# Chemical Reactions Ms. Haut Biology Engineering 2.

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Chemical Reactions Ms. Haut Biology Engineering 2

Chemical Reactions Reactants—starting material Products—ending material Chemical Equation Reactants → Products

Chemical Reactions Occur when bonds between the outermost parts of atoms are formed or broken Involve changes in matter, the making of new materials with new properties, and energy changes

Chemical Equations Show the kind of reactants and products and their relative amounts in the reaction 4 Al + 3 O 2 → 2 Al 2 O 3

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

Balancing Chemical Equations notYou may add coefficients in front of the compounds to balance the reaction, but you may not change the subscripts. 3 CO 2 Changing subscripts changes the compound.

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

4 Al + 3 O 2 → 2 Al 2 O 3 This equation means 4 Al atoms + 3 O 2 molecules →produces→ 2 molecules of Al 2 O 3

Four basic steps to balancing a chemical equation. Write the correct formula for the reactants and the products in an unbalanced equation. Inventory the number of each kind of atom on both sides of the unbalanced equation. Determine where to place coefficients in front of formulas to balance the equation. Take another inventory to determine if: –The numbers of atoms on both sides of the equation are now balanced. –The coefficients are in the lowest possible whole number ratios.

Write and balance the chemical equation for: methane reacting with oxygen gas to produce carbon dioxide and water 1) What are the formulas of the reactants and products? REACTANTPRODUCT Methane: CH 4 Carbon dioxide: CO 2 Oxygen gas: O 2 Water: H 2 O 2) Write the general formula of the reaction CH 4 + O 2  CO 2 + H 2 O Let’s Practice…

CH 4 (l) + O 2 (g)  CO 2 (g) + H 2 O(l) Count the number of same elements on both sides ELEMENTREACTANTPRODUCT C11 H42 O23 C is balanced by H and O are not…H is off by 2 and O is off by 1

CH 4 + O 2  CO 2 + H 2 O C is balanced but H and O are not…H is off by 2 and O is off by 1 For H: What number do you multiply the 2 H on the product by to get 4H? 2 CH 4 + O 2  CO 2 + 2 H 2 O So now there are 4 H’s on both sides Now there are 2 O on the reactant side and 4 O on the product side For O: what number do you multiply the 2O on the reactant side to get 4O? 2 CH 4 + 2 O 2  CO 2 + 2 H 2 O

ELEMENTREACTANTPRODUCT C11 H44 O44 Reaction is balanced

Now Balance this Reaction N 2 + H 2 → NH 3 N = 2 H = 2 N = 1 H = 3 2 2 6 3 6

Now Balance this Reaction Na + Cl 2 → NaCl Na = 1 Cl = 2 Na = 1 Cl = 1 2 2 2 2 2

Now Balance this Reaction Al + O 2 → Al 2 O 3 Al = 1 O = 2 Al = 2 O = 3 4 2 6 2 2 3 6 4 4