NOTES: 11.1 – Writing and Balancing Chemical Equations Chemical Reactions

Effects of chemical reactions: • Chemical reactions rearrange atoms in the reactants to form new products. • The identities and properties of the products are completely different from that of the reactants.

What is a Chemical Reaction? • Chemical Reaction – one or more substances change into new substances • Process involves reactants and products • Reactant – a starting substance • Product – a substance formed • Example: Nitrogen and hydrogen gas can react to form ammonia under certain conditions. Reactants Yield Products N2 (g) + H2 (g)  NH3 (g)

How Can You Tell Whether or Not a Chemical Reaction Has Taken Place? • Chemical Change – alters a given material by changing its chemical composition • Production of gases and color changes are signs of chemical reactions • Examples: burn, rust, decompose, corrode, explode

Chemical Reactions: • we can describe a chemical reaction with words: “Iron metal reacts with oxygen gas to produce iron(III) oxide, or rust.” • we can then write a word equation: Iron + oxygen  iron(III) oxide

Chemical Equations: 2H2 + O2  2H2O • Chemical equations are used to represent or describe chemical reactions. • Chemical equations use chemical symbols and formulas for the reactants and products • For example when hydrogen, H2,burns, it reacts with oxygen, O2, in the air to form water. We write the chemical equation for this reaction as follows: 2H2 + O2  2H2O

Chemical Equations: An equation shows…  Chemical formulas of reactants;  Chemical formulas of products;  Molecule / Mole ratios of all compounds in the reaction.

Chemical Equations: We read the (+) sign as “reacts with” and the arrow ( ) as “produces” or “yields”. 2H2 + O2  2H2O Reactants Products

To show physical states of each substance: • (s) = solid • (l) = liquid • (g) = gas • (aq) = aqueous **aqueous means dissolved in water

• Consider again the reaction of iron with oxygen to form iron(III) oxide, or rust. Fe(s) + O2 (g)  Fe2O3 (s) (unbalanced) **this is a skeleton equation in that is NOT “balanced” and does not show the relative amounts of reactants and products

Coefficients & Subscripts COEFFICIENTS: numbers in front of compound that represents the number of molecules/moles of that compound SUBSCRIPTS: small numbers within a formula that help define the compound. 2H2SO4 Subscript Coefficient

H2O: One molecule of water 2H2O: Two molecules of water H2O2: One molecule of Hydrogen Peroxide

• During a chem. rxn. atoms are rearranged (NOT created or destroyed!) • Chemical equations must be BALANCED to show the relative amounts of all substances. • Balanced means: each side of the equation has the same # of atoms of each element. CH4 + O2  H2O + CO2 Unbalanced CH4 + 2O2  2H2O + CO2 Balanced

In order to balance… • Write correct formulas for all reactants and products • Reactants  Products • Count the number of atoms of each element in reactants & products. • Balance one at a time using coefficients. • Check for balance • Are the coefficients in the lowest possible ratio?

Balancing Equations NOTE: When balancing equations, you may change coefficients as much as you need to, but you may never change subscripts because you can’t change what substances are involved.

Fe(s) + O2(g)  Fe2O3(s) (unbalanced) 4Fe(s) + 3O2(g)  2Fe2O3(s) (balanced)

Sample Problem 1: ● Water is decomposed (broken down) to form the gaseous products hydrogen, H2, and oxygen, O2. Write the balanced equation for this reaction. ● H2O  H2 + O2 **O is not balanced ● 2H2O  2H2 + O2 **The equation is balanced!

Sample Problem 2: ● Chlorine gas, Cl2, reacts with potassium bromide, KBr, to form potassium chloride and bromine, Br2. Write the balanced equation for this reaction, ● Cl2 + KBr  KCl + Br2 **Cl and Br are not balanced ● Cl2 + 2KBr  2KCl + Br2 **The equation is balanced!

Examples: (3:2:3:2) CuCl2 (aq) + Al (s)  Cu (s) +AlCl3 (aq)

Balance C – then H – then O Examples: Propane, C3H8, burns in oxygen, O2, to form carbon dioxide and water. C3H8 + O2  CO2 + H2O Balance C – then H – then O C3H8 + 5O2  3CO2 + 4H2O (1:5:3:4)

Balance C – then H – then O Examples: Pentane, C5H12, burns in oxygen, O2, to form carbon dioxide and water. C5H12 + O2  CO2 + H2O Balance C – then H – then O C5H12 + 8O2  5CO2 + 6H2O (1:8:5:6)

Examples: Silver nitrate reacts with copper to produce silver and copper (II) nitrate. AgNO3 + Cu  Ag + Cu(NO3)2 2AgNO3 + Cu  2Ag + Cu(NO3)2 (2:1:2:1)

Examples: Phosphorus reacts with oxygen gas to produce diphosphorus pentoxide. P + O2  P2O5 4P + 5O2  2P2O5 (4:5:2)

Balance C – then H – then O Examples: C7H14 + O2  CO2 + H2O Balance C – then H – then O C7H14 + 10½O2  7CO2 + 7H2O 2C7H14 + 21O2  14CO2 + 14H2O (2:21:14:14)