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.

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.

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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