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Chemical Reactions Comprehensive Science Mr. Knighton.

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Presentation on theme: "Chemical Reactions Comprehensive Science Mr. Knighton."— Presentation transcript:

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2 Chemical Reactions Comprehensive Science Mr. Knighton

3 Chemical Reactions are all around us! Examples : Growth, cooking, decaying, burning,…… Food + O 2 energy + CO 2 + H2O This is a Chemical Reaction!! It is what takes place in your body when you eat!

4 Another Example Cooking: Yeast + Sugar + Flour Bread (CO2 + acid) Chemical Reaction: Substances undergo a chemical change to form new substances.

5 Signs of a Chemical Reaction Production of gas –Ex) CO2 bubbles are produced to make the bread rise. Color Change –Ex) The White dough changes color to a golden brown after cooking.

6 Chemical Reactions Rearrange Atoms Example) Photosynthesis! The Chemical Equation: CO 2 + H 2 O C 6 H 12 O 6 + O 2

7 Parts of a Chemical Equation CO 2 + H 2 O C 6 H 12 O 6 + O 2 To the Left of the Arrow are the REACTANTS (what goes in) The Arrow is called a YIELD SIGN To the Right of the yield sign are the PRODUCTS (what comes out)

8 Rearranging the Atoms CO 2 + H 2 O C 6 H 12 O 6 + O 2 Notice that no new atoms appear when the products are made. The atoms just rearrange themselves to form new compounds!

9 How does this happen? Energy must be added to break the bonds of the reactants. When new bonds form the products, energy is then released.

10 2 Major Types of Reactions Exothermic: more energy is released then was absorbed. This produces HEAT! Endothermic: More energy is abosrbed the released. Results in COLD

11 Classifying Chemical Reactions There are 5 general types of chemical reactions. 1.Synthesis 2.Decomposition 3.Combustion 4.Single Displacement 5.Double Displacement

12 1.Synthesis Reaction A reaction of at least 2 substances that forms a new, more complex compound. (Synthesis means to make) Example: Hydrogen + oxygen water General Form: A+ BAB 2Na + Cl 2 2NaCl

13 2. Decomposition Reaction A reaction in which one compound breaks into at least 2 products. Example: digestion (breaking down your food molecules) General Form: AB A + B Ex.) 2H 2 O 2H 2 + O 2 The Decomposition of Water

14 3. Combustion A reaction in which a compound and oxygen burn. Oxygen is a reactant in every combustion reaction. Example) the burning of methane 2CH 4 + 4O 2 2CO 2 + 4H 2 O

15 4. Single-Displacement Reaction A reaction in which atoms of one element take the place of atoms of another element in a compound. General Form: XA + B AB + X Example: CuCl 2 + Al AlCl 3 + Cu

16 5. Double-displacement reaction A reaction where a compound is formed from the exchange of ions between 2 compounds. General form: AX + BY AY + BX Example: Pb(NO 3 ) 2 + K 2 CrO 4 PbCrO 4 + 2KNO 3

17 Balancing Equations

18 Review…. What is a chemical Equation? The use of chemical formulas and symbols to show the reactants and products in a chemical reaction. Write an equation and label the reactants, products, and yield sign.

19 Why do we need to balance equations? Equations must be balanced to account for the Conservation of Mass!

20 For Example….. CH 4 + O 2 CO 2 + H 2 O This equation is not balanced. It appears that we have lost Hydrogens and gained Oxygens…..does this agree with the Law of Conservation of Energy?

21 How to Balance Equations CH 4 + O 2 CO 2 + H 2 O 1.Count the # of atoms on each side of the yield sign.C = 1 H = 4H = 2 O = 2O = 3

22 Now what? You can NOT change the subscripts! 2. Place Coefficients to balance the equation. Coefficient = a # in front of atoms or compounds.

23 Using Coefficients CH 4 + O 2 CO 2 + H 2 O There are 4 Hydrogens on the left side, how many more do you need to get 4 on the right side? Answer = 2

24 Using Coefficients Place a 2 in front of H 2 O. CH 4 + O 2 CO 2 + 2H 2 O 3. Now Re-count atoms for both sides!C = 1H = 4 O = 2O = 4

25 How to Balance the Oxygen We need to add another Coefficient. CH 4 + O 2 CO 2 + 2H 2 O Oxygen needs 2 more on the left side. Place a 2 in front of the O 2.

26 4. Re-count atoms for both sides. CH 4 + 2O 2 CO 2 + 2H 2 OC = 1H = 4O = 4 All is balanced!

27 Try this! 1.Mg + O 2 MgO 2.KI + Br 2 KBr + I 2 3. What types of reactions are these? 4. P.168 # 1, 2a., 3


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