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Evidence & Types of Chemical Reactions GPS 2a & 2b

Evidence & Types of Chemical Reactions GPS 2a & 2b. Fall 2011. Part I: Describing Chemical Reactions chemical reaction the process by which one or more substances are changed into one or more different substances.

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Evidence & Types of Chemical Reactions GPS 2a & 2b

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  1. Evidence & Types of Chemical ReactionsGPS 2a & 2b Fall 2011

  2. Part I: Describing Chemical Reactions chemical reaction the process by which one or more substances are changed into one or more different substances. in any chemical reaction, the original substances are known as the reactantsand the resulting substances are known as the products. steel wool + oxygen (reacting) rust Fe O2 Fe2O3 reactantsproduct Click for Bill Nye Video

  3. Part I: Describing Chemical Reactions • according to the law of conservation of mass: • total mass of reactants = total mass of products • 223.38 g Fe + 96.00 g O2 287.38 g Fe2O3 • chemical reactions are described by chemical equations. • chemical equation = represents, with symbols and formulas, the identities and molar amounts of the reactants and products in a chemical reaction.

  4. Example of a chemical equation for the breakdown of ammonium dichromate. • “The reactant ammonium dichromateyields the products nitrogen,chromium (III) oxide, andwater.” is the same as: (NH4)2Cr2O7(s)N2(g)+ Cr2O3(s)+ H2O(g) • [This equation is UNBALANCED. We will balance it using coefficients later.] Click Image for Cool Video

  5. Part II: Evidence of Chemical Reactions • To know for certain that a reaction has taken place requires evidence that one or more substances have undergone a change in identity. • Evolution of heat and light.A change in matter that releases energy as both heat and light is strong evidence that a chemical reaction has taken place. An example would be the decomposition you just viewed. • Temperature Change. If a reaction gets hot, we say it is exothermic (releasing heat energy to the environment). However, a reaction can absorb energy from the environment (get cold); this is called an endothermicreaction. Check out this “cool” reaction!

  6. 3. Production of a gas. The evolution of gas bubbles when two substances are mixed is often evidence of a chemical reaction. Ex: bubbles of CO2 gas form immediately when baking soda is mixed with vinegar; the classic middle school volcano experiment. (baking soda) (vinegar) (carbon dioxide) NaHCO3(s)+ CH3COOH(l)CO2(g)+ NaC2H3O2(aq)+ H2O(l) Try this one for yourself! (Click the image to watch the video.)

  7. 4. Formation of a precipitate.Many reactions take place between substances that are dissolved in liquids. If a solid appears after two solutions are mixed, a reaction has likely occurred. This solid is called a precipitate. precipitate = a solid that is produced as a result of a chemical reaction in solution & it separates from the solution. Another video: This one you can’t try at home. The precipitate, lead iodide, is toxic but still used in the paint that lines the roads we travel.

  8. Color change. A change in color is often an indication of a chemical reaction. Ex: Bleach breaks down dye molecules attached to fabric, altering the wavelength of light they reflect, thus changing the color. Evolution of an odor. Odors that suddenly appear indicate new, aromatic chemicals have been produced. Examples would be synthesis of perfumes & flavorings. (Sorry, hard to demo this online! ) Check out these awesome color-changing reactions!

  9. Part III: Types of Chemical Reactions • There are 5 distinct types of chemical reactions. Each has a general formula that • represents the types of substances that react within each type of reaction. • Synthesis • Decomposition • Single replacement • Double replacement • Combustion The first 4 types of reactions often involve ionic compounds. It’s helpful to think of the cations (A and B) as boys and the anions (X and Y) as girls.

  10. Synthesis = combines 2 small compounds or atoms into one larger compound the “marriage” reaction Ex: Na + Cl2NaClK + CO3K2CO3Al + O2Al2O3 A + XAX Making Salt Synthesis Explained!!

  11. 2. Decomposition= splits 1 compound into 2 or more smaller compounds or atoms • the “divorce” reaction • Ex)NaClNa+ Cl2H2O2  H2O + O2 • hydrogen peroxide breaks down • into water & oxygen gas AXA+ X Watch this video!!

  12. 3. Single replacement= Replaces the cation or anion of a compound with another free cation or anion; the “one-cheater” reaction. • Ex)Cation Replacement • NaCl+ KKCl + NaAl2(CO3)3+ LiLi2CO3+ Al • AgNO3 + Cu Ag + Cu(NO3)2 AX+ BBX + A Click image to watch video of this reaction.

  13. Ex) Single Replacement Rxns • Anion Replacement • NaBr+ Cl2NaCl+ Br2 NaI+ Cl2NaCl+ I2 AX+ YAY + X Watch the video of these reactions.

  14. Double replacement = the cation of one compound is combined with anion of the other compound, creating 2 new compounds the “switching partners” reaction are often precipitation reactions Ex: NaCl + KBrNaBr + KClLi2S + TiF2LiF + TiS Sn3P4+ Zn(NO3)2 Zn3P2+Sn(NO3)4 AX+ BYAY + BX What is the yellow stuff? You’ve seen this one before!

  15. Combustion= Hydrocarbon + O2 produces CO2/CO, H2O plus energy • This reactions “involves hydrocarbons” reaction (not ionic compounds) • Sometimes results in CO2 as a product, other times CO • Ex) C2H6+ O2CO2+ H2O C7H14+ O2CO2+ H2O C10H22+ O2CO2+ H2O C7H14+ O2CO+ H2O CnHn+ O2CO2+ H2O Some of these demos could be classified as synthesis also. Watch closely & see if you can find them in the video! Burn, baby, burn!

  16. For more help with types of reactions, watch the tutorial below: Tutorial Lesson More Demos!

  17. let’s take a closer look at the single-replacement reaction... the activity serieshelps determine whether or not one element can replace another in a single-replacement rxtn. if an element is above another, it can replace it. so, can these reactions happen or not? Ex: NaCl + KKCl + Na Al2(CO3)3 + LiLi2CO3 + Al NaCl + Br2NaBr + Cl2 Part IV: Symbols Found in Chemical Equations Li Rb K Ba Sr Ca Na Mg Al Mn Zn Cr Fe Cd Co Ni Sn Pb H2 Sb Bi Cu Hg Ag Pt Au F2 Cl2 Br2 I2 NaHCO3 (s)+CH3COOH(l)CO2+NaC2H3O2(aq)+ H2O(l)

  18. Part IV: Symbols Found in Chemical Equations Part V: Other Info About Chemical Reactions some elements occur only in diatomic form, meaning they are bonded to another atom of that element. H2 N2 O2 F2 Cl2 Br2 I2 these gases are never found by themselves in any chemical formula or equation!!! NaHCO3 (s)+CH3COOH(l)CO2+NaC2H3O2(aq)+ H2O(l) solid ( for precipitate) gas ( for gaseous product) liquid in aqueous solution yields or produces reversible reaction heat applied to reactants specific pressure for rxtn specific temp for reaction catalyst needed for rxtn

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