CHEMICAL REACTIONS Reactants: Zn + I 2 Product: Zn I 2

2 –Chemical reactions occur when bonds between the valence electrons of atoms are formed or broken, these atoms then reform bonds with different atoms in new arrangements. –Chemical reactions involve changes in matter, making new materials with new properties.Introduction

3 The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide. The chemical equation for this reaction, C + O 2  CO 2, contains the same information as the English sentence but is shorter, easier & shows the number of atoms.

4 reactants products –Chemical equations show the conversion of reactants (the molecules shown on the left of the arrow) into products (the molecules shown on the right of the arrow). A “+” sign separates molecules on the same side The arrow is read as “yields” Example C + O 2  CO 2 This reads “carbon plus oxygen react to yield carbon dioxide” Parts of a Chemical Reaction

5 Chemical Equations – Know your terms! 4 Al + 3 O 2 ---> 2 Al 2 O 3 4 Al + 3 O 2 ---> 2 Al 2 O 3 Product Reactants Chemical Symbol (abbreviation of element) Coefficient (number of molecules) Chemical Formula (combination of element symbols in a compound) Subscripts (Number of an element’s atoms in a molecule) Subscripts (Number of an element’s atoms in a molecule)

6 Because of the principle of the conservation of matter, an equation must be balanced. It must have the same number of atoms of the same kind on both sides. Lavoisier, 1788 Chemical Equations

7 not –When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but you may not change the subscripts. Changing the subscripts changes the compound. Balancing Equations

8 Subscripts vs. Coefficients The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.

The Combustion of Methane (natural gas: a fossil fuel) __CH 4 + __O 2  __CO 2 + __H 2 O Each person in a group of 4 should build one molecule listed in the above equation. Use the color key to find the correct atoms. All of the holes represent empty valence electrons in the atoms that need to be filled, so you may need to use double bonds which are the longer more flexible connectors in the model kits. Put the reactants on one side of the table and the products on the other. Count the atoms. Do the number of atoms in the reactants match the number of atoms in the products? Which molecules do you need more of to balance the equation? Remember you can only change the number of molecules, not the subscripts! Make the molecules and count the atoms again. Write the balanced equation in your notebook. The combustion of methane in air generates CO 2, which is a significant green house gas. However, methane itself is 20 X more potent as a greenhouse gas than carbon dioxide.

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The Synthesis of Water __H 2 + __O 2  __H Construct the reactants and products with the model set. 2.Count the atoms on each side of the equation. What do you need more of? 3.Make more molecules until the number of atoms is the same on each side of the equation. 4.Write the formula for the balanced equation in your notebook. ___________________________________________________ This equation looks too simple to be interesting, but the reaction is fast and explosively energetic. The ignition temperature for the reaction is O C. (youtube video)

12 Balancing Equations ___ H 2 + ___ O 2 ---> ___ H 2 O 22 What Happened to the Other Oxygen Atom????? This equation is not balanced! Two hydrogen atoms from a hydrogen molecule (H 2 ) combines with one of the oxygen atoms from an oxygen molecule (O 2 ) to form H 2 O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H 2 molecule) to make a second H 2 O molecule.

Bombardier Beetle spray! (a decomposition reaction) __H 2 O 2  __H 2 O + __O 2 1.Make the molecules of hydrogen peroxide, water and oxygen. 2.Count the atoms in the reactants and products. 3.Make more molecules until the equation is balanced. 4.Write the balanced equation in your notebook. This reaction helps to pressurize the toxin spray from the back end of a beetle.

Carbonic acid: Blood buffer (used to transport CO 2 in blood) H 2 CO 3  H CO 2 1.Set out all the atoms needed to make carbonic acid. Look at the number of holes available and try to work out the structure. 2.Make the other molecules and balance the equation. 3. Write the equation in your notebooks. 4.This is a decomposition reaction that is important for maintaining blood pH and telling our brain that we need to breath!

15 Balancing Equations ____C 3 H 8 (g) + _____ O 2 (g) ----> _____CO 2 (g) + _____ H 2 O(g) ____B 4 H 10 (g) + _____ O 2 (g) ----> ___ B 2 O 3 (g) + _____ H 2 O(g)