Reactions

Five Main Types of Reactions

• Single Replacement

• Double Replacement

• Synthesis

• Decomposition

• Combustion

• http://

www.youtube.com/watch

?v=tE4668aarck&safety_

mode=true&persist_safet

y_mode=1&safe=active

Single Replacement

• A + BX AX + B• A and B are cations and X is an anion. The cations exchange

position.

Double Replacement

• AX + BY AY + BX• A and B are cations & X and Y are anions.• Cations trade anions to form two new

products.Colorless solutions of lead (II) nitrate and potassium iodide react to produce the yellow precipitate lead (II) iodide and the colorless solution of potassium nitrate.

Synthesis

• A + X AX• Elements react to form molecule.• Also molecules react to form larger molecules.• http://

www.youtube.com/watch?v=F54rqDh2mWA&safety_mode=true&persist_safety_mode=1&safe=active

Decomposition

• AX A + X• Molecule breaks apart into smaller products,

elements.

Combustion

• CnH2nO + O2 CO2 + H2O• Reaction of organic molecule containing

carbon and hydrogen with oxygen.• Products are carbon dioxide and water.

Writing Reaction Equations

• Write a sentence describing the reaction that includes the names and states of all reactants and products, and any special conditions the reaction requires. – "Ozone gas decomposes into oxygen gas when

exposed to ultraviolet light.“

Add symbols

• Separate reactants & products with a process symbol: – reaction goes to completion; – sometimes used for one-step reactions reaction

is reversible; at equilibrium, reactants and products are mixed

– = net reaction; reaction occurs in several steps.– Catalysts are usually written on top of the process

symbol. • Ozone gas oxygen gas

Replace names with formulas

• Replace names with formulas; put plus signs between formulas.

• O3 (gas) O2 (gas)

Add symbols for phase state

• Include symbols for physical states: – (g) gas – (l) liquid – (s) solid – (aq) aqueous (dissolved in water)

• O3 (g) O2(g)

Balance atoms and charges

• "Balance" the equation by adding coefficients so atoms and charge are conserved. Each side of a balanced equation must have: – the same number of atoms of each type – the same total charge

• 2 O3 (g) 3 O2(g)

Law of Conservation of Mass

• Lavoisier’s quantified research into combustion showed the mass of hydrogen and oxygen reactants equals the mass of H2O produced.

• “Mass is neither created nor destroyed in chemical reactions.”

• This law requires that reaction equations balance, every atom accounted for.

Balancing Equations

• Elements are included in equations in the form that they exist in nature: diatomic gases, Hg2

2+.• Most elements exist as single atoms for the simplest

form.• The number and kind of atoms on each side of the

reaction arrow are the same, and therefore the equation is balanced.

• Formula units (atom, ion, or molecule) must be identified for each substance that participates in the reaction.

Process for balancing

• Write the formulas for the reactant(s) and product(s).

• Find suitable coefficients for each substance.• Reduce the coefficients to their smallest

whole number values. – Divide by a common divisor if necessary.Check your work to be sure the number and type of

atom is the same on both sides of equation.

Coefficients

• Strategy—start with most complex molecule.– Balance for each kind of atom in the molecule,

one at a time.KClO3 + C12H22O11 KCl + CO2 + H2O

1. C2. H3. O4. K5. Cl

12

Start with C in largest molecule, 12 on the left. Multiply CO2 by 12.

Next H, 22 on the left. Multiply H2O by 11.

11

Next O, 3 + 11 on the left, and 24 + 11 on the right Left side needs 21 more. Multiply KClO3 by 8 (It has 3 O’s plus 7*3 O’s needed = 8 coefficient.Next K, 8 on the left. Multiply KCl by 8.

Next Cl, 8 on the left. Have 8 on the right.

8 8

Try these?• C6H12O6 C2H6O + CO2 fermentation of sugar to yield

alcohol.

• Fe + O2 Fe2O3 rusting of iron.

• NH3 + Cl2 N2H4 + NH4Cl synthesis of hydrazine for

rocket fuel.

• KClO3 KCl + O2 decomposition reaction used to produce

oxygen for the emergency breathing masks in airliners.

2 2Start with sugar on left. Balance carbon(remember to add up all C), then oxygen.

Start with iron (III) oxide. Add 2 in front of iron. LCM for O is 6. Add 3 to O2 and 2 in front of Fe. Recheck the tally. Left has 2 Fe and right side has 4. Adjust left side Fe to 4.

2 3 24

24Start with ammonium chloride on right. Balance chlorine; multiply left by 4.. Balance N(remember to add up all CN), then hydrogen.

Start with potassium chlorate on left. Balance oxygen; find least common multiple for oxygen on both sides. Multiply left side KClO3by 2 and the right O2 by 3. Balance potassium, and multiply right side by 2.

2 2 3

Predicting reactions

• A world of water—many reactions occur in aqueous solutions.

• Precipitation reactions—proceed in the direction of a stable precipitate formation as the ions are removed from the solution. – Common for single or double replacement

reactions.Pb(NO3) 2(aq) + 2KI(aq) 2KNO3(aq) + PbI2(s)

Predicting reactions

• Acid-base neutralization reactions are processes in which an acid(H+) & a base(OH-) react to yield water and a salt (ionic compound)– Double replacement reactionHCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

The driving force behind the neutralization reaction is the production of the stable covalent water molecule by removing the H+ and OH- out of solution.

Predicting reactions

• Oxidation-reduction or redox reactions—are processes in which one or more electrons are transferred between reaction partners.– Common to synthesis or decomposition reactions.Mg(s) + I2(g) MgI2(s)

The driving force is the reduction in electrical potential (analogous to what happens when a live electrical wire is grounded and electrons flow from the wire to the ground.) Charges on the atoms change.

Activity SeriesLiKBaCaNaMgAlMnZnCrFeCoNiSnH2

CuAgHgPtAu

These elements react rapidly with aqueous H+ ions (acid) or with liquid H2O to release H2 gas.

These elements react with aqueous H+ ions or with steam to release H2 gas.

These elements react with aqueous H+ ions to release H2 gas.

These elements do not react with aqueous H+ ions to release H2.

More reactiveStrongly reducing

Significance of Equations

• On a microscopic level, the equation describes the behavior of individual atoms and molecules.

• On a macroscopic level, formulas and equations represent the large scale behaviors of atoms and molecules that give rise to visible properties.– The symbols represent vast numbers of molecules

that together have measurable physical properties.• Example: a single H2O molecule is neither solid, liquid, nor

gas. A huge collection of H2O molecules appears as a colorless liquid that freezes at 0oC.

Resources

• http://antoine.frostburg.edu/chem/senese/101/reactions/index.shtml notes-equations

• http://antoine.frostburg.edu/chem/senese/101/compounds/index.shtml notes-ions

• http://phet.colorado.edu/en/simulation/balancing-chemical-equations java based game