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Chemical Reactions

Learn about chemical changes, conservation of mass, writing equations, balancing chemical reactions, classifying different reaction types, and the role of energy exchange in reactions. Discover how catalysts and inhibitors impact reaction rates.

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Chemical Reactions

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  1. Chemical Reactions

  2. Section 1 Chemical Changes

  3. A. Describing chemical reactions- change of one or more substances 
into new substances

  4. 1. Reactants are substances that combine or change

  5. 2. New substances that are 
produced are called products.

  6. B. Conservation of Mass- a law which states that, in a chemical 
reaction, matter is not created or 
destroyed; it stays the same

  7. 1. Antoine Lavoisier- experimented with mercury(II) oxide and heat

  8. 2. Found mass of products (liquid 
mercury and oxygen gas) equaled mass of reactants

  9. C. Writing equations- a chemical equation uses chemical formulas and symbols to describe a chemical 
reaction and the product(s) it 
produces

  10. 1. Chemical formula expresses the 
relationship between elements in 
the compounds and molecules they 
make up

  11. 2. Coefficients- numbers which represent the number of units of 
each substance in a reaction

  12. 3. Knowing coefficients of chemical 
reactions allows chemists to use 
the correct amounts of reactants to predict the amounts of products

  13. 4. Subscripts- numbers which 
represent the number of atoms in a molecule of a particular element.

  14. 5. Symbols used to show state of 
reactants: (s)solid, (aq) aqueous, (g)gas, (clear) liquid. When a chemical has been dissolved in water, 
this is denoted by writing (aq) after the chemical name.

  15. D. Metals react with atmosphere in different ways.

  16. Section 2 Chemical Equations

  17. A. Checking for balance- law of conservation of mass requirement

  18. 1. A balanced chemical reaction- both sides of equation have the 
same number of atoms of each 
element

  19. 2. Choosing coefficients- becomes 
easier with practice; trial and error 
at first

  20. B. Writing balanced chemical reactions- a four step process

  21. 1. Describe the reaction in words.

  22. 2. Write the equation using formulas and symbols.

  23. 3. Check for balance.

  24. 4. Add coefficients where needed for balance.

  25. Section 3 Classifying Chemical 
Reactions

  26. A. Synthesis reaction- two or 
more substances form a new 
substance; A + B ---> C

  27. -> +

  28. B. One substance breaks down 
into two or more substances in a 
decomposition reaction AB ---> A + B The starting compound is ammonium dichromate. When heated, it begins to 
decompose into nitrogen gas, water vapor and powdered chromium (III) oxide. It 
looks like a volcano with ash being spread all over the place. http://www.youtube.com/watch?v=jowikn6tsyY

  29. -> +

  30. C. Single-displacement reaction- one element replaces another one 
in a compound: A + BC ---> AC + B or D + BC ---> 
BD + C A + BC --> AC + B

  31. -> + +

  32. D. A double-displacement reaction results if a precipitate, water, or a 
gas forms when two ionic 
compounds in solution are 
combined; AB + CD --> AD + CB AB + CD --> AD + CB

  33. + -> +

  34. Combustion- O2 as a reactant and CO2 and H2O as the product MgCO3 + 2HCl --> MgCl2 + H2O + CO2

  35. Section 4 Chemical Reactions and 
Energy

  36. A. Chemical reactions involve 
energy exchange. Reaction of sulfuric acid and sugar.

  37. 1. Breaking chemical bonds 
requires energy. Dr. Pyenta melted some potassium chlorate in a test tube over a Bunsen burner in a hood. Then he dropped in a sugar 
cube. The result was a considerable jet of flame that burned for 30 seconds or more.

  38. 2. Forming chemical bonds releases energy.

  39. B. More energy out

  40. 1. Exergonic reactions- energy required to break bonds is less than 
the energy released from new 
bonds; energy given off is usually 
light

  41. 2. Exothermic reactions- energy given off in the form of heat

  42. C. More energy in

  43. 1. Energonic reactions- more energy is required to break bonds 
than to form new ones; need 
energy for the reaction to occur

  44. 2. If energy needed is heat; the 
reaction is endothermic. vinegar & baking soda

  45. 3. A catalyst speeds up a chemical reaction without itself being 
permanently changed. Enzymes speed up 
chemical reactions

  46. 4. An inhibitor prevents or slows a chemical reaction or interferes with 
a catalyst's action

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