Intro to Balancing Equations

Q: How could you represent this reaction (rusting bicycle), besides just describing it in words? Look at this rusty bike wheel. It has been left outside in damp weather too many times, so the iron in the metal parts has rusted. Iron rusts when it combines with oxygen in the air. Iron rusting is an example of a chemical reaction. In a chemical reaction, substances change into entirely different substances. For example, the iron in the bike and the oxygen in the air have changed into rust.

Fe + O2  Fe2O3 A: Scientists use a standard method to represent a chemical reaction, called a chemical equation. Fe + O2  Fe2O3 (Rust) Look at this rusty bike wheel. It has been left outside in damp weather too many times, so the iron in the metal parts has rusted. Iron rusts when it combines with oxygen in the air. Iron rusting is an example of a chemical reaction. In a chemical reaction, substances change into entirely different substances. For example, the iron in the bike and the oxygen in the air have changed into rust.

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

Chemical equations are used to represent CHEMICAL REACTIONS Reactants: Zn + I2 Product: Zn I2

Q: What is the general equation for the reaction in which iron rusts? Iron + Oxygen → Iron Oxide Or, using symbols: Fe + O2 → Fe2O3 (rust)

Q: What are the reactants and products? Fe + O2 → Fe2O3 (new product - rust) Reactants Products

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

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

Learning Check: A chemical reaction is balanced when The reactants are the same as the products The same atoms and molecules appear on each side of the equation The same number of each type of atom appears on both sides of the arrow. 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 O2 (g) ---> 2 Al2O3 (s) The numbers in the front are called coefficients subscripts The letters (s), (g), and (l) are the physical states of compounds. (aq) – aqueous solution The arrow  means: to yield = to produce = to form

Learning Check – Cold Call What does “2“ mean in front of LiOH? 2Li (s) + 2H2O (l)  2LiOH (aq) + H2 (g) Number of molecules What does “2“ to the right of “H2” mean? Number of atoms of H What does “(aq)“ to the right of LiOH mean? Aqueous solution What does ““ mean? Yield, form, produce

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

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

Classroom discussion O=C=O C + O 2 → CO 2 C + O=O → Bonds Break New Bonds Form 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.

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.

10 sec  Cold Call Q: Is this chemical reaction correct? Explain why. 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. Count the number of atoms of each element below: O2 H2O HCl Mg(NO3)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) 2x1 =2 1 atom of magnesium (Mg), 2 atoms of nitrogen (N) & 6 atoms of oxygen (O) 2x3 =6

Count the number of molecules below: A MOLECULE is a pure substance composed of two or more atoms. Count the number of molecules below: 1 O2 2H2O 17HCl One molecule of oxygen gas Two molecules of water Seventeen molecules of an acid

Chemical Equations 4 Al(s) + 3 O2(g) ---> 2 Al2O3(s) + This equation means: 2 molecules of Al2O3 4 Al atoms + 3 O2 molecules --->

Learning Check NaHCO3 # of “O” atoms? 3 Mg(OH)2 # of “H” atoms? 2 C6H12O6 # of “C” atoms? 6 Or, use response cards instead (1-10)

Complete Practice – Counting Atoms 5 min

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

Which of the following equations is balanced? 3Fe + 3O2  3Fe2O3 2Fe + O2  Fe2O3 4Fe + 2O2  3Fe2O3 4Fe + 3O2  2Fe2O3 When called on, be ready to justify your answer

Does this equation obey the Law of Conservation of Mass? (Is it balanced?) 2H2O2  2H2O + O2 YES What is the coefficient ratio? 2:2:1

Does this equation obey the Law of Conservation of Mass? (Is it balanced?) 2Al + O2  Al2O3 NO Why? Al is balanced 2 atoms of Al of left side 2 atoms of Al on right side 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. O2 – oxygen, if a subscript changed to 3, O3 = ozone

Balancing using shapes ____H2 + ____O2  ____H2O 2 1 2 Is this reaction balanced? Now fix the H on the left. Double Hydrogen too. Coefficient ratio: 2:1:2 BALANCED !

Balancing using shapes You try! __Al2O3 ___Al + ___O2 2 4 3 + Now fix the Al on the right. Coefficient ratio: 2:4:3 BALANCED !

Balancing Equations using T-chart ___ H2(g) + ___ O2(g) ---> ___ H2O(l) R P Count the number of atoms of each element found in the reaction. 2 H 2 H 2 O 1 O What Happened to the Other Oxygen Atom????? This equation is NOT balanced!

Balancing Equations using T-chart 2 1 ___ H2(g) + ___ O2(g) ---> ___ H2O(l) 2 2. Add coefficients to balance the atoms R P 2 H 2 H 2 O 1 O 4 4 2 3. Don’t forget to recount. Now it is balanced! Now fix the H on the left. Don’t forget there is a 1 in front of O2.

RULE: Balance H and O last Easy Balancing RULE: Balance H and O last Al + O2  Al2O3 2 3 2 1 R P 4 2 1 Al 2 Al 2 x 2 = 4 2 x 3 = 6 2 O 3 O 3 x 2 = 6 2:3 – find a common multiple Coefficient ratio: 4:3:2

Is this a balanced equation? 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 + O2  CO2, R P Count the number of atoms of each element found in the reaction. 1 C 1 C 2 O 2 O Is this a balanced equation? YES

You Try! Use shapes or T-cart method Fe + O2  Fe2O3 2x2 = 4 _____Fe + ____O2  ____Fe2O3 2 3 2 3x2 = 6 Common multiple Coefficient ratio: 4:3:2