Presentation on theme: "NOTES: 11.1 – Writing and Balancing Chemical Equations"— Presentation transcript:
1 NOTES: 11.1 – Writing and Balancing Chemical Equations Chemical Reactions
2 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.
3 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 ProductsN2 (g) H2 (g) NH3 (g)
4 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
5 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
6 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
7 Chemical Equations: An equation shows… Chemical formulas of reactants; Chemical formulas of products; Molecule / Mole ratios of all compounds in the reaction.
8 Chemical Equations:We read the (+) sign as “reacts with” and the arrow ( ) as “produces” or “yields”.2H2 + O2 2H2OReactantsProducts
9 To show physical states of each substance: • (s) = solid• (l) = liquid• (g) = gas• (aq) = aqueous**aqueous means dissolved in water
10 • 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
11 Coefficients & Subscripts COEFFICIENTS: numbers in front of compound that represents the number of molecules/moles of that compoundSUBSCRIPTS: small numbers within a formula that help define the compound.2H2SO4SubscriptCoefficient
12 H2O: One molecule of water 2H2O: Two molecules of waterH2O2: One molecule of Hydrogen Peroxide
13 • 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 UnbalancedCH4 + 2O2 2H2O + CO2 Balanced
14 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?
15 Balancing EquationsNOTE: 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.
17 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!
18 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!
19 Examples: (3:2:3:2) CuCl2 (aq) + Al (s) Cu (s) +AlCl3 (aq)
20 Balance C – then H – then O Examples:Propane, C3H8, burns in oxygen, O2, to form carbon dioxide and water.C3H8 + O2 CO2 + H2OBalance C – then H – then OC3H8 + 5O2 3CO2 + 4H2O(1:5:3:4)
21 Balance C – then H – then O Examples:Pentane, C5H12, burns in oxygen, O2, to form carbon dioxide and water.C5H12 + O2 CO2 + H2OBalance C – then H – then OC5H12 + 8O2 5CO2 + 6H2O(1:8:5:6)
22 Examples:Silver nitrate reacts with copper to produce silver and copper (II) nitrate.AgNO3 + Cu Ag + Cu(NO3)22AgNO3 + Cu 2Ag + Cu(NO3)2(2:1:2:1)
23 Examples:Phosphorus reacts with oxygen gas to produce diphosphorus pentoxide.P + O2 P2O54P + 5O2 2P2O5(4:5:2)
24 Balance C – then H – then O Examples:C7H14 + O2 CO2 + H2OBalance C – then H – then OC7H ½O2 7CO2 + 7H2O2C7H O2 14CO2 + 14H2O(2:21:14:14)