Chemical reactions Revision Notes

Understanding Chemical Symbols and Formulae

Every element has a unique chemical symbol, typically one or two letters, with the first letter always capitalised (e.g., O for Oxygen, Na for Sodium, Cl for Chlorine). Compounds are formed when two or more elements chemically combine. Their chemical formula represents the type and number of atoms in one molecule or formula unit. For instance, H₂O means two hydrogen atoms and one oxygen atom make up a water molecule. CO₂ means one carbon atom and two oxygen atoms make up a carbon dioxide molecule. Correctly writing these symbols and formulae is the first step in constructing chemical equations. An incorrect formula will lead to an incorrect equation, regardless of balancing.

The Law of Conservation of Mass

The Law of Conservation of Mass is a fundamental principle in chemistry. It states that mass is neither created nor destroyed in any chemical reaction. This means the total mass of the reactants before a reaction must exactly equal the total mass of the products after the reaction. The number of atoms of each distinct element must also be identical on both sides of a chemical equation. If you start with 10 grams of reactants, you must end with 10 grams of products. This law forms the basis for balancing chemical equations, as it requires that all atoms present in the reactants are accounted for in the products.

Writing and Balancing Chemical Equations

A chemical equation uses chemical symbols and formulae to represent a chemical reaction. Reactants are written on the left side of an arrow (→), and products are written on the right. For example, A + B → C + D. To balance an equation, you place whole numbers called coefficients in front of the chemical formulae. These coefficients indicate the number of molecules or formula units involved. For example, in 2H₂ + O₂ → 2H₂O, the '2' in front of H₂ means two molecules of hydrogen, and the '2' in front of H₂O means two molecules of water. Never change the subscripts within a chemical formula (e.g., changing H₂O to H₄O), as this changes the substance itself. Always balance by adjusting coefficients until the number of atoms of each element is the same on both sides of the reaction arrow.

What is Electrolysis?

Electrolysis is a process that uses electrical energy to cause non-spontaneous chemical reactions, breaking down compounds. It involves an electrolyte (molten or dissolved ionic compound) and two electrodes connected to a power supply. Cations (positive ions) move to the cathode (negative electrode) and gain electrons (reduction). Anions (negative ions) move to the anode (positive electrode) and lose electrons (oxidation). The products formed depend on the ions present and the type of electrodes used.

Inert Electrodes

Inert electrodes do not react chemically during electrolysis; they only act as a surface for electron transfer. Examples include graphite (carbon) and platinum. When inert electrodes are used, the products at the anode and cathode are determined exclusively by the electrolyte ions and, if present, the solvent (like water). For instance, in the electrolysis of molten salts with inert electrodes, the metal is produced at the cathode and the non-metal at the anode.

Non-inert (Active) Electrodes

Non-inert, or active, electrodes participate in the chemical reactions. They can be oxidised or reduced themselves. A common example is using copper electrodes in copper(II) sulfate electrolysis. At the copper anode, the anode itself oxidises, forming copper(II) ions, instead of anions in the solution reacting. At the cathode, copper(II) ions from the solution deposit onto the electrode. This is used for electrorefining and electroplating processes.

Key Differences and Applications

The choice between inert and non-inert electrodes is crucial as it dictates the products and purpose of electrolysis. Inert electrodes are typically used for extracting elements from molten compounds or purifying water. Non-inert electrodes are essential for processes like electroplating (coating one metal with another) or electrorefining (purifying a metal, such as copper, where the anode itself dissolves to supply metal ions).

Definition of Activation Energy

Activation energy is the minimum amount of energy that colliding reactant particles must possess for a chemical reaction to take place. It acts as an energy barrier which must be overcome to break existing bonds and form new ones. Without reaching this energy threshold, collisions between molecules will not result in a reaction.

Role in Chemical Reactions

Every chemical reaction requires an input of energy to initiate the process. This energy is used to destabilise existing chemical bonds in reactant molecules, making them available to rearrange and form new bonds in product molecules. If particles collide with energy below the activation energy, they rebound without converting into products.

Factors Affecting Activation Energy

The activation energy is a characteristic property of a specific chemical reaction and does not change with external factors like temperature or concentration. However, a catalyst can lower the activation energy by providing an alternative reaction pathway. This allows a reaction to proceed more quickly at a given temperature, as more particles will have sufficient energy to react.

Understanding Chemical Symbols

Every element on the Periodic Table has a unique chemical symbol. These symbols are universally recognised and are used to represent elements in chemical formulae. They consist of one or two letters; the first letter is always uppercase, and the second (if present) is lowercase. For example, hydrogen is H, helium is He, carbon is C, and chlorine is Cl. Memorising common element symbols is fundamental to writing chemical formulae correctly. These symbols are the building blocks for all chemical representations.

Formulae for Elements

The formula for an element depends on how its atoms exist in their natural state. Many metallic elements, such as iron (Fe), copper (Cu), and sodium (Na), are simply represented by their symbol. This is because they form giant metallic structures, and the symbol indicates a single atom. However, several non-metal elements exist as diatomic molecules, meaning they are composed of two atoms bonded together. Examples include hydrogen (H₂), oxygen (O₂), nitrogen (N₂), fluorine (F₂), chlorine (Cl₂), bromine (Br₂), and iodine (I₂). It is important to know which elements form diatomic molecules.

Formulae for Simple Covalent Compounds

Simple covalent compounds are formed when non-metal atoms share electrons to form discrete molecules. The chemical formula for a simple covalent compound indicates the exact number of each type of atom present in one molecule. For instance, water has the formula H₂O, meaning one molecule contains two hydrogen atoms and one oxygen atom. Carbon dioxide is CO₂, indicating one carbon atom and two oxygen atoms. Methane is CH₄, meaning one carbon atom and four hydrogen atoms. Subscripts are used to denote the number of atoms; if no subscript is present, it means there is one atom of that element.

Formulae for Ionic Compounds

Ionic compounds are typically formed between a metal and a non-metal, involving the transfer of electrons to form ions. The chemical formula for an ionic compound represents the simplest whole-number ratio of positive and negative ions that results in an electrically neutral compound. For example, sodium chloride has the formula NaCl because one sodium ion (Na⁺) combines with one chloride ion (Cl⁻). Magnesium chloride is MgCl₂ because one magnesium ion (Mg²⁺) combines with two chloride ions (Cl⁻) to balance the charges. The valency (combining power) of each ion is crucial in determining these ratios. You need to be able to use valencies to deduce the correct formula.