Characterizing and Modeling Chemical Reactions

WS – Defining and Differentiating Chemical, Physical and Nuclear Changes

What is a Chemical Reaction? Chemical Reactions involve change, but what kind of change?

PHYSICAL CHANGE

Is a change of state (gas, liquid or solid) or appearance without (s) (l) changing the formula

NUCLEAR CHANGE 4 H  He If the nucleus is changed (new element or isotope)

Chemical change

is the reordering and rearrangement of atoms to make new compounds (bonds break and bonds are formed). A

change in the chemical formula

indicates a

chemical reaction

.

8Ni + S

8 

8NiS

Each type of change ( Nuclear, Chemical, Physical ) can happen fast/slow, with little/lots of energy, be dramatic/unexciting

WS – H2 Balloon Rxn – Describing & Modeling A Couple Chemical Reactions

Hindenburg Disaster

WS – Chemical Reactions – Lecture and Demo

Overview of Chemical Change / Rxn 1) Activation energy is used to break bonds or attraction between ions 2) The reactants now having incomplete valence shells (we call them activated complexes) are seeking to reform their valence shell

3) Upon forming new bonds and completing their valence shells, the become products. The products will have new chemical properties and possible new physical properties

At the end of the reaction the chemical potential energy is less and / or the atoms are in a more disordered state (increased entropy).

Common Conditions Necessary for Reactions to Occur

• Reactants will need some energy (

Activation Energy

heat, light, motion) to break the existing bonds •

Activated Complexes / Reactants

come in

Contact

need to • Reactants need to be in the correct orientation especially if the reactants are large like polymers

Contact between reactants

How close the reactants are to each other & the activation energy is important

Orientation of Reactants

Reaction NO Reaction

How do we Model of Energy in Chemical Reactions?

Reaction Energy Pathway Diagrams

Types of Energy in Chem Rxns

•

Potential Energy is Bond Energy

–This is the energy caused by charge attraction and repulsions •

Kinetic is Heat and Light Energy

–Heat and light are the

energy of atomic motion

–Heat is motion of atoms –Light is motion of charge

Energy Diagrams Only Show Potential Energy

Rxn Pathway for Exothermic Rxn - Energy Released

Rxn Pathway for Endothermic Rxn - Energy Absorbed

Energy Absorbed Potential Energy in Compounds Reaction Pathway

Why do chemical reactions stop?

Reactants run out

Reactions stop when there is not enough activation energy

Law of

Entropy:

Chemical reactions obey the law of entropy which means either matter or energy (or both) tends to be naturally more disordered at the end of a process.

Therefore, entropy can stop reactions because the reactants or energy are scattered.

Representing Chemical Reactions

Chemical EQUATIONS Reactants  Products Conservation of Matter and Energy

Examples of Chemical Reactions and Equations C + O 2 CO 2 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 Reactants, Products, Yields Arrow, Coefficients, Subscripts, Combined w/ (And)

What’s wrong with this equation?

H 2 + O 2  H 2 O + Energy • Violating the conservation of mass law

2

H 2 + O 2 

2

H 2 O + Energy

LAW of CONSERVATION of Matter Matter isn’t created or destroyed in

chemical

reactions or

physical

changes. It is in NUCLEAR RXNS E = MC 2 for nuclear reactions.

Coefficients take care of C of M

2

H 2 + O 2 

2

H 2 O + Energy Coefficients indicate so many of the whole compound Adding or changing coefficients does not change the reaction because it does not change the chemical formulas.

Keep a tally of reactant atoms. The products should have the same number and type of atoms.

The equation is still not complete 2H 2 + O 2  2H 2 O + Energy 2H 2 (g) + O 2 (g)  2H 2 O (l) + Energy • Include states of matter

States of Matter

g – gas l – liquid s – solid

Aq – Aqueous (dissolved in H 2 O)

Tips for Balancing • Look for odd and even conflicts • Limit coefficient possibilities C 6 H 6 + O 2  H 2 O + CO 2 • Balance polyatomic ions instead of atoms • Use fractions initially

Note of Caution

• Just because you can write out a balanced chemical reaction does not mean it will occur • The rate of a reaction is not indicated by the balanced chemical reaction either

Practice WS

of Student Expectations and Required Content

Types of Reactions •

Direct combination (synthesis)

• •

Decomposition Single-Replacement

•

Double-Replacement

•

Combustion

•

Acid-Base

•

Dissolution

Synthesis

A + B



AB

Burning magnesium metal to make magnesium oxide 2 Mg (s) + O 2 (g)  2 MgO (s) Synthesis rxns can be Ionic or Covalent Rusting and making polymers

Typical Synthesis Reactions • • • • • • Oxidation of Metals Sulfur and Metal Active metal oxides and water to produce hydroxides Nonmetal oxides and water to produce acids Acids and oxygen to produce oxidized acids Metal oxides and nonmetal oxides to produce salts

Synthesis Reaction – Burning of Mg to Make MgO

Decomposition Reaction AB  A + B 2H 2 O 2  2H 2 O + O 2 MOST DECOMPOSITION REACTION REQUIRE ENERGY

Decomposition of H

2

O

2

with KI catalyst & soap

Electrolysis Reaction

Electricity is used to break down a substance 2H 2 O  2H 2 + O 2

Who’s tough?

Single Replacement Reaction

A + BX



AX + B (Driven by attacking element) Al foil and aqueous copper (III) chloride 2Al (s) + 3CuCl 2 (aq)



2AlCl 3 (aq) + 3Cu (s) 2Al (s) + 3Cu +2 (aq) + 6Cl -1 (aq)



2Al +3 (aq) + 3Cu (s) + 6Cl -1 (aq)

You need a precipitate or evolution of a gas to occur in a single replacement reaction.

Single Replacement Lab

Double Replacement

AX + BY  AY + BX K(IO 4 ) + Pb(NO 3 ) 2

+

2K(IO 4 ) (aq) + Pb(NO 3 ) 2 (aq)  2K(NO 3 ) (aq) + Pb(IO 4 ) 2 (s) 2K +1 + IO 4 -1 + Pb +2 + 2NO 3 -1  2K +1 + 2NO 3 -1 + Pb(IO 4 ) 2 (solid)

When you look at the ionic equations for double replacement reactions (like lead nitrate and potassium iodate) it is obvious that a precipitate is necessary for real change.

Therefore the key to predicting a double replacement reaction is in predicting a precipitate.

Need your silverware cleaned? Al foil, baking soda & hot water What kind of Reactions are taking place?

Double Replacement Lab

Decomposition of Sugar

C 6 H 12 O 6  6C + 6H 2 O

Dissolving NaCl(s)



NaCl (aq) NaCl (aq) = Na + + Cl -

decomposition ≠ dissolving

A dissolving can be reversed by removing the solvent (sea water evaporation for sea salt) Decomposition reactions aren’t as easy to reverse (rotting fruit does not become fresh again).

Acid-Base Reactions H + + OH  H 2 O H Cl + Na OH  H 2 O + NaCl H 2 SO 4 + Ca (OH) 2  2H 2 O + CaSO 4

RedOx Reaction Charges of an atom change 4FeO + O 2  2Fe 2 O 3

Combustion Reactions

C x H y + O 2



CO 2 + H 2 O O 2 is always a reactant and always exothermic

Combustion Reactions are more generally reactions in which oxygen as a reactant breaks down another reactant into oxides.

Polymerization reactions

Two major types of polymerization reactions • Radical (breaking a double bond and creating a unpaired electron) • Condensation (acid / base rxn which makes water as byproduct)

Radical Reaction making PVC

Dacron is a condensation polymer obtained from ethylene glycol and terephthalic acid. It has high tensile strength, good stretching, both wet and dry, and good resistance to degradation by chemical bleaches and to abrasion. It is used in curtains, dress fabrics, high-pressure fire hoses, men's shirts, and thread. It is ideal for mixing with wool in men's and women's suits, as well as in dress fabrics, knitted wear, and washable woven sportswear.

Nylon

Littlest Christmas Tree Lab

Use K sp to predict double replacement reactions p. 566 [Pb +2 ][SO 4 -2 ] K sp = [PbSO 4 ]

Very low number means very low solubility and probably a precipitate.

Gas Producing Reactions Lab

Enlighten us with your knowledge of one chemical reaction poster day.

Process of a Chem Rxn • All matter has motion. Atomic collisions are happening all the time • Bonds have varying strength • If the collisions are strong enough to break bonds, atoms are free and may react with something near

• The atoms want a full valence shell so they will find partners which help them achieve this aim.

• The new partnership may be unstable and another reaction occurs

FLOW CHART

CONCEPT MAP for chemical reactions

Catalog of assessments • • • • Section Review 9-1 Q. 1, 2, 3 Balancing worksheet Section Review 9-3 Q. 1-4 Laboratory write-up (hydrogen bubbles • Test

No Reaction: Insufficient energy

No Reaction: Enough energy, but no reason to reorganize

REACTION HAPPENS

Reactions yield a different distribution of kinetic and potential energy More Kinetic  More Potential  Exothermic Endothermic

Sometimes the destabilized reactants (called

activated complexes

) may simply return to original state.

No net effect in atom arrangements or energy.

Chemical Reaction Pretest

• What is a chemical reaction?

• What conditions must exist for a reaction to take place?

• What happens during a chem rxn?

• How do we represent chem rxns?

• How do we change reaction rates (the speed of a chemical reaction)