Chemistry Unit 3 Flashcards

Chemical Equilibrium, Acids + Bases

Two requirements for an effective collision (Collision Theory)

Particles must collide with the correct orientation and with energy equal to or greater than the activation energy ($E_a$).

Activation energy ($E_a$)

The minimum kinetic energy required by colliding reactant particles for a reaction to occur.

How does a catalyst increase reaction rate

Provides an alternative reaction pathway with a lower activation energy ($E_a$) without being permanently consumed.

Effect of increasing temperature on reaction rate

Increases particle kinetic energy, leading to more frequent collisions and a significantly greater proportion of collisions with energy $\ge E_a$.

Effect of increasing concentration or surface area on reaction rate

Increases the frequency of collisions between reactant particles.

Relative energy of products vs reactants in an exothermic reaction

Products have lower chemical potential energy than reactants because enthalpy is released to the surroundings.

Relative energy of products vs reactants in an endothermic reaction

Products have higher chemical potential energy than reactants because enthalpy is absorbed from the surroundings.

Open system vs. closed system

An open system exchanges both matter and energy with surroundings, whereas a closed system exchanges energy only.

Dynamic equilibrium

A state in a closed system where the forward and reverse reaction rates are equal and concentrations of reactants and products remain constant.

Equilibrium expression for $aA + bB \rightleftharpoons cC + dD$

$$K = \frac{[C]^c[D]^d}{[A]^a[B]^b}$$

Interpretation of an equilibrium constant $K \gg 1$

Equilibrium position strongly favours the formation of products.

Interpretation of an equilibrium constant $K \ll 1$

Equilibrium position strongly favours the reactants.

Only condition that changes the numerical value of $K$

Temperature.

Le Châtelier's principle

When a system at equilibrium is subjected to an external change, the system shifts its equilibrium position to oppose or minimise that change.

Response of an equilibrium mixture to an increase in reactant concentration

Shifts to the right (forward direction) to consume the added reactant.

Response of a gaseous equilibrium system to an increase in pressure (decrease in volume)

Shifts toward the side with fewer stoichiometric moles of gas to reduce pressure.

Effect of increasing temperature on an exothermic equilibrium system

Shifts in the endothermic direction (to the left) to absorb the added heat energy.

Effect of increasing temperature on an endothermic equilibrium system

Shifts in the endothermic direction (to the right) to consume the added heat energy.

Effect of a catalyst on equilibrium position

No effect on position; it increases the rates of both forward and reverse reactions equally, reaching equilibrium faster.

Effect of changing the mass of a pure solid or liquid on equilibrium position

No effect, because the concentrations of pure solids and pure liquids remain constant and are omitted from $K$.

Arrhenius definition of acids and bases

Acids produce hydrogen ions ($H^+$) in aqueous solution; bases produce hydroxide ions ($OH^-$) in aqueous solution.

Brønsted–Lowry definition of acids and bases

An acid is a proton ($H^+$) donor; a base is a proton ($H^+$) acceptor.

Conjugate acid-base pair

Two chemical species that differ from each other by the presence or absence of a single proton ($H^+$).

Conjugate pairs in $NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-$

$NH_3$ (base) and $NH_4^+$ (conjugate acid); $H_2O$ (acid) and $OH^-$ (conjugate base).

Amphoteric substance

A chemical species that can act as either an acid or a base (e.g., $H_2O$, $Al_2O_3$, $ZnO$).

Difference between a strong acid and a weak acid

A strong acid completely ionises in aqueous solution, whereas a weak acid only partially ionises.

Acid strength vs. acid concentration

Strength refers to the degree of ionisation in water, whereas concentration refers to the amount of solute dissolved per unit volume.

Three common strong acids

Hydrochloric acid ($HCl$), sulfuric acid ($H_2SO_4$), and nitric acid ($HNO_3$).

Two common weak acids

Ethanoic acid ($CH_3COOH$) and phosphoric acid ($H_3PO_4$).

Three common strong bases

Sodium hydroxide ($NaOH$), potassium hydroxide ($KOH$), and calcium hydroxide ($Ca(OH)_2$).

Two common weak bases

Ammonia ($NH_3$) and sodium carbonate ($Na_2CO_3$).

Acid dissociation constant expression ($K_a$) for a weak acid $HA$

$$K_a = \frac{[H^+][A^-]}{[HA]}$$

Relationship between $K_a$ value and acid strength

A larger $K_a$ value indicates greater extent of ionisation and therefore a stronger acid.