# Intro to Balancing Equations Q: How could you represent this reaction (rusting bicycle), besides just describing it in words?

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Intro to Balancing Equations

Q: How could you represent this reaction (rusting bicycle), besides just describing it in words?

A: Scientists use a standard method to represent a chemical reaction, called a chemical equation. Fe + O 2  Fe 2 O 3 (Rust)

ANALOGY for a Chemical Equation FRAME WHEEL HANDLEBAR PEDAL BYCICLE - New product

Chemical equations are used to represent CHEMICAL REACTIONS Reactants: Zn + I 2 Product: Zn I 2

Q: What is the general equation for the reaction in which iron rusts? Iron + Oxygen → Iron Oxide Or, using symbols: Fe + O 2 → Fe 2 O 3 (rust)

Q: What are the reactants and products? Fe + O 2 → Fe 2 O 3 (new product - rust ) ReactantsProducts

Because of the principle Because of the principle of the conservation of matter, an equation must be balanced. of the conservation of matter, an equation must be balanced. Lavoisier, 1788 Chemical Equations

When is an equation called balanced? when an equation has the same number of atoms of the same kind on both sides. when an equation has the same number of atoms of the same kind on both sides.

Learning Check: A chemical reaction is balanced when a.The reactants are the same as the products b.The same atoms and molecules appear on each side of the equation c.The same number of each type of atom appears on both sides of the arrow. d.None of the above

The substances that begin a chemical reaction are called REACTANTS

The substances that form in a chemical reaction are called PRODUCTS

Chemical Equations 4 Al (s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s)  The numbers in the front are called  The letters (s), (g), and (l) are the physical states of compounds. physical states of compounds. (aq) – aqueous solution (aq) – aqueous solution coefficientssubscripts  The arrow  means: to yield = to produce = to form to yield = to produce = to form

Learning Check – Cold Call What does “2“ mean in front of LiOH? 2Li (s) + 2H 2 O (l)  2LiOH (aq) + H 2 (g) What does “2“ to the right of “H 2 ” mean? What does “(aq)“ to the right of LiOH mean? Number of molecules Number of atoms of H Aqueous solution What does “  “ mean? Yield, form, produce

Q: What is the chemical equation for this reaction? A: The chemical equation is: C + O 2 → CO 2 1 min – answer in your IAN

Q: How have the atoms of the reactants been rearranged in the products of the reaction? C + O 2 → CO 2 Classroom discussion A: Carbon atom chemically bonds with two oxygen atoms. HOW ? What bonds are broken and what new bonds have formed?

A: Bonds between the oxygen atoms in the oxygen molecule have been broken, and new bonds have formed between the carbon atom and the two oxygen atoms. C + O=O → Classroom discussion C + O 2 → CO 2 O=C=OO=C=O Bonds Break New Bonds Form

Learning Check 1 30 sec - Turn to your elbow partner….. Q: What needs to happen for a chemical reaction (change) to take place? A: Bonds between atoms of the reactants break and NEW bonds form, producing new substance.

Learning Check 2 10 sec  Cold Call Q: Is this chemical reaction correct? Explain why. A: No. There are no “pink atoms” on the reactant side – NEW matter (pink) on the product side. Matter canNOT be created. +  +

Learning Check 2 Why is this reaction correct? B/C atoms must REARRANGE for a chemical reaction to take place +  + NEW PRODUCTS

 Count the number of atoms of each element below:  An ATOM is the smallest particle of an element. O2O2 H2OH2O HCl Mg(NO 3 ) 2 2 atoms of oxygen (O) 2 atoms of hydrogen & 1 atom of oxygen (O) 1 atom of hydrogen (H) & 1 atom of chlorine (Cl) 1 atom of magnesium (Mg), 2 atoms of nitrogen (N) & 6 atoms of oxygen (O) 2x3 =6 2x1 =2

 Count the number of molecules below:  A MOLECULE is a pure substance composed of two or more atoms. O2O2 2H 2 O 17HCl One molecule of oxygen gas Two molecules of water Seventeen molecules of an acid 1

Chemical Equations 4 Al(s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s) 4 Al atoms + 3 O 2 molecules ---> 2 molecules of Al 2 O 3 This equation means:

Learning Check NaHCO 3 # of “O” atoms? 3 Mg(OH) 2 # of “H” atoms? 2 C 6 H 12 O 6 # of “C” atoms? 6

Complete Practice – Counting Atoms 5 min

Q: Is this reaction Balanced? When called on, be ready to justify your answer C + O 2 → CO 2 Cold Call

Which of the following equations is balanced? a.3Fe + 3O 2  3Fe 2 O 3 b.2Fe + O 2  Fe 2 O 3 c.4Fe + 2O 2  3Fe 2 O 3 d.4Fe + 3O 2  2Fe 2 O 3 When called on, be ready to justify your answer

Does this equation obey the Law of Conservation of Mass? (Is it balanced?) 2H 2 O 2  2H 2 O + O 2 What is the coefficient ratio? 2:2:1 YES

Does this equation obey the Law of Conservation of Mass? (Is it balanced?) 2Al + O 2  Al 2 O 3 Why? 2 atoms of Al of left side 2 atoms of Al on right side NO Al is balanced 2 atoms of O on left side 3 atoms of O on right side O is NOT balanced

REMEMBER: To balance chemical equation, you must add COEFFICIENTS (big numbers) in front of the chemical formulas in the equation. You cannot add or change subscripts!!! Why? Because it will alter the identity of the substance otherwise. Ex. O 2 – oxygen, if a subscript changed to 3, O 3 = ozone

Balancing using shapes ____H 2 + ____O 2  ____H 2 O Is this reaction balanced? Now fix the H on the left. Coefficient ratio: 2:1:2 BALANCED ! Double Hydrogen too. 221

Balancing using shapes You try! __Al 2 O 3  ___Al + ___O 2 Now fix the Al on the right. Coefficient ratio: 2:4:3 BALANCED ! 2 3 4 +

Balancing Equations using T-chart ___ H 2 (g) + ___ O 2 (g) ---> ___ H 2 O(l) What Happened to the Other Oxygen Atom????? This equation is NOT balanced! 1.Count the number of atoms of each element found in the reaction. RP 2 H 2 O1 O

Balancing Equations using T-chart ___ H 2 (g) + ___ O 2 (g) ---> ___ H 2 O(l) 2 2 3. Don ’ t forget to recount. 2. Add coefficients to balance the atoms 1 2 44 Now fix the H on the left. R P 2 H 2 H 2 O 1 O Don ’ t forget there is a 1 in front of O2. Now it is balanced!

Easy Balancing Al + O 2  Al 2 O 3 3 RULE: Balance H and O last RP 1 Al 2 Al 2 O3 O 2 2 3 x 2 = 6 2:3 – find a common multiple 2 2 x 2 = 4 4 1 2 x 3 = 6 Coefficient ratio: 4:3:2

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, Is this a balanced equation? YES RP Count the number of atoms of each element found in the reaction. 1 C 2 O

You Try! Use shapes or T-cart method Fe + O 2  Fe 2 O 3 _____Fe + ____O 2  ____Fe 2 O 3 22 3x2 = 6 Common multiple 3 2x2 = 4 Coefficient ratio: 4:3:2

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