AP Chemistry Crash Course

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

Answer 2

SO₂ is the product.

S + O₂ ⇒ SO₂

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

Question 3

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

Answer 3

A combustion reaction.

Ex: The equation for the combustion of glucose is:

C₆H₁₂O₆ + 6O₂ ⇒
6CO₂ + 6H₂O

Question 4

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 4

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 5

Hydrogen peroxide, H₂O₂, is exposed to UV light and a chemical reaction occurs. What products result? What is the balanced equation for this reaction?

Answer 5

O₂ and H₂O are the products.

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 6

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

Answer 6

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 7

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

Answer 7

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 8

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

Answer 8

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 9

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

Answer 9

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 10

Define:

nuclear force

Answer 10

The nuclear force holds protons and neutrons together in the nucleus.

In very large molecules, or ones with an over-abundance of neutrons, the nuclei can become unstable and decay.

Question 11

Define:

alpha particle and alpha decay

Answer 11

An alpha particle is a helium nucleus, consisting of 2 protons and 2 neutrons (without the electrons).

Alpha decay is a nuclear decay and occurs when a nucleus emits an alpha particle. The daughter nucleus of alpha decay will have an atomic number 2 less and mass number 4 less than the parent nucleus.

Question 12

What will the daughter atom be when uranium 238 undergoes a single alpha decay?

Answer 12

²³⁴₉₀Th

In single alpha decay, an alpha particle is emitted. To identify the daughter nucleus, therefore, simply subtract 2 from the parent nucleus’ atomic number, and 4 from its mass. Subtracting 2 protons from uranium (92) shifts it to thorium (90).

Question 13

Under what conditions would alpha decay be an isotope’s preferred form of radioactive decay?

Answer 13

Alpha decay is typical only in large nuclei (atomic number 60 or greater).

Many of the most famous radioactive elements, such as radium (88), uranium (92), and plutonium (94), decay primarily via alpha decay.

Question 14

Define:

beta particle and beta decay

Answer 14

A beta particle is simply an electron, a massless negatively-charged particle.

Beta decay is a nuclear decay and occurs when a nucleus emits a beta particle. The daughter nucleus of beta decay will have an atomic number 1 more* and mass number identical to the parent nucleus.

*When the electron is ejected, total charge must remain constant, so a neutron is converted to a new proton.

Question 15

What will the daughter atom be when carbon 14 undergoes a single beta decay?

Answer 15

¹⁴₇N

In single beta decay, a beta particle (electron) is emitted. To identify the daughter nucleus, therefore, simply add 1 to the parent nucleus’ atomic number, without changing its mass. The element with an atomic number one larger than carbon is nitrogen.

Question 16

Under what conditions would beta decay be an isotope’s preferred form of radioactive decay?

Answer 16

Beta decay is typical in smaller nuclei, when the isotope’s mass number is greater than the element’s atomic weight.

Ex: The carbon 16 isotope, which is heavier than carbon’s atomic weight of 12.011, decays via beta decay.

Question 17

Define:

positron and positron decay

Answer 17

A positron is an anti-electron, a massless positively-charged particle.

Positron decay is a nuclear decay and occurs when a nucleus emits a positron. The daughter nucleus of positron decay will have an atomic number 1 less* and mass number identical to the parent nucleus.

*When the positron is ejected, total charge must remain constant, so a proton is converted to a new neutron.

Question 18

What will the daughter atom be when carbon 11 undergoes positron decay?

Answer 18

¹¹₅B

In positron decay, a positron is emitted. To identify the daughter nucleus, therefore, simply subtract 1 from the parent nucleus’ atomic number, without changing its mass. The element with an atomic number one less than carbon is boron.

Question 19

Define:

electron capture

Answer 19

Electron capture is a nuclear decay and occurs when a nucleus captures one of its own electrons. The electron merges with a proton to form a neutron. The daughter nucleus of electron capture will have an atomic number 1 less and mass number identical to the parent nucleus.

Question 20

What will the daughter atom be when beryllium 7 decays via electron capture?

Answer 20

⁷₃Li

In electron capture, an electron merges with a proton, forming a neutron. To identify the daughter nucleus, therefore, simply subtract 1 from the parent nucleus’ atomic number, without changing its mass. The element with atomic number one less than beryllium is lithium.