Chemistry 101: Descriptive Chemistry

Combination reactions are the only possible result of two standard elements being mixed.

Ex: N2 + 3H2 ⇒ 3NH3

In general, make sure you are keeping track of the standard states of the elements involved and their standard oxidation numbers in order to predict the product and write a balanced equation.

1. SO2 is the product.

2. S + O2 ⇒ SO2

Note: A combination reaction where molecular oxygen is one of the reactants is also known as a combustion reaction.

This reaction is both a combustion and a redox reaction. Both S and O are in their standard states, hence must start with oxidation number 0. S is then oxidized to +4, and O is reduced to -2.

A combustion reaction.

Ex: The equation for the combustion of glucose is:
C6H12O6 + 6O2 ⇒
6CO2 + 6H2O

Decomposition reactions are the only possible reactions with only one reactant.

Ex: CaCO3 ⇒ CaO + CO2

Note: one of the products of a decomposition reaction is almost always an oxide gas. General practice is to figure out possible oxide gas products first, then proceed to other products.

1. O2 and H2O are the products.

2. 2H2O2 ⇒ O2 + 2H2O

Note: even though it’s conceptually possible for H2 to be produced here; H2 is NOT an oxide gas hence is not expected.

1. CO2 and H2O are the products.

2. H2CO3 ⇒ CO2 + H2O

CO2 is the common oxide gas expected to be produced, and H2O is the only possible remaining product.

When a solid appears out of solution, a precipitation reaction has occurred.

Ex: AgNO3(aq) + NaCl(aq) ⇒
AgCl (s) + NaNO3 (aq)

Note: solubility rules help to predict the products of precipitation reactions. In this case, alkali metal salts are soluble, as are nitrate salts, so AgCl is the only expected precipitation product.

Iron (III) hydroxide, Fe(OH)3, will be the precipitate.

The balanced reaction is:

3NaOH (aq) + FeCl3 (aq) ⇒
3NaCl (aq) + Fe(OH)3 (s)

Note: the only soluble hydroxide salts are alkali metal or ammonium hydroxides.

Calcium sulfate, CaSO4, will be the precipitate.

The balanced reaction is:

Ca(NO3)2 (aq) + Na2SO3 (aq) ⇒
NaNO3 (aq) + CaSO4 (s)

Note: the insoluble sulfate salts are Ag+, Ca+2, Sr+2, Ba+2, Pb+2, and Hg2+2 sulfates.

Metals react with water to produce a base and hydrogen gas.

Ex: 2K (s) + 2H2O ⇒
2KOH (aq) + H2 (g)

Note: In the case of alkali metals, this reaction is sufficiently exothermic that it can ignite the hydrogen released, setting it on fire.

Metal hydrides react with water to produce a base and hydrogen gas.

Ex: CaH2 + 2H2O ⇒
Ca(OH)2 (aq) + 2H2 (g)

Note: because the reaction of metal hydrides with water is less vigorous than that of bare metals, metal hydrides are sometimes seen being used as desiccants (drying agents) on many chemistry tests like the AP Chem exam.

The pH will be neutral.

When identical molar equivalents of strong base and strong acid are combined, they neutralize each other perfectly and the resultant solution will be neutral.

The pH will be basic.

When identical molar equivalents of strong base and weak acid are combined, the strong base has a dominant effect and the pH of the resultant solution will be basic.

The pH will be acidic.

When identical molar equivalents of weak base and strong acid are combined, the strong acid has a dominant effect and the pH of the resultant solution will be acidic.

Zn will be oxidized to Zn+2, while the Cu+ ions will be reduced to solid Cu and precipitate out of solution.

Cu’s positive reduction potential indicates that Cu(I) ions will favorably reduce, while Zn’s negative reduction potential indicates that Zn(s) will preferentially oxidize.
The net voltage gain from this process is +1.28V.

No reaction will occur.

The fact that Ag’s reduction potential is more positive than that of Cu(II) means that Ag will preferentially stay in its solid, reduced state. The amount of energy liberated by reducing the Cu is insufficient to oxidize the Ag.
There is no voltage gain, but if .46V were applied (the difference) a reaction could be generated.

Mg(s) + Cl2(g) ⇒ MgCl2(s)

Cl2’s positive reduction potential indicates that Cl2 will prefer to reduce to Cl-, while Mg+2’s negative reduction potential indicates that Mg(s) will preferentially oxidize to Mg+2, and the net reaction will be formation of the ionic salt.
The net voltage gain is 3.73V through this process.

Colored chemical compounds will contain either transition metals with positive oxidation numbers or large, conjugated systems.

In either case, the color comes from the absorption of visible photons when electrons change energy levels.

A copper(II) sulfate solution will be blue-green.

Most solutions of copper ions will be blue or green in color. Unprotected copper left outside will develop a green patina on its surface as it oxidizes.

An iron(III) oxide solution will be red-brown in color.

Red-brown is the indicative color of oxidized iron (also known as rust).

Solutions containing free Co+2 ions tend to be pink in color.

Most Co+2 salts are blue or purple in the solid phase, but turn pink when dissolved in water.

Titanium dioxide salt is bright white in color.

Titanium dioxide is used in white paint and sunscreen for its reflective qualities.

Silver chloride salt is white in color.

Nearly all ionic salts of small, simple ions are white crystalline solids.

Potassium chromate salt is yellow-orange in color.

Many complex ions, which feature a transition metal and other ligands, are colored. Chromium-containing complexes tend to be yellow and orange in color.

• Li makes a red flame

• Na makes an orange flame

• K makes a purple flame

• Cs makes a blue flame

These metals are responsible for the colors in many fireworks.

• Be imparts no color

• Mg imparts no color

• Ca makes a red-orange flame

• Sr makes a deep red flame

• Ba makes a green-yellow flame

The latter three are responsible for the colors in many fireworks.

This molecule is called butane.

A molecule with nothing but carbons and hydrogens, combined with single bonds, is known as an alkane. The prefix for carbon chains of one through six carbons are meth-, eth-, prop-, but-, pent-, and hex-.

This molecule is called 1-butene.

A molecule with nothing but carbons and hydrogens, with a carbon-carbon double bond, is known as an alkene.

The carbon chain is numbered from one end. The end that is chosen as carbon #1 is the end that results in the double bond being attached to the lower numbered carbon. In this case, the chain is numbered from the right.
This molecule would never be called 3-butene.

This molecule is called 1-butyne.

A molecule with nothing but carbons and hydrogens, with a carbon-carbon triple bond, is known as an alkyne. The carbon chain is numbered using the lowest possible numbering/order for the carbon chain.
This molecule would never be called 3-butyne.

This molecule is called 2-methyl pentane.

The longest unbranched carbon chain is 5 carbons long, hence the molecule is a pentane derivative, and numbered from the left so that the methyl group is on carbon 2 (lowest possible numbering).

The molecule is called propanal.

A molecule with a terminal (on the end) carbonyl group is known as an aldehyde, and is named by taking the parent alkane, and replacing the 'ane' suffix with 'al'.

The molecule is called 2-propanone.

A molecule with an internal (not on the end) carbonyl group is known as a ketone, and is named by taking the parent alkane, and replacing the 'ane' suffix with 'one'.

Propanone is commonly known as acetone.

The molecule is called ethanol.

A molecule with a hydroxyl functional group is known as an alcohol, and is named by taking the parent alkane, and replacing the 'ane' suffix with 'ol'.

Note: this molecule does not need to be named as 1-ethanol since it only has two carbons. Whichever carbon has the hydroxyl group on it will be carbon #1, by definition.

The molecule is called 1-propanamine.

A molecule with an amino (-NH2) functional group is known as an amine, and is named by the longest hydrocarbon chain bound to the nitrogen.

The molecule is called butanoic acid.

A molecule with a COOH functional group is known as a carboxylic acid, and is named by taking the parent alkane and replacing the 'ane' suffix with 'oic acid'.

Carboxylic acids are weak organic acids.

Propanal has the higher boiling point.

When estimating relative boiling points, there are two factors to consider: higher molecular size and greater intermolecular attractive forces. Propane and propanal are roughly the same size, but propane molecules are bound only by dispersion forces, while propanal will have stronger dipole-dipole interactions, hence a higher boiling point.

Propanol has the higher boiling point.

Propanal and propanol are roughly the same size, but propanal molecules are bound by dipole-dipole forces, while propanol's hydroxide functional groups are bound by stronger hydrogen bonds, hence it has a higher boiling point.

Butane has the higher melting point.

To determine relative melting points, there are two factors to consider: greater molecular weight, and lower degree of branching. Butane and isobutane are roughly the same size, but isobutane is more branched hence lower. The butane molecules can "pack" together more tightly, and thus require more energy to break out of the solid phase - they have a higher melting point.

This solution is acidic.

Carboxylic acids are weak organic acids. The hydroxyl proton can be donated to the solvent, leaving behind a negative charge on the oxygen that is stabilized via resonance.

The final solution will have a basic pH.

Carboxylic acids are weak acids, so if they are neutralized by a strong base, the final solution will have a basic pH.