chemical reactions reactants: zn + i 2 product: zn i 2 chem i: chapter 6 chem ih: chapter 11

CHEMICAL REACTIONSCHEMICAL REACTIONS

Reactants: Zn + IReactants: Zn + I22 Product: Zn IProduct: Zn I22

Chem I: Chapter 6Chem IH: Chapter 11

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 EquationsUsing EquationsEquations 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 productsEx:aluminumaluminum+ + oxygenoxygenaluminum aluminum

oxideoxide

2 Ways to Write an Equation

1. Word Equations-

• use names of compounds & elements.

Ex: Ex: aluminumaluminum++oxygenoxygenaluminum oxidealuminum oxide

• don’t tell you how much reactant or product is involved.

• Write this equation first

2. 2. Chemical EquationsChemical Equations• Show the relative amounts of Show the relative amounts of

reactantsreactants and and productsproducts in a reaction in a reaction• Symbols represent elements

Ex: Al• Formulas represent compounds

Ex: OO22 & AlAl22OO33

• Coefficients are the numbers in front of the symbols or formulas. They show how many units (atoms, molecules, etc.)

44 AlAl (s)(s) + + 33 OO22 (g) (g) ---> ---> 22 AlAl22OO33 (s) (s)

Additional Rules Chemical Equations

Follow• Physical state Physical state of chemicals is shown after of chemicals is shown after the formula or symbolthe formula or symbol

(s) = solid(s) = solid (l) = liquid(l) = liquid(g) = gas(g) = gas (aq) = in aqueous solution(aq) = in aqueous solution

• EnergyEnergy is shown when it is an important is shown when it is an important part of a rxn.part of a rxn.

• With the With the reactantsreactants in endothermic rxns in endothermic rxns• With the With the products products in exothermic rxnsin exothermic rxns

• BalanceBalance-equations must be balanced-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.

Balancing Chemical Balancing Chemical EquationsEquations

Because of the principle of Because of the principle of the the conservation of conservation of

mattermatter, an , an equation equation

must be balancedmust be balanced..

Def: It must have the Def: It must have the

same number of atoms of the same number of atoms of the same kind on both sides.same kind on both sides.

Lavoisier, 1788Lavoisier, 1788

Balancing Chemical Balancing Chemical Equations, Equations, cont.cont.

Balancing Chemical Balancing Chemical Equations, Equations, cont.cont.

44 Al(s)Al(s) + + 33OO22(g)--->(g)--->22 AlAl22OO33(s)(s)

This equation meansThis equation means4 4 AlAl atoms +atoms + 33 OO22 molecules molecules

---produces--->---produces--->

2 2 units of units of AlAl22OO33

Balancing Equations, Balancing Equations, cont.cont.• When balancing a chemical

equation you may ONLY add coefficients in front of the compounds & elements to balance the reaction.

• You may notnot change subscripts b/c this changes the compound.

Subscripts vs. Subscripts vs. CoefficientsCoefficients• SubscriptsSubscripts tell tell

you how many you how many atoms of an atoms of an element are in a element are in a compound.compound. Ex: Ex: the “2” in Othe “2” in O22..

• CoefficientsCoefficients tell tell you the quan-you the quan-tity, or number, tity, or number, of units of a of units of a chemical. chemical. Ex: Ex: the “3” in front the “3” in front of COof CO22.←.←

4 Steps to Balancing 4 Steps to Balancing EquationsEquations

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 4 Steps to Balancing EquationsEquations

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 4 Steps to Balancing EquationsEquations

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 EquationsSteps 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 Some Suggestions to Help YouYouHelpful 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.

Practice Problem

Balancing Balancing EquationsEquationsBalancing Balancing EquationsEquations

__ H__ H22(g) + ___ O(g) + ___ O22(g) ---> __ H(g) ---> __ H22O(l)O(l)2

What Happened to the Other Oxygen Atom?????

This equation is not balanced!

LEFT SIDE RIGHT SIDEH=2 H=2O=2 O=1

2

ANSWER TO PRIOR SLIDE

• Re-Count the atoms

__22_H_H22(g) + ___O(g) + ___O22(g) --->_(g) --->_22_H_H22O(l)O(l)

• LEFT SIDE RIGHT SIDE

• H=4 H=4• O=2 O=2

Balancing Balancing Equations EquationsBalancing Balancing Equations Equations

___ Al(s) + ___ Br___ Al(s) + ___ Br22(l) ---> ___ Al(l) ---> ___ Al22BrBr66(s)(s)2 3

Practice Practice Balancing Balancing EquationsEquations

Practice Practice Balancing Balancing EquationsEquations

____C____C33HH88(g) + _____ O(g) + _____ O22(g) ---->(g) ---->

_____CO_____CO22(g) + _____ H(g) + _____ H22O(g)O(g)

____B____B44HH1010(g) + _____ O(g) + _____ O22(g) ---->(g) ---->

___ B___ B22OO33(g) + _____ H(g) + _____ H22O(g)O(g)

Balancing EquationsBalancing EquationsBalancing EquationsBalancing EquationsSodium phosphate + iron (III) oxide Sodium phosphate + iron (III) oxide

sodium oxide + iron (III) phosphatesodium oxide + iron (III) phosphate

NaNa33POPO44 + Fe + Fe22OO33 ----> ---->

NaNa22O + FePOO + FePO44

5 Types of Reactions1. 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. Combustiontion-the-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. Combustiontion• 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. Na1. Na33POPO44 + Fe + Fe22OO33 NaNa22O +FePOO +FePO44

2. C2. C33HH88(g)+O(g)+O22(g)(g)COCO22(g) + H(g) + H22O(g)O(g)

3. 2Al(s) + 3Br3. 2Al(s) + 3Br22(l) ---> Al(l) ---> Al22BrBr66(s)(s)

Answers to Practice Problems:

What type of reaction?1.1. NaNa33POPO44 + Fe + Fe22OO33 NaNa22O +FePOO +FePO44

Double replacementDouble replacement

2. C2. C33HH88(g)+O(g)+O22(g)(g)COCO22(g) + H(g) + H22O(g)O(g)

combustioncombustion

3. 2Al(s) + 3Br3. 2Al(s) + 3Br22(l) ---> Al(l) ---> Al22BrBr66(s)(s)

synthesissynthesis

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

Activity Series of Halogens

Practice Problems• Complete Q#1- from p

____ of text.• Q& A please!