of reactants and products, and how energy changes occur during these processes. Key indicators of a chemical change, such as color change, gas production, and energy release, are discussed. Techniques to speed up reactions are highlighted, along with the importance of balancing chemical equations for accurate representation. The Law of Conservation of Mass and various types of chemical equations—word, skeleton, and formula—are also explained.

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Understanding Chemical Reactions: Processes, Clues, and Balancing Equations

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  1. Chapter 4.1 Chemical Reactions

  2. Get it? http://images.mnn.com/sites/default/files/user/134790/chemistry-cat-mhBjFn.jpg

  3. Chemical Reactions • Chemical Change– the rearrangement of the atoms in a substance that creates new products with different chemical properties • Chemical Reaction-- the process in which one or more substances undergo a chemical change • Change in energy Energy can be released (heat, light, sound) or absorbed • Occur at different rates: a reaction can be slow or fast (a catalyst is used to speed up reactions)

  4. Demos Q: How do we know a chemical change has occurred? Observe: Demo 1– lead nitrate + potassium iodide Demo 2– baking soda + citric acid solution

  5. Clues to a Chemical Reaction • Change in colour • Precipitate forms • Energy is released or absorbed • Gas is produced • Difficult to reverse REMEMBER: A change in state or dissolving a substance is a physical change-- NOT a chemical change

  6. How can we speed up a chemical reaction? • Increase the temperature • Increase the surface area • Increase reactant concentration • CATALYST: A substance that makes a chemical reaction go faster WITHOUT being consumed.

  7. Chemical Equations • Using words or symbols and formulas to represent a chemical reaction • Starting materials= “reactants” • New substances= “products” • An arrow is read as “produces” • States of matter are in brackets (s)= solid (l)= liquid (g)= gas (aq)= dissolved in water REACTANTS  PRODUCTS

  8. Word Equations • Words represent the chemical reaction REMEMBER: any compound that is not molecular, check the criss cross rule first. Example: Lithium + Aluminum Chloride  Aluminum + Lithium Chloride

  9. Skeleton Equations • An unbalanced equation that represents the chemical formulas of reactants and products Ex. Na + O2 Na2O Rules: • Write the symbol of a metal NOT in a compound • If the non-metal is part of hockey stick and puck, write it as a diatomic molecule (H2, N2, O2, F2, Cl2, Br2, I2) • Write the formula of other compounds using rules you have learned

  10. Law of Conservation of Mass • In a chemical reaction…The total mass of reactants EQUALS the total mass of products because elements cannot be created nor destroyed in a chemical reaction. They can only be rearranged!

  11. Formula Equations • Formulas represent reactions • Include states of matter • Coefficients show how the ratios of different substances in the chemical reaction Ex. 2 N2 (g) + 5 O2 (g)  2 N2O5 (s)

  12. Balancing Chemical Formulas Some tips: • Balance polyatomic ions first if the same ion is on both sides of the equations • Balance all other elements other than H and O • Balance H • Balance O

  13. Examples • FeCl3 + NaOH Fe(OH)3 + NaCl • Cu + AgNO3 Cu(NO3)2 + Ag • C5H12 + O2  CO2 + H2O • C2H6 + O2  CO2 + H2O

  14. Some Helpful Videos • Elephant toothpaste (catalyst example): http://www.youtube.com/watch?v=4N0m95PExHY • Balancingchemicalequations:http://www.youtube.com/watch?v=HB6cG7bQew0 • Practicebalancingchemicalequations:http://www.sky-web.net/science/balancing_chemical_equations_examples.htm

  15. Homework - Balancing chemical equations worksheet - p. 155 # 3, 5

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