Descriptive Chemistry

Question 1

If two elements in their standard states are going to be reacted together, what type of reaction must be occurring?

Ex: N₂ and H₂ are mixed in a catalytic environment. What product results?

Answer 1

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.

Question 2

Solid sulfur is burned in an oxygen atmosphere.

1. What product results?
2. What is the balanced equation for this reaction?

Answer 2

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.

Question 3

Solid sulfur is burned in an oxygen atmosphere. What are the starting and finishing oxidation numbers for the atoms in the reaction?

S + O₂ ⇒ SO₂

Answer 3

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.

Question 4

What is the name for any reaction where a material is combined with oxygen and heat is applied?

Answer 4

A combustion reaction.

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

Question 5

If one substance is the only reactant, what type of reaction must be occurring?

Ex: CaCO₃ is heated and a reaction occurs creating multiple products?

Answer 5

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.

Question 6

Hydrogen peroxide, H₂O₂, is exposed to UV light and a chemical reaction occurs.

1. What products result?
2. What is the balanced equation for this reaction?

Answer 6

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.

Question 7

Carbonic acid, H₂CO₃, is heated and a chemical reaction occurs.

1. What products result?
2. What is the balanced equation for this reaction?

Answer 7

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.

Question 8

If two solutions are mixed, and a solid compound results, what sort of reaction occurred?

Answer 8

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.

Question 9

Solutions of sodium hydroxide and iron (III) chloride are mixed. What will the precipitate, if any, be?

Answer 9

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.

Question 10

Solutions of calcium nitrate and sodium sulfate are mixed. What will the precipitate, if any, be?

Answer 10

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.

Question 11

If a pure metal is added to water, what will the resultant products be?

Answer 11

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.

Question 12

If a metal hydride is added to water, what will the resultant products be?

Answer 12

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.

Question 13

Equal moles of NaOH and HNO₃ are combined in water. Will the pH of the resultant solution be acidic, neutral, or basic?

Answer 13

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.

Question 14

Equal moles of NaOH and CH₃COOH are combined in water. Will the pH of the resultant solution be acidic, neutral, or basic?

Answer 14

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.

Question 15

Equal moles of NH₃ and HCl are combined in water. Will the pH of the resultant solution be acidic, neutral, or basic?

Answer 15

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.

Question 16

A strip of solid zinc is placed in a solution of copper(I) sulfate. Will there be a chemical reaction? If so, what will happen?

E^o_red (Cu⁺⇒Cu) = +0.52 V
E^o_red(Zn⁺²⇒Zn) = -0.76 V

Answer 16

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.

Question 17

A strip of solid silver is placed in a solution of copper(II) sulfate. Will there be a chemical reaction? If so, what will happen?

E^o_red (Ag⁺⇒Ag) = +0.80 V
E^o_red(Cu⁺²⇒Cu) = +0.34 V

Answer 17

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.

Question 18

What reaction, if any, will occur when magnesium is burned in an atmosphere of chlorine gas?

E^o_red (Cl₂⇒2Cl^-) = +1.36 V
E^o_red(Mg⁺²⇒Mg) = -2.37 V

Answer 18

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.

Question 19

What properties will a colored chemical compound have?

Answer 19

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.

Question 20

What will be the color of an aqueous copper(II) sulfate solution?

Answer 20

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.

Question 21

What will be the color of an iron(III) oxide solution?

Answer 21

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

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

Question 22

What is the color of solutions containing free Co⁺² ions?

Answer 22

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.

Question 23

What is the color of titanium dioxide salt?

Answer 23

Titanium dioxide salt is bright white in color.

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

Question 24

What is the color of silver chloride salt?

Answer 24

Silver chloride salt is white in color.

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

Question 25

What is the color of potassium chromate salt?

Answer 25

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.

Question 26

What color do these alkali metals impart when heated in a flame?

• Li
• Na
• K
• Cs

Answer 26

• 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.

Question 27

What color do these alkali earth metals impart when heated in a flame?

• Be
• Mg
• Ca
• Sr
• Ba

Answer 27

• 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.

Question 28

What is the IUPAC name for the below molecule, C₄H₁₀?

Answer 28

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-.

Question 29

What is the IUPAC name for the below molecule, C₄H₈?

Answer 29

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.

Question 30

What is the IUPAC name for the below molecule, C₄H₆?

Answer 30

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.

Question 31

What is the IUPAC name for the below molecule, C₆H₁₄?

Answer 31

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).

Question 32

What is the IUPAC name for the below molecule, C₃H₆O?

Answer 32

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’.

Question 33

What is the IUPAC name for the below molecule, C₃H₆O?

Answer 33

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.

Question 34

What is the IUPAC name for the below molecule, C₂H₆O?

Answer 34

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.

Question 35

What is the IUPAC name for the below molecule, C₃H₉N?

Answer 35

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.

Question 36

What is the IUPAC name for the below molecule, C₃H₇COOH?

Answer 36

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.

Question 37

Which molecule has the higher boiling point, propane or propanal?

Answer 37

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.

Question 38

Which molecule has the higher boiling point, propanal or propanol?

Answer 38

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.

Question 39

Which molecule has the higher melting point, butane or isobutane?

Answer 39

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.

Question 40

What is the pH of a solution of propanoic acid: acidic, neutral, or basic?

Answer 40

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.

Question 41

Identical equivalents of butanoic acid and sodium hydroxide solutions are mixed together. What will the final pH of the mixture be?

Answer 41

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.