Equations & Reactions

8.1 Describing Chemical ReactionsA. Chemical Changes and Reactions

1. New substances are producedproduced.2. Chemical reaction – chemical bonds

between atoms or ions breakbreak, and new new bondsbonds form between atoms or ions.

B. Evidence of a Chemical Reaction1. color changecolor change2. formation of a precipitateformation of a precipitate3. temperature changetemperature change4. formation of a gasformation of a gas

C. Mechanics of a Chemical Reaction1. Starting Materials – reactantsreactants2. Ending Materials - productsproducts3. reactantsreactants → productsproducts

Arrow = yieldsyields or producesproduces4. Many reactions occur to complete a set of a set of

valence electronsvalence electrons.

5. Symbols above the yield sign represent conditions conditions necessary for a reaction to proceed. Ex)

= delta = heatheat

= electrolysiselectrolysis

Δ

elec

6. Some reactions occur spontaneouslyspontaneously.7. Symbols represent the statestate of the reactants and

products.

Liquid = ll Gas = gg Solid = ss Crystal = crcr

Aqueous = aqaq (solids in water water solutionsolution)

DEMO

Ex) Al(s) + CuCl2(aq) → AlCl3(aq) + Cu(s)

Ex) 2Al(s) + 3CuCl2(aq) → 2AlCl3(aq) + 3Cu(s)

8. Complete chemical equations include the subscript to indicate the physical statestate of each substance.

9. Diatomic molecules – certain elements exist in nature as diatomic molecules (X2) List them: N2 O2 F2 Cl2 Br2 I2 H2

a. Tetra-atomic element = Phosphorous (P4)b. Octa-atomic element = Sulfur (S8)

8.2 Balancing Equations

A. Equations in Chemistry1. Chemical equation: an expression that uses

symbolssymbols and formulasformulas to describe a chemical

reaction.2. ++ means “reacts with”3. → means produces (Called the yieldyield sign)

B. Balancing Chemical Equations

1. Conservation of massmass leads to balancing equations – the number of atoms of each element must be the same before & after the before & after the reactionreaction.

2. The Law of Conservation of Mass also states that the total massmass before and after the reaction must be the samesame. You cannot lose or gain mass.

3. Therefore the MASS OF THE PRODUCTS = MASS OF REACTANTSMASS OF REACTANTS

4. Subscript – indicates number of atomsatoms of an element present in a compound.

5. Coefficient – indicates the number of atomsatoms or moleculesmolecules involved in the reaction.

6. Steps to Balance Equations:

A. Write equation with symbols.

B. Count # of atoms on each side of the reaction.

C. Balance atoms using coefficients. D. General Rule: Balance all elements first.

Then, balance C, H, and O. E. General Rule: If there is an odd number of an element, double everything, but that element.

F. NEVER EVER EVER, CHANGE THE SUBSCRIPTS!!!!

H2 + N2 → NH3

P + O2 → P2O5

NaNO3 → NaNO2 + O2

C8H18 + O2 → CO2 + H2O

8.3 Classifying Chemical Reactions

A. Synthesis Reactions (direct direct combinationcombination)1. Two or more elements or compoundselements or compounds combine

to form a more complexcomplex product. A + B → AB

2. Ex. Fe + S → FeSCaO + H2O → Ca(OH)2

sodium + chlorine → sodium chloride 2 Na + Cl2 → 2 NaCl

Synthesis Reaction

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Sodium Metal plus Chlorine Gas Video

Sodium Metal plus Chlorine Gas Video

2 Na + Cl2 2 NaCl

Synthesis Reaction

B. Decomposition Reactions (analysisanalysis)

1. A singlesingle reactant breaks down into simpler substancessimpler substances.

AB → A + B2. The oppositeopposite of a synthesis reaction.3. Ex. 2 HgO → 2 Hg + O2

CaCO3 → CaO + CO2

Decomposition Reaction

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C. Single Replacement Reactions

1. Atoms of an uncombined element replacereplace atoms of another element in a compound.

A + BX → AX + B2. A moremore active element will replace a less

active element. (See activity series.)

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3CuCl2 + 2Al 2AlCl3 + 3Cu

Single Replacement Reaction3CuCl2 + 2Al 2AlCl3 + 3Cu

3. An Activity Series is a way of rankingranking elements (usually metals) in order from greatest to least reactivity. It can be used to predictpredict whether a reaction will occur or not.

Fe + CuSO4 → FeSO4 + Cu FeSO4 + Cu → No Reaction

Mg + CuSO4 → MgSO4 + Cu

MgSO4 + Cu → No Reaction

D. Double-Replacement Reactions

Atoms or ions from 2 different2 different compounds replace each other.

AX + BY → AY + BX

Ex. CaCO3 + 2HCl → CaCl2 + H2CO3

Ex. NaOH + HCl → NaCl + HOH

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E. Combustion Reactions

1. One substance reacts with oxygen, oxygen, OO22 to produce oxide compounds.

2. Occurs during burningburning.3. Some combustion reactions can

often be classified as synthesissynthesis reactions.

Ex) S + O2 → SO2

Hydrogen Burning Video

Hydrogen Burning Video

Synthesis Reaction

2H2 + O2 2 H2O

4. These reactions are usually exothermicexothermic, releasing a large amount of energy as light, heat, or sound.

5. When a hydrocarbon (compound containing H & C) is involved in a combustion reaction, COCO22 and HH22OO are always the products. (also a vast amount of heat.)

6. Ex. __CH4 + __O2 → __CO2 + __H2O + 803 kJ

C2H6 + O2 →

CO2 + H2O + heat

2 2

2 7 4 6

Combustion Reaction

Combustion Reaction

5 Types of Chemical Reactions Video