3. Stoichiometry (Part 1) (1/6) (Cambridge IGCSE Chemistry 0620 for 2023, 2024 & 2025)
Introduction to Stoichiometry
Overview of Chemistry Topics
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Understanding Chemical Formulas
Elements and Their Symbols
- A chemical formula represents elements and their proportions in a compound, e.g., NaCl for table salt (one sodium atom, one chlorine atom).
- Each element has a unique symbol from the periodic table; for example, H for hydrogen, O for oxygen, and Na for sodium.
Molecular vs Empirical Formulas
- The molecular formula indicates the actual number of atoms in a molecule (e.g., CH4 for methane), while empirical formulas show the simplest whole number ratio of atoms (e.g., C2H5O is ethanol's molecular formula).
- Examples illustrate that some compounds have identical molecular and empirical formulas (e.g., CO2 and H2O).
Valency: The Key to Bonding
Definition of Valency
- Valency refers to an atom's ability to bond with other atoms by gaining, losing, or sharing electrons to achieve stability. For instance, an atom with valency one can lose or share one electron.
Group Trends in Valency
- Elements' groups on the periodic table indicate their outer shell electrons:
- Groups 1–3 typically lose electrons.
- Groups 5–7 usually gain electrons.
- Group 4 can either gain or lose electrons.
Formulating Ionic Compounds
Determining Compound Formulas
- To find ionic compound formulas, crisscross each atom's valency; e.g., Al has a valency of three while O has two leading to Al2O3 as aluminum oxide.
Charge Balance in Ionic Compounds
- In ionic compounds, positive and negative charges must balance out; this results in a neutral compound overall. Examples include potassium chloride (KCl) where K+ balances Cl-.
Examples of Ionic Compounds
Potassium Chloride Example
- Potassium loses one electron forming K+, while chlorine gains one electron forming Cl-. The resulting ratio is 1:1 leading to KCl as the formula.
Boron Oxide Example
Understanding Ionic Compounds and Their Formulas
Formation of Neutral Ionic Compounds
- The process of creating a neutral ionic compound involves balancing positive and negative charges. In this case, Boron contributes electrons to Oxygen atoms to achieve stable electron configurations.
- To ensure neutrality, the total positive charge from Boron (+6) must equal the total negative charge from Oxide (-6), leading to a simple ratio of 2:3 for the ions involved.
Crisscross Method for Determining Formulas
- A practical method for determining the formula of an ionic compound is the crisscross method, where ion charges are crossed over to find the simplest ratio needed for neutrality. If both ions have a charge of one, they combine in a 1:1 ratio without writing '1' in the formula.
- For example, when calculating calcium nitrate's formula, Calcium (+2) and Nitrate (-1) require brackets around Nitrate since more than one is needed in the final compound representation (Ca(NO₃)₂).
Examples and Applications
- When using ammonium carbonate as another example, it’s crucial to enclose any compound ion that appears more than once in brackets; however, if only one is required (like Carbonate), no brackets are necessary. This highlights how different compounds can vary in their structural requirements based on their ionic composition.
Conclusion of Part One