Comprehensive Science Mr. Knighton Chemical Reactions Comprehensive Science Mr. Knighton

Chemical Reactions are all around us! Examples: Growth, cooking, decaying, burning,…… Food + O2 energy + CO2 + H2O This is a Chemical Reaction!! It is what takes place in your body when you eat!

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

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.

Chemical Reactions Rearrange Atoms Example) Photosynthesis! The Chemical Equation: CO2 + H2O C6H12O6 + O2

Parts of a Chemical Equation CO2 + H2O C6H12O6 + O2 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)

Rearranging the Atoms CO2 + H2O C6H12O6 + O2 Notice that no new atoms appear when the products are made. The atoms just rearrange themselves to form new compounds!

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.

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

Classifying Chemical Reactions There are 5 general types of chemical reactions. Synthesis Decomposition Combustion Single Displacement Double Displacement

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+ B AB 2Na + Cl2 2NaCl

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.) 2H2O 2H2 + O2 The Decomposition of Water

3. Combustion A reaction in which a compound and oxygen burn. Oxygen is a reactant in every combustion reaction. Example) the burning of methane 2CH4 + 4O2 2CO2 + 4H2O

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: CuCl2 + Al AlCl3 + Cu

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(NO3)2 + K2CrO4 PbCrO4 + 2KNO3

Balancing Equations

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.

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

For Example….. CH4 + O2 CO2 + H2O 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?

How to Balance Equations CH4 + O2 CO2 + H2O Count the # of atoms on each side of the yield sign. C = 1 C = 1 H = 4 H = 2 O = 2 O = 3

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

Using Coefficients CH4 + O2 CO2 + H2O There are 4 Hydrogens on the left side, how many more do you need to get 4 on the right side? Answer = 2

Using Coefficients Place a 2 in front of H2O. CH4 + O2 CO2 + 2H2O 3. Now Re-count atoms for both sides! C = 1 C = 1 H = 4 H = 4 O = 2 O = 4

How to Balance the Oxygen We need to add another Coefficient. CH4 + O2 CO2 + 2H2O Oxygen needs 2 more on the left side. Place a 2 in front of the O2.

4. Re-count atoms for both sides. CH4 + 2O2 CO2 + 2H2O C = 1 C = 1 H = 4 H = 4 O = 4 O = 4 All is balanced!

Try this! Mg + O2 MgO KI + Br2 KBr + I2 3. What types of reactions are these? 4. P.168 # 1, 2a. , 3