1. Stoichiometric relationships

1. Stoichiometric relationships

1.1 Introduction to the particulate nature of matter and chemical change

Nature of science:

  • Making quantitative measurements with replicates to ensure reliability - definite and multiple proportions.

Understandings:

  • Atoms of different elements combine in fixed ratios to form compounds, which have different properties from their component elements.

  • Mixtures contain more than one element and/or compound that are not chemically bonded together and so retain their individual properties.

  • Mixtures are either homogeneous or heterogeneous.

Applications and skills:

  • Deduction of chemical equations when reactants and products are specified.

  • Application of the state symbols (s), (l), (g) and (aq) in equations.

  • Explanation of observable changes in physical properties and temperature during changes of state.

1.2 The mole concept

Nature of science:

  • Concepts - the concept of the mole developed from the related concept of “equivalent mass” in the early 19th century.

Understandings:

  • The mole is a fixed number of particles and refers to the amount, n, of substance.

  • Masses of atoms are compared on a scale relative to ¹²C and are expressed as relative atomic mass (A r) and relative formula/molecular mass (M r).

  • Molar mass (M) has the units g mol⁻¹.

  • The empirical formula and molecular formula of a compound give the simplest ratio and the actual number of atoms present in a molecule respectively.

Applications and skills:

  • Calculation of the molar masses of atoms, ions, molecules and formula units.

  • Solution of problems involving the relationships between the number of particles, the amount of substance in moles and the mass in grams.

  • Interconversion of the percentage composition by mass and the empirical formula.

  • Determination of the molecular formula of a compound from its empirical formula and molar mass.

  • Obtaining and using experimental data for deriving empirical formulas from reactions involving mass changes.

1.3 Reacting masses and volumes

Nature of science:

  • Making careful observations and obtaining evidence for scientific theories - Avogadro's initial hypothesis.

Understandings:

  • Reactants can be either limiting or excess.

  • The experimental yield can be different from the theoretical yield.

  • Avogadro’s law enables the mole ratio of reacting gases to be determined from volumes of the gases.

  • The molar volume of an ideal gas is a constant at specified temperature and pressure.

  • The molar concentration of a solution is determined by the amount of solute and the volume of solution.

  • A standard solution is one of known concentration.

Applications and skills:

  • Solution of problems relating to reacting quantities, limiting and excess reactants, theoretical, experimental and percentage yields.

  • Calculation of reacting volumes of gases using Avogadro’s law.

  • Solution of problems and analysis of graphs involving the relationship between temperature, pressure and volume for a fixed mass of an ideal gas.

  • Solution of problems relating to the ideal gas equation.

  • Explanation of the deviation of real gases from ideal behaviour at low temperature and high pressure.

  • Obtaining and using experimental values to calculate the molar mass of a gas from the ideal gas equation.

  • Solution of problems involving molar concentration, amount of solute and volume of solution.

  • Use of the experimental method of titration to calculate the concentration of a solution by reference to a standard solution.

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