物质的状态

Where gas pressure comes from

Gas molecules move fast in all directions. They keep hitting — colliding 碰撞 with — the walls of their container. Each hit gives the wall a tiny push. The pressure 压强 of the gas is the overall result of these many collisions on the walls.

Ideal gases

An ideal gas 理想气体 is a simple model. We assume two things:

• the particles themselves take up zero volume.
• there are no intermolecular forces 分子间作用力 of attraction between the particles.

A real gas 实际气体 follows this model closely at low pressure and high temperature. It behaves least like an ideal gas at high pressure and low temperature, when the particles are squeezed close together and the forces between them start to matter.

The ideal gas equation

The ideal gas equation 理想气体方程 links pressure, volume, amount and temperature:

where $p$ is the pressure in Pa, $V$ is the volume in $\text{m}^3$, $n$ is the amount in moles, $T$ is the temperature in kelvin 开尔文 (K), and $R$ is the gas constant 气体常量 ($8.31\ \text{J K}^{-1}\,\text{mol}^{-1}$).

Always change the units first: °C to K (add 273), and $\text{cm}^3$ or $\text{dm}^3$ to $\text{m}^3$.

Worked example. Find the volume of $0.50\ \text{mol}$ of an ideal gas at $27\ ^{\circ}\text{C}$ and $100\ \text{kPa}$. ($R = 8.31\ \text{J K}^{-1}\,\text{mol}^{-1}$.)

Convert first: $T = 300\ \text{K}$, $p = 1.00 \times 10^{5}\ \text{Pa}$. Then

You can also use the equation to find a molar mass 摩尔质量. Since $n = m/M$:

This lets you work out $M_r$ from the mass (or the density) of a gas.

Worked example. A flask holds $0.96\ \text{g}$ of a gas in $600\ \text{cm}^3$ at $100\ \text{kPa}$ and $27\ ^{\circ}\text{C}$. Find the molar mass of the gas. ($R = 8.31\ \text{J K}^{-1}\,\text{mol}^{-1}$.)

Convert: $V = 6.00 \times 10^{-4}\ \text{m}^3$, $T = 300\ \text{K}$. Then

How a substance behaves depends on how its particles are joined. There are four main structures of a crystalline solid 晶体.

Giant ionic

A giant ionic 离子晶体 structure is a huge regular lattice 晶格 of positive and negative ions 离子, held together by strong attraction in every direction. Examples are sodium chloride and magnesium oxide.

Simple molecular

A simple molecular 分子晶体 structure is made of small molecules 分子. The bonds inside each molecule are strong, but the intermolecular forces between the molecules are weak. Examples are iodine ($\text{I}_2$), fullerene 富勒烯 ($\text{C}_{60}$) and ice.

Giant molecular

A giant molecular 原子晶体 structure (also called giant covalent) is a huge network of atoms joined by strong covalent bonds 共价键. Examples are silicon(IV) oxide 二氧化硅, graphite 石墨 and diamond 金刚石.

Giant metallic

A giant metallic 金属晶体 structure is a lattice of positive metal ions in a "sea" of delocalised electrons 离域电子. An example is copper.

Physical properties

The structure decides the physical properties:

• melting point 熔点 and boiling point 沸点 are high when strong forces (ionic, covalent or metallic) must be broken, and low when only weak intermolecular forces break.
• electrical conductivity 导电性 needs charged particles that can move — ions that are free (when molten or dissolved) or delocalised electrons. Graphite conducts because some of its electrons are delocalised.
• solubility 溶解度 in water is usually high for ionic solids and low for molecular and giant covalent solids.

You can work backwards too: from the melting point, conductivity and solubility of an unknown substance, deduce the type of structure and bonding it has.

Common mistakes.

• Using kPa and $\text{dm}^3$ with $R = 8.31$. With this value of $R$, pressure must be in Pa and volume in $\text{m}^3$: $600\ \text{cm}^3 = 6.00 \times 10^{-4}\ \text{m}^3$.
• Using °C. Temperature must be in kelvin; at $27\ °\text{C}$, $T = 300\ \text{K}$.
• Saying real gases deviate at high temperature. Deviation is greatest at high pressure and low temperature, when the particles are close together and move slowly, so their volume and attractions matter.
• Saying graphite does not conduct because it is covalent. Each carbon uses three electrons in bonds, and the fourth is delocalised between the layers, so graphite conducts along the layers.
• Calling ice or iodine a giant structure because they are solids. They are simple molecular: small molecules held together by weak intermolecular forces.
• Saying ionic solids conduct when solid. The ions are fixed in the lattice; they conduct only when molten or dissolved, when the ions can move.

• Use SI units in $pV = nRT$: pressure in Pa, volume in $\text{m}^3$ ($\text{cm}^3 \times 10^{-6}$), temperature in K ($^\circ\text{C} + 273$).
• State the ideal-gas assumptions (negligible molecular volume, no intermolecular forces) and when real gases deviate (high pressure, low temperature).
• Link each property to structure: giant ionic (high m.p., conducts molten), giant covalent (very high m.p.), simple molecular (low m.p.), giant metallic (conducts, malleable).
• Graphite conducts because each carbon has a delocalised electron; diamond does not — a favourite comparison.