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Chemistry 1: free practice, theory and problems
The atom has a nucleus with protons and neutrons, and electrons around it. The number of protons decides which element it is, and the electrons decide its chemistry.
Contents
- Atoms and the periodic table
- Chemical bonds
- Amount of substance and concentration
- Acids, bases and pH
- Redox reactions
1. Atoms and the periodic table
What is it about?
The atom has a nucleus with protons and neutrons, and electrons around it. The number of protons decides which element it is, and the electrons decide its chemistry.
Concepts
- Atomic number = number of protons (= number of electrons in a neutral atom).
- Mass number = protons + neutrons. Isotopes have the same number of protons but different numbers of neutrons.
- Electrons are arranged in shells (2, 8, 8, …). The valence electrons in the outer shell control how the substance reacts.
- The periodic table: groups (columns) have the same number of valence electrons, periods (rows) have the same number of shells.
- Group 1: alkali metals. Group 17: halogens. Group 18: noble gases (full outer shell, not very reactive).
2. Chemical bonds
What is it about?
Atoms bond together to get a more stable electron arrangement. The type of bond determines properties such as melting point and solubility.
Concepts
- Ionic bond: a metal gives electrons to a non-metal. Example: NaCl.
- Covalent bond: non-metals share electron pairs. Example: H₂O and CH₄.
- Electronegativity: how strongly an atom attracts the electrons. A large difference gives a polar bond or an ionic bond.
- Metallic bond: positive metal ions in a "sea" of free electrons. Gives conductivity.
- Between molecules: hydrogen bonds (strongest, for example between water molecules), dipole–dipole forces and van der Waals forces.
3. Amount of substance and concentration
What is it about?
Chemists count particles in moles. 1 mole is particles (Avogadro's number).
Concepts and formulas
- Amount of substance: (mass divided by molar mass, g/mol).
- You find the molar mass by adding atomic masses: H 1.008, C 12.01, N 14.01, O 16.00, Na 22.99, Cl 35.45.
- Concentration: (mol/L).
- Dilution: .
- The reaction equation gives the ratio between the amounts of substance (stoichiometry).
4. Acids, bases and pH
What is it about?
An acid donates H⁺ (protons), a base accepts them. The pH scale shows how acidic a solution is.
Concepts and formulas
- and .
- pH 7 is neutral (at 25 °C), below 7 is acidic, above 7 is basic.
- One pH unit is a tenfold change in the concentration of oxonium ions.
- Strong acids (HCl, HNO₃) are completely protolysed: .
- Neutralisation: acid + base → salt + water.
- Buffer: a mixture of a weak acid and its conjugate base, which resists changes in pH.
5. Redox reactions
What is it about?
In a redox reaction, electrons are transferred. Batteries, corrosion (rust) and combustion are redox reactions.
Concepts
- Oxidation: loss of electrons (the oxidation number increases). Reduction: gain of electrons (the oxidation number decreases).
- Mnemonic: OIL RIG, oxidation is loss, reduction is gain.
- The oxidising agent is itself reduced, the reducing agent is itself oxidised.
- Oxidation numbers: elements in pure form have 0, O usually has −2, H usually has +1, and the sum in a compound is 0.
- The electrochemical series shows which metals most easily give up electrons. A less noble metal can protect a nobler one (sacrificial anode).
Example problems with solutions
Here are some of the problems in chemistry 1. In the app, calculation problems get new numbers every time, so you can practise until it sticks – and take a graded practice exam before the real one.
Atoms and the periodic table: Carbon-14 has atomic number 6. How many neutrons does it have?
Answer: 8
Neutrons = mass number − protons = 14 − 6 = 8.
Chemical bonds: What type of bond is found in table salt (NaCl)?
Answer: Ionic bond
Sodium (a metal) gives an electron to chlorine (a non-metal).
Amount of substance and concentration: What is the molar mass of water (H₂O)?
Answer: 18.02 g/mol
g/mol.
Acids, bases and pH: What is the pH of 0.01 mol/L HCl?
Answer: 2
Strong acid: , so .