AP Chemistry — Stoichiometry & Reactions

How do you calculate the **molar mass** of a compound?

The molar mass is calculated by summing the atomic masses of all atoms in the molecular formula, expressed in grams per mole.

What is an **empirical formula**?

An empirical formula represents the simplest whole-number ratio of atoms of each element in a compound.

Describe the process to determine the **empirical formula** from percent composition.

1. Convert percent composition to grams assuming 100 g of compound. 2. Convert grams to moles using molar masses. 3. Divide moles by the smallest number of moles to find the simplest ratio. 4. Write the empirical formula.

What is a **balanced equation**?

A balanced equation has equal numbers of each type of atom on both sides of the reaction, satisfying the law of conservation of mass.

How do you identify the **excess reactant**?

The excess reactant is the reactant that remains after the limiting reactant is completely consumed, having more moles than required by the stoichiometry.

What is a **molecular formula**?

A molecular formula shows the actual number of each type of atom in a molecule, which can be a multiple of the empirical formula.

Explain the difference between **ionic** and **covalent** compounds in the context of stoichiometry.

Ionic compounds consist of ions and are often involved in reactions with dissociation, whereas covalent compounds involve sharing of electrons and do not dissociate into ions in the same way.

What is **Avogadro's number** and its significance?

Avogadro's number is $6.022 \times 10^{23}$, representing the number of atoms, molecules, or particles in one mole of a substance.

Define a **combustion reaction**.

A combustion reaction is a chemical reaction where a substance reacts rapidly with oxygen, releasing heat and light, typically producing CO$_2$ and H$_2$O.

What is the role of a **catalyst** in a chemical reaction?

A catalyst speeds up a chemical reaction without being consumed by lowering the activation energy required for the reaction to proceed.

What is a **single displacement reaction**?

A single displacement reaction is a type of chemical reaction where one element replaces another element in a compound, typically represented as $A + BC \rightarrow AC + B$.

Describe a **double displacement reaction**.

A double displacement reaction involves the exchange of ions between two compounds to form two new compounds, typically represented as $AB + CD \rightarrow AD + CB$.

What does the **law of conservation of mass** state?

The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction; the mass of the reactants equals the mass of the products.

What is the **enthalpy change ($\Delta H$)** in a chemical reaction?

Enthalpy change ($\Delta H$) is the heat change at constant pressure, representing the difference in enthalpy between products and reactants.

What is a **neutralization reaction**?

A neutralization reaction is a chemical reaction between an acid and a base that produces water and a salt.

What is a **precipitation reaction**?

A precipitation reaction occurs when two aqueous solutions react to form an insoluble solid, known as a precipitate.

Define the term **standard solution**.

A standard solution is a solution of known concentration, used for titrations or other analytical methods.

What is the purpose of a **titration**?

Titration is a laboratory technique used to determine the concentration of a solute in a solution by reacting it with a standard solution.

Explain the concept of **equivalence point** in titration.

The equivalence point in a titration is the point at which the amount of titrant added exactly neutralizes or reacts completely with the analyte.

What is the **end point** in a titration?

The end point in a titration is the point at which a visible indicator signals that the equivalence point has been reached.

Describe **Le Chatelier's Principle**.

Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by changing conditions, the system adjusts to counteract the change and restore equilibrium.

What is an **oxidation-reduction (redox) reaction**?

A redox reaction involves the transfer of electrons between two species, where oxidation is the loss of electrons and reduction is the gain of electrons.

How is **oxidation state** determined?

Oxidation state is determined by assigning electrons based on a set of rules, such as elements in their standard state having an oxidation state of zero, and hydrogen typically being +1.

What is the **activity series** of metals?

The activity series is a list of metals ranked by their reactivity, with the most reactive metals at the top, used to predict the products of displacement reactions.

How is the **Gibbs free energy ($\Delta G$)** change related to reaction spontaneity?

Gibbs free energy change ($\Delta G$) indicates the spontaneity of a reaction: $\Delta G < 0$ implies a spontaneous reaction, while $\Delta G > 0$ implies a non-spontaneous reaction.

What is the **Hess's Law**?

Hess's Law states that the total enthalpy change in a reaction is the same regardless of whether the reaction occurs in one step or multiple steps.

Define **reaction rate**.

Reaction rate is the speed at which reactants are converted into products, usually expressed as the change in concentration of a reactant or product per unit time.

What is a **reaction mechanism**?

A reaction mechanism is a step-by-step sequence of elementary reactions by which overall chemical change occurs.

Explain the **collision theory** of reaction rates.

Collision theory states that chemical reactions occur when reactant molecules collide with sufficient energy and proper orientation to overcome activation energy and form products.

What is **activation energy** ($E_a$)?

Activation energy ($E_a$) is the minimum energy required for a chemical reaction to occur, allowing reactants to transform into products.

Describe the significance of a **rate law**.

A rate law expresses the relationship between the rate of a chemical reaction and the concentration of its reactants, usually in the form $\text{Rate} = k[A]^m[B]^n$.