Physical and Thermal Properties of Water, Acids, and Bases

Flashcards covering the physical and thermal properties of water (Module 9A) and the properties, reactions, and pH calculations for acids and bases (Modules 10A, 10B, and 10C).

What is the molecular structure and shape of a water ($H_2O$) molecule?

Water is a V-shaped (bent) molecule containing two polar covalent $O-H$ bonds.

Describe the polarity within a water molecule.

Oxygen is significantly more electronegative than hydrogen, resulting in a $\delta-$ charge on Oxygen and a $\delta+$ charge on Hydrogen.

What intermolecular forces are present between water molecules?

Water molecules form strong hydrogen bonds and weaker dispersion forces.

How do the boiling and melting points change down Group 16 hydrides ($H_2S \rightarrow H_2Se \rightarrow H_2Te$)?

Boiling and melting points increase due to increasing molar mass and stronger dispersion forces.

Why does $H_2O$ have an abnormally high melting and boiling point compared to other Group 16 hydrides?

Because it forms strong hydrogen bonds, whereas the other hydrides primarily experience weaker dipole-dipole and dispersion forces.

What is the definition of Specific Heat Capacity ($c$)?

The amount of heat energy required to raise the temperature of $1\,g$ of a substance by $1\,^\circ\text{C}$.

What is the specific heat capacity value of water, $c(H_2O)$?

$4.18\,J\,g^{-1}\,^\circ\text{C}^{-1}$.

Why is ice less dense than liquid water?

In solid ice, water molecules bond into an open hexagonal crystal lattice held rigid by hydrogen bonds, creating empty space.

At what temperature does water reach its maximum density?

$4\,^\circ\text{C}$.

Define Latent Heat ($L$).

The heat energy required to change the physical state of a substance at its constant melting or boiling temperature without changing its temperature.

What is the Latent Heat of Fusion ($L_f$) for water?

$6.02\,kJ\,mol^{-1}$.

What is the Latent Heat of Vaporisation ($L_v$) for water?

$44.0\,kJ\,mol^{-1}$.

List the chemical formulas for Hydrochloric acid, Nitric acid, and Sulfuric acid.

$HCl$, $HNO_3$, and $H_2SO_4$.

List the chemical formulas for Ethanoic acid and Phosphoric acid.

$CH_3COOH$ and $H_3PO_4$.

List the chemical formulas for Sodium hydroxide, Ammonia, and Sodium carbonate.

$NaOH$, $NH_3$, and $Na_2CO_3$.

How are acids and bases defined under the Brønsted-Lowry theory?

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

What is the chemical equation for the formation of a hydronium ion when a general acid ($HA$) reacts with water?

$$HA(aq) + H_2O(l) \rightarrow A^{-}(aq) + H_3O^{+}(aq)$$

What is a conjugate acid-base pair?

Two species that differ by a single proton ($H^+$).

In the reaction $HNO_3 + H_2O \rightleftharpoons NO_3^{-} + H_3O^{+}$, identify the conjugate pairs.

$HNO_3/NO_3^{-}$ (Acid / Conjugate Base) and $H_2O/H_3O^{+}$ (Base / Conjugate Acid).

What are amphiprotic substances?

Substances that can act as either an acid (donate $H^+$) or a base (accept $H^+$), depending on the reaction conditions.

What are polyprotic acids?

Acids capable of donating more than one proton per molecule in successive ionisation steps.

Provide an example of a triprotic acid and its three ionisation steps.

Phosphoric acid ($H_3PO_4$); steps: (1) $H_3PO_4(aq) + H_2O(l) \rightleftharpoons H_2PO_4^{-}(aq) + H_3O^{+}(aq)$, (2) $H_2PO_4^{-}(aq) + H_2O(l) \rightleftharpoons HPO_4^{2-}(aq) + H_3O^{+}(aq)$, (3) $HPO_4^{2-}(aq) + H_2O(l) \rightleftharpoons PO_4^{3-}(aq) + H_3O^{+}(aq)$.

State the equation for the self-ionisation of water.

$$2H_2O(l) \rightleftharpoons H_3O^{+}(aq) + OH^{-}(aq)$$

What are the products of a reaction between an Acid and a Metal Carbonate?

$$Salt + Water + Carbon Dioxide$$

What are the products of a reaction between an Acid and a Reactive Metal?

$$Salt + Hydrogen Gas$$

What are antacids?

Weak bases such as $Mg(OH)_2$ or $NaHCO_3$ used to neutralise excess stomach acid ($HCl$) and relieve heartburn.

What is the ionic product constant of water ($K_w$) at $25\,^\circ\text{C}$?

$$K_w = [H_3O^{+}][OH^{-}] = 1.0 \times 10^{-14}\,M^2$$

What is the concentration of $H_3O^{+}$ and $OH^{-}$ in a neutral solution at $25\,^\circ\text{C}$?

$$[H_3O^{+}] = [OH^{-}] = 1.0 \times 10^{-7}\,M$$

What is the fundamental equation for calculating pH?

$pH = -\log_{10}[H_3O^{+}]$ or $pH = -\log_{10}[H^{+}]$

How do you calculate the hydronium ion concentration for a diprotic acid like $H_2SO_4$?

$$[H_3O^{+}] = 2 \times \text{c(Acid)}$$

Distinguish between Strength and Concentration in acids and bases.

Strength refers to how readily an acid/base ionises in water (completely vs partially), whereas Concentration refers to the amount of solute present per unit volume ($c = n/V$).