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Chemical Reactions Chapter 7

Chemical Reactions Chapter 7

Tro, 2nd ed.

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SSME’s on SSTS Combustion Reaction

H2(white) & O2(red) 2 H2O

Plus LOTs of energy!!!

A typical chemical reaction powers the liftoff of a space shuttle, along with the more complex solid rocket boosters combustion.

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Evidence of Chemical Reactions

There are five main observations that denote a chemical reaction may have occurred.

Try to list them yourself first:Color changePrecipitateGas or odor detectedHeat evolved or absorbedChange in pH (later in semester)

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Chemical Equations for Chemical Reactions

Chemical reactions always involve changeAtoms, molecules or ions rearrange to form new

substances: chemical bonds are broken and new bonds are formed

Chemical equations describe reactions observed in the laboratory or in nature; we use the chemical symbols and formulas of the reactants and products and other symbolic terms to represent a chemical reaction.

Chemical equations provide us with the means to: - summarize the reaction - display the substances that are reacting (reactants) - show the products - indicate the amounts of all component substances

in a reaction

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Chemical Reactions

(Unbalanced) Chemical Equation

Al + Fe2O3 Fe + Al2O3

reactants products

iron oxygen aluminum oxygen

bonds break bonds form

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Chemical Reactions

Coefficients (whole numbers) are placed in front of substances to balance the equation and to indicate the number of units (atoms, molecules, moles, or ions) of each substance that are reacting.

2 Al + Fe2O3 2 Fe + Al2O3

Coefficient CoefficientConditions required to carry out the reaction may be

placed above or below the arrow. = Heat added.

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Law of Conservation of Mass

Matter cannot be created or destroyed- Therefore the total mass cannot change,

and the total mass of the reactants will be the same as the total mass of the products

In a chemical reaction, all the atoms present at the beginning are still present at the end- if all the atoms are still there, then the

mass will not change

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Symbols used in chemical equations

Learn the state symbols: s, l, g, aq

Delta, , means heat

means “yields” or makes or produces

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THREE STEPS TO WRITING CHEMICAL EQUATIONS:

1. PREDICT WHAT PRODUCTS WILL BEIf two elements react, you can decide what

compound will form based on whether it’s ionic or covalent.

EXAMPLE: Al metal reacts with Cl2 gas (nonmetal) to make an ionic cmpd. What ions are likely to form? Al3+ and Cl-, so the product will be AlCl3.

If two compounds react, you need to know about types of chemical reactions (coming up soon).

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THREE STEPS TO WRITING CHEMICAL EQUATIONS:

2. WRITE CORRECT CHEMICAL FORMULA FOR EACH REACTANT AND PRODUCT**Remember the seven diatomic elements**Remember the naming rules

This is the step where most of the mistakes are made!!!

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THREE STEPS TO WRITING CHEMICAL EQUATIONS:

3. BALANCE EQUATION USING STOICHIOMETRIC COEFFICIENTS

**Never change formulas’ subscripts to mass-balance an equation!

Start with the most complex formula or with the formula that contains the most of an atom

**Treat polyatomic ions as single units unless they have broken up in the reaction

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THREE STEPS TO WRITING CHEMICAL EQUATIONS: An Example

Write the chemical equation for the reaction of solid aluminum with chlorine gas to form aluminum chloride salt

Predict the product – has already been given, but we’ll learn how to do this later

Write the correct chemical formulas – keep working on this

__Al(s) + __Cl2(g) __AlCl3(s) Not mass balanced Balance equation using the correct stoich coefficients

1 Al & 2 Cl 1 Al & 3 Cl2 Cl vs. 3 Cl: Find least common denominator

(6) ___Al(s) + 3 Cl2(g) 2 AlCl3(s) Not balanced yet1 Al & 6 Cl 2 Al & 6 Cl2 Al(s) + 3 Cl2(g) 2 AlCl3(s) Now it’s balanced!

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Practice writing and balancing chemical equations

1. Glucose and oxygen produce carbon dioxide and water

2. Sulfuric acid and potassium hydroxide produce potassium sulfate and water

1. C6H12O6(s) + 6 O2(g) 6 CO2(g) + 6 H2O(l)

2. H2SO4(aq) + 2KOH(aq) K2SO4(aq) + 2 H2O(l)

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The meaning of a formulais context-dependent.

The formula H2O can mean:

1. 2 H atoms and 1 O atom2. 1 molecule of water3. 1 mole of water4. 6.0221 x 1023 molecules of

water5. 18.015 g of water

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FormulasNumber of molecules

Number of atomsNumber of molesMolar masses

In an equation formulas can represent units of individual chemical entities or moles.

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Aqueous SolutionsSome chemicals in a reaction are

dissolved in water; they are called aqueous solutions

Dissolving chemicals in water helps them to react together faster- water separates chemicals into

individual molecules or ions, called dissociation

- the particles come in contact more frequently speeding the reaction up

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DissociationWhen ionic compounds

dissolve in water, the anions and cations are separated from each other - this is called dissociation- however not all ionic

compounds are soluble in water!

When compounds containing polyatomic ions dissociate, the polyatomic group stays together as one ion

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Dissociation

When potassium iodide dissociates in water into potassium cations and iodide anions:

KI(aq) K+1(aq) + I-1(aq)

Copper(II) sulfate dissociates in water into copper(II) cations and sulfate anionsCuSO4(aq) Cu+2

(aq) + SO4-2

(aq)

Potassium sulfate dissociates in water into potassium cations and sulfate anions

K2SO4(aq) 2 K+1(aq) + SO4

-2(aq)

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Electrolytes

Electrolytes are substances whose water solution is a conductor of electricity

All electrolytes have ions dissolved in water

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ElectrolytesStrong electrolytes: all

electrolyte “units” are separated into ions; includes soluble salts, strong acids, strong bases

Nonelectrolytes: none are separated into ions; includes covalent solutes

Weak electrolytes: a small percentage are separated into ions; includes weak acids & weak bases

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Types of ElectrolytesSalts = ionic compounds; soluble salts

are all strong electrolytesAcids = form H+ ions in water solution.

Strong acids dissoc 100% strong electrolytes; weak acids dissoc <10% weak electrolytes

Bases = water soluble metal hydroxides. Strong bases dissoc 100% strong electrolytes; weak bases dissoc <10% weak electrolytes

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When will a Salt Dissolve?

Soluble: if it dissolves in a specified solvent- NaCl is soluble in water, but

AgCl is not

Insoluble: if a significant amount does not dissolve in the specified solvent- AgCl is insoluble in water

(though there is a very small amount dissolved, but not enough to be significant)

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Compounds Containing the following Ions are Generally Soluble

Exceptions

Li+, Na+, K+, NH4+ none

NO3–, C2H3O2

– none

Cl–, Br–, I– Ag+, Hg22+, Pb2+

SO42– Ca2+, Sr2+, Ba2+, Pb2+

Solubility RulesCompounds that are Generally Soluble in

Water

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Compounds Containing the following Ions are Generally Insoluble

Exceptions(slightly soluble)

OH– Li+, Na+, K+, NH4+,

Ca2+, Sr2+, Ba2+

S2– Li+, Na+, K+, NH4+,

Ca2+, Sr2+, Ba2+

CO32–, PO4

3– Li+, Na+, K+, NH4+

Solubility RulesCompounds that are Generally Insoluble

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Five Main Types of Chemical Equations

CombinationDecompositionSingle-ReplacementDouble-ReplacementCombustion (I added this!)(Ignore sections 9 & 10 and use our

lecture notes!)

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TYPES OF CHEMICAL REACTIONS:

1. COMBINATION (OR SYNTHESIS): A + B C

2 K + S K2S 2 Na + Br2 2 NaBr

MEMORIZE THESE TWO: H2O + CO2 H2CO3

NH3 + H2O NH4OH

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TYPES OF CHEMICAL REACTIONS:

2. DECOMPOSITION: C A + B2 KClO3 2 KCl + 3 O2

2 H2O2 2 H2O + O2

Memorize these two:H2CO3 H2O + CO2

NH4OH NH3 + H2O(What is similar about these two

equations and the ones in the first type?)

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TYPES OF CHEMICAL REACTIONS:

3. SINGLE REPLACEMENT: A + BC B + ACK(s) + ZnSO4(aq) Zn(s) + K2SO4(aq)

Al(s) + FeCl3(aq) Fe(s) + AlCl3(aq)

Replacement occurs only if solid metal element is more reactive than metal ion which is part of the compound

ACTIVITY SERIES (partial): K Ca Mg Al Zn Fe Sn H Cu Ag Au

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Metals KCaNaMgAlZnFeNiSnPbHCuAgHg

An atom of an element in the activity series will displace an atom of an element below it from one of its compounds .

Sodium (Na) will displace an atom below it from one of its compounds.

increasing activity

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Mg(s) + PbS(s) MgS(s) + Pb(s)

Metal Higher in Activity Series Displacing Metal Below It

Magnesium is above lead in the activity series.

Metals MgAlZnFeNiSnPb

EXAMPLES USING ACTIVITY SERIES:

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Ag(s) + CuCl2(aq) no reaction

Metal Lower in Activity Cannot Displace Metal Above It

Metals PbHCuAgHg

Silver is below copper in the activity series.

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Cl2(g) + CaBr2(s) CaCl2(aq) + Br2(aq)

Halogen Higher in Activity Series Displaces Halogen Below It

Halogens F2Cl2Br2I2

Chlorine is above bromine in the activity series.

EXAMPLES USING ACTIVITY SERIES:

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Using the Activity Series

SEE IF THESE REACT: if they do, then determine the product formulas and balance the equations!

___Ca(s) + ___MgCO3(aq)

___Zn(s) + ___HCl(aq)

___Mg(s) + ___AgNO3(aq)

___Ca(s) + ___HOH(l)

___Cu(s) + ___Fe(NO3)3(aq)

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TYPES OF CHEMICAL REACTIONS: (continued)

4. DOUBLE REPLACEMENT: trades partners cation to anion! AB + CD AD + CBA. Precipitation reactions: must learn how to use the solubility rules! NaCl(aq) + AgNO3(aq) NaNO3(aq) + AgCl(s)

B. Gas-forming reactions:2 HNO3(aq) + MgS(aq) H2S(g) + Mg(NO3)2(aq)

C. Neutralization (acid-base) reactions: HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

IN EVERY DOUBLE REPLACEMENT REACTION, A SOLID, LIQUID OR GAS HAS FORMED!

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TYPES OF CHEMICAL REACTIONS: (continued)

4.A. A precipitate is two ions combining to make a compound that is not soluble in water

MgCl2(aq) + 2 NaOH(aq) Mg(OH)2(?) + 2 NaCl(?)

One of the two products

will be insoluble

Use tables and rules to determine which is insoluble and will precipitate.

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Types of Chemical Reactions (continued)

Practice finding which product is insoluble__Pb(NO3)2(aq) + __KI(aq) __PbI2(?) + ___KNO3(?)

Practice: what compound precipitates when aqueous solutions of silver nitrate and lithium bromide are mixed?

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TYPES OF CHEMICAL REACTIONS: (continued)

4.B. Gas-forming reactions: may be double replacement, but also others.

Na2S(aq) + 2 HCl(aq) 2 NaCl(aq) + H2S(g)

All carbonate compounds react with any acid to form CO2(g)

If two compounds react to form H2CO3,then it decomposes: H2CO3(aq) H2O(l) + CO2(g)

EXAMPLE: NaHCO3(aq) + HCl(aq) NaCl(aq) + H2CO3(aq)

NaCl(aq) + H2O(l) + CO2(g)

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TYPES OF CHEMICAL REACTIONS: (continued)

4.C. Acid-base (neutralization) reactions usually form water and a salt

HCl(aq) + NaOH(aq) HOH(l) + NaCl(aq)

H2SO4(aq) + 2 KOH(aq) 2 HOH(l) + K2SO4(aq)

You try:___H3PO4(aq) + ___Ca(OH)2(aq) _____

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TYPES OF CHEMICAL REACTIONS:

5. COMBUSTION: reacting very rapidly and exothermally with O2 gas

S(s) + O2(g) SO2(g) (+ HEAT)

CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)

(+HEAT)2 H2(g) + O2(g) 2 H2O(g) (+ HEAT)

C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g)

(+HEAT)***Memorize: all hydrocarbon combustion

products are the same: CO2 and H2O

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PRACTICE: WRITE THE COMPLETE BALANCED CHEMICAL EQUATION

& ID THE TYPE OF REACTION1. Aqueous solutions of magnesium chloride &

sodium hydroxide form magnesium hydroxide and sodium chloride (you decide on phases)

2. Decomp of HCl gas to its elements3. Zinc metal reacts with aqueous nitric acid4. Ammonia gas plus water (Hint:

combination)5. Aqueous solutions of sulfuric acid + calcium

hydroxide react6. Aqueous solutions of aluminum chloride and

zinc bicarbonate react 7. Methane gas burns in air

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PRACTICE: WRITE THE COMPLETE BALANCED CHEMICAL EQUATION

& ID THE TYPE OF REACTION

1. MgCl2(aq) + 2 NaOH(aq) Mg(OH)2(s) + 2 NaCl(aq)

Double replacement - ppt2. 2 HCl(g) H2(g) + Cl2(g) Decomposition3. Zn(s) + 2 HNO3(aq) Zn(NO3)2(aq) + H2(g)

Single replacement: Zn > H4. NH3(g) + H2O(l) NH4OH(aq) Combination (memorize)5. H2SO4(aq) + Ca(OH)2(aq) CaSO4(aq) + 2 H2O(l)

Double replacement - neut6. 2AlCl3(aq) + 3Zn(HCO3)2(aq) 2 Al(HCO3)3(s) + 3 ZnCl2(aq)

Double replacement - ppt7. CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) Combustion HEAT!

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NET IONIC EQUATIONS

REFER TO HANDOUTS!!SUBSTITUTE “normal chemical

equation” for “molecular equation in your textbook

PRACTICE, PRACTICE, PRACTICE!Memorize the six strong acids (HCl,

HBr, HI, HNO3, H2SO4, HClO4) and what makes a strong base (Group I and Group II oxides and hydroxides)

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Heat (Enthalpy) inHeat (Enthalpy) inChemical Reactions (take good notes, not in Chemical Reactions (take good notes, not in

your chapter)your chapter)

Energy changes always accompany chemical reactions.

When this occurs energy is released to the surroundings.

The amounts of substances are expressed in moles.

H2(g) + Cl2(g) → 2 HCl(g) + 185 kJ (exothermic)

N2(g) + O2(g) + 185 kJ → 2 NO(g) (endothermic)

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Energy of Activation

A certain amount of energy is always required for a reaction to occur.

The energy required to start a reaction is called the energy of activation.CH4 + 2 O2 → CO2 + 2 H2O + 890 kJ

This reaction will not occur unless activation energy is supplied.

The activation energy can take the form of a spark or a flame.

458.18.2