4 3 Balancing Chemical Equations A chemical reaction

4. 3 – Balancing Chemical Equations A chemical reaction… • is what happens when ≥ 1 chemical changes occur @ the same time • obeys the LOCOM • The reactant(s) change into new substance(s) with different properties, called product(s). • Chemical reactions can be written in different ways. 1. A word equation: nitrogen monoxide + oxygen nitrogen dioxide State of Matter 2. A symbolic equation: - letters indicate the state of each § 2 NO(g) + O 2(g) 2 NO 2(g) pure substance Coefficients (aq) = aqueous/dissolved in water - indicate the ratio of the pure substances in the reaction (*lowest whole #s) (s) = solid -there is twice as much NO and NO 2 as there is O 2 (g) = gas ( ) = liquid

Evidence of Chemical Change Some clues that may indicate that chemical change has occurred are… • • precipitate formed gas produced heat produced light produced colour change in smell energy consumed / given off (temp. change) the change is irreversible

Law of Conservation of Mass (LOCOM) in Chemical Change • chemical change means that new compounds are created w BUT… atoms are neither created nor destroyed w Dalton realized that atoms simply rearrange themselves during chemical reactions. w total # of atoms of an element among reactants = total # of atoms of that element among products w this is why chemical equations must be balanced! • LOCOM (Lavoisier, 18 th c. ) w In a chemical reaction, the total mass of the products is equal to the total mass of the reactants; mass is conserved. http: //www. brainpop. com/science/matterandchemistry/conservationofmass/

Writing and Balancing Chemical Equations • a word equation just shows the elements / compounds involved • a balanced chemical equation shows all atoms in their correct quantities w make sure that the # of each atom is the same on both sides of the reaction arrow w always use the smallest whole number ratio skeleton equation: K + O 2 K 2 O balanced equation: 4 K(s) + O 2 (g) 2 K 2 O(s)

Practice Writing & Balancing Chemical Equations Ex. Use the following word equations to write balanced chemical equations. 1. iron metal + bromine liquid iron (III) bromide solid 2. carbon dioxide + water glucose + oxygen 3. methane gas + oxygen gas water + carbon dioxide gas

Hints for Writing Word Equations • Be careful of molecules of diatomic and polyatomic elements such as O 2, P 4 and S 8 • The “special seven” are all diatomic elements – when they occur on their own, they are written as… w H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2 • use the chemical symbol for elements when they are not in a compound (ie. Cu represents pure / solid copper) • Several common covalent molecules containing H have common names that do not help in writing chemical formulae. Ex. methane: CH 4 glucose: C 6 H 12 O 6 ethane: C 2 H 6 ammonia: NH 3

Strategies for Balancing Chemical Equations 1. do not change subscripts! 2. balance compounds first, elements last 3. if hydrogen and oxygen occur in several places, balance them last 4. you can sometimes balance polyatomic ions as a group, if they exist of both sides of the reaction arrow 5. always double-check once you think you are finished

Practice Balancing Equations, then Write the Equations in Words 1. __Na. Cl + __Be. F 2 __Na. F + __Be. Cl 2 1. __Fe. I 3 + __Be 3(PO 4)2 __Be. I 2 + __Fe. PO 4 2. __Ag. NO 3 + __Li. OH __Ag. OH + __Li. NO 3 3. __CH 4 + __O 2 __CO 2 + __H 2 O 4. __Mg + __Mn 2 O 3 __Mg. O + __Mn 5. __Sn(NO 2)4 + __K 3 PO 4 __KNO 2 + __Sn 3(PO 4)4