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Flashcards · 8462 · Topic 11
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Topic 11: Key ideas
- Element
- A substance made of one type of atom; all its atoms have the same number of protons.
- Relative atomic mass
- An average value that takes account of the abundance of the isotopes of the element.
- Activation energy
- The minimum amount of energy that particles must have to react.
- Catalyst
- Changes the rate of reaction but is not used up; it gives a pathway with a lower activation energy.
- The three ways in which chemical reactions take place, and which one neutralisation is
- Proton transfer, electron transfer and electron sharing. Neutralisation is proton transfer: H⁺ ions react with OH⁻ ions to form water.
- Why do ionic compounds conduct electricity only when molten or dissolved?
- The ions must be free to move to carry charge; in the solid they are held in fixed positions in the lattice.
- Why does graphite conduct electricity but diamond does not?
- In graphite each carbon forms three bonds, leaving one delocalised electron per atom that can move. In diamond all four outer electrons are used in covalent bonds.
- Why does a small molecular substance have a low melting point?
- Only the weak intermolecular forces between molecules are overcome, which needs little energy. The strong covalent bonds inside the molecules do not break.
- Number of protons, neutrons and electrons in ³⁷Cl⁻ (atomic number 17)
- 17 protons, 37 − 17 = 20 neutrons, 17 + 1 = 18 electrons.
- How many times greater is [H⁺] at pH 2 than at pH 6?Higher
- 10⁽⁶⁻²⁾ = 10⁴ = 10 000 times.