CHEMICAL REACTIONS Chem I: Chapter 6Chem IH: Chapter 11 Reactants: Zn + I2 Product: Zn I2
Chemical Reactions… • occur when bonds between valence electrons of atoms are formed or broken • involve changes in matter, the making of new materials with new properties & energy changes
Signs of Chemical Reactions (These signs usually indicate a chemical rxn has occurred.) 1. Color change (ex: KMnO4) 2. Formation of precipitate 3. Odor change (ex: spoiled milk) 4. Gas release (baking soda & vinegar) 5. Energy change- (vermiculite) • heat absorbed • heat or light released
Using Equations Equations represent chemical rxns. They show what is happening.
Rules All Equations Follow • Show reactants (“before”) on the left, products (“after”) on the right • Use an arrow() to separate reactants from products • Use + signs between reactants • Use + signs between products Ex:aluminum+ oxygenaluminum oxide
2 Ways to Write an Equation • Word Equations- • use names of compounds & elements. Ex: aluminum+oxygenaluminum oxide • don’t tell you how much reactant or product is involved. • Write this equation first
2. Chemical Equations • Show the relative amounts of reactants and products in a reaction • Symbols represent elements Ex: Al • Formulasrepresent compounds Ex: O2 & Al2O3 • Coefficients are the numbers in front of the symbols or formulas. They show how many units (atoms, molecules, etc.) 4Al(s) + 3O2 (g) ---> 2Al2O3 (s)
Additional Rules Chemical Equations Follow • Physical state of chemicals is shown after the formula or symbol (s) = solid (l) = liquid (g) = gas (aq) = in aqueous solution • Energyis shown when it is an important part of a rxn. • With the reactants in endothermic rxns • With the products in exothermic rxns • Balance-equations must be balanced
Charcoal, which is basically carbon, reacts with oxygen to yield carbon dioxide. • Word eq’n: carbon + oxygen carbon dioxide +energy • Chemical equation: C(s) + O2(g) CO2(g) +energy has a quantitative meaning as well.
Lavoisier, 1788 Balancing Chemical Equations Because of the principle of the conservation of matter, an equation must be balanced. Def: It must have the same number of atoms of the same kind on both sides.
Balancing Chemical Equations, cont. 4Al(s) + 3O2(g)--->2Al2O3(s) This equation means 4 Alatoms +3O2molecules ---produces---> 2 units of Al2O3
Balancing Equations, cont. • When balancing a chemical equation you may ONLY add coefficients in front of the compounds & elements to balance the reaction. • You may not change subscripts b/c this changes the compound.
Subscripts vs. Coefficients • Subscriptstell you how many atoms of an element are in a compound. Ex: the “2” in O2. • Coefficientstell you the quan-tity, or number, of units of a chemical. Ex: the “3” in front of CO2.←
4 Steps to Balancing Equations Step #1: Write the correct symbols & formulas for the reactants and the products. • (DO NOT TRY TO BALANCE IT YET! Just write the correct formulas first. • And most importantly, once you write them correctly, DO NOT CHANGE THE FORMULAS!)
HINT: WHEN COMPLETING STEP #1 When writing formulas, remember • The 7 diatomic elements • H2, N2, O2, F2, Cl2, Br2, I2
4 Steps to Balancing Equations Step #2. Count the number of atoms for each element on the left side. Count the number of the atoms of each element on the right side.
4 Steps to Balancing Equations Step #3: Determine where to place coefficients in front of symbols & formulas so that the left side has the same number of atoms as the right side for EACH element.
Steps to Balancing Equations Step # 4: Check your answer to see if: • The numbers of atoms on both sides of the equation are now balanced. • The coefficients are in the lowest possible whole number ratios. (reduced)
Some Suggestions to Help You Helpful Hints for balancing equations: • Take one element at a time, working left to right except for H and O. Save H for next to last, and O until last. • For polyatomic ions that appear on both sides of the equation count ions, not atoms. • Water may be considered ionic, made up of H+ and OH-, if helpful.
Balancing Equations 2 2 LEFT SIDE RIGHT SIDE H=2 H=2 O=2 O=1 __ H2(g) + ___ O2(g) ---> __ H2O(l) What Happened to the Other Oxygen Atom????? This equation is not balanced!
ANSWER TO PRIOR SLIDE • Re-Count the atoms _2_H2(g) + ___O2(g) --->_2_H2O(l) • LEFT SIDE RIGHT SIDE • H=4 H=4 • O=2 O=2
Balancing Equations ___ Al(s) + ___ Br2(l) ---> ___ Al2Br6(s) 2 3
Practice Balancing Equations ____C3H8(g) + _____ O2(g) ----> _____CO2(g) + _____ H2O(g) ____B4H10(g) + _____ O2(g) ----> ___ B2O3(g) + _____ H2O(g)
Balancing Equations Sodium phosphate + iron (III) oxide sodium oxide + iron (III) phosphate Na3PO4 + Fe2O3 ----> Na2O + FePO4
5 Types of Reactions • Combination(Synthesis)-2 or more reactants combine to form 1 product. Ex: 2H2 + O2 2H2O 2. Decomposition- 1 reactant breaks down into 2 or more products. Ex: 2H2O 2H2 + O2
5 Types of Reactions, cont. 3. Single Replacement- 1 element replaces another in a compound Ex: 2K + H2O 2KOH + H2 (K replaced H in water)
5 Types of Reactions, cont. 4. Double Replacement-exchange of cations be-tween 2 ionic compounds Ex: 2NaCN +H2SO42HCN + Na2SO4
5 Types of Reactions, cont. 5. Combustion-the rapid combination of a substance w/oxygen to form heat & light. • Reactants are made of C,H, and sometimes O. • Products are CO2, H2O, (and energy).
5 Types of Reactions, cont. 5. Combustion • Ex 1: CH4 + 2O2 CO2 + 2H2O + energy • Ex 2: Mg + O2 2MgO +energy (note: Ex2 is also a synthesis rxn)
Practice Problems: What type of reaction? 1. Na3PO4 + Fe2O3Na2O +FePO4 2. C3H8(g)+O2(g)CO2(g) + H2O(g) 3. 2Al(s) + 3Br2(l) ---> Al2Br6(s)
Answers to Practice Problems:What type of reaction? • Na3PO4 + Fe2O3Na2O +FePO4 Double replacement 2. C3H8(g)+O2(g)CO2(g) + H2O(g) combustion 3. 2Al(s) + 3Br2(l) ---> Al2Br6(s) synthesis
Activity Series (Single Replacement) • Not all combinations of chemicals will produce a reaction. • An ion that is more reactive will replace a less reactive ion in a compound. • Copy the Activity Series of Metals Table onto your P. Table. (also on p333 of text)
Activity Series of Metals
Practice Problems • Complete Q#1- from p ____ of text. • Q& A please!