Chemical reactions Revision Notes

The Core of Neutralisation

Neutralisation reactions always involve an acid reacting with a base. When these reactions occur in aqueous solutions, the chemical essence boils down to a simpler interaction: hydrogen ions (H⁺) from the acid combine with hydroxide ions (OH⁻) from the base. This combination forms water (H₂O). The other ions present, known as spectator ions, do not participate in the formation of water but combine to form a salt. This is the fundamental principle behind all aqueous neutralisation reactions, providing a unified way to understand what happens chemically at the ionic level.

Ionic Equation for Neutralisation

The universal ionic equation for the neutralisation of an aqueous acid by an aqueous base is: H⁺(aq) + OH⁻(aq) → H₂O(l). Here, '(aq)' denotes that the ions are dissolved in water (aqueous solution), and '(l)' denotes that water is in the liquid state. This equation shows that the hydrogen ions, which cause acidity, and the hydroxide ions, which cause alkalinity, are consumed to produce neutral water. This simplified equation strips away the specific identities of the acid and base, highlighting only the species directly involved in the neutralisation process.

Examples of Neutralisation Reactions

Consider the reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH). The full equation is HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l). Breaking this down into ions, we have H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l). Removing the spectator ions (Na⁺ and Cl⁻, which appear on both sides of the equation unchanged), leaves us with the net ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l). This demonstrates how the general ionic equation applies to specific examples. Another example is nitric acid (HNO₃) reacting with potassium hydroxide (KOH) to form potassium nitrate (KNO₃) and water, with the same net ionic equation.

Understanding Element Names and Symbols

Every element has a unique name and a distinct one or two-letter symbol. You will always be supplied with a Periodic Table in your exams, which lists these names and symbols. It is important to be able to locate elements on the Periodic Table and identify their corresponding symbols. For example, 'Na' is the symbol for sodium, 'Cl' is for chlorine, and 'Ca' is for calcium. The first letter of a symbol is always capitalised, and the second letter (if present) is always lowercase. This consistency helps to avoid confusion between elements or with multiple atoms of the same element.

Writing Chemical Formulae

A chemical formula represents a compound, showing the types of elements present and the ratio of atoms of each element. The symbols of the elements are used, and small numbers called subscripts are written after the element symbol to indicate the number of atoms. For example, methane is CH₄, meaning one carbon atom and four hydrogen atoms. Sodium chloride is NaCl, meaning one sodium atom and one chlorine atom. If there is no subscript next to a symbol, it implies there is only one atom of that element in the compound. For simple covalent compounds, you need to remember the common combining power (valency) to deduce the formula, for example, water (H₂O) and carbon dioxide (CO₂).

Introduction to Word and Symbol Equations

Chemical reactions can be represented in two main ways: word equations and symbol equations. A word equation uses the full names of the reactants and products, like 'Hydrogen + Oxygen → Water'. A symbol equation uses the chemical symbols and formulae, like 'H₂ + O₂ → H₂O'. Symbol equations are much more precise and informative. The arrow in an equation means 'reacts to form' or 'produces'. Reactants are always on the left of the arrow, and products are always on the right. Understanding how to translate between word equations and symbol equations is a fundamental skill.

Balancing Chemical Equations

The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction. This means the number of atoms of each element must be the same on both sides of a chemical equation. To balance an equation, you adjust coefficients – the large numbers placed in front of the chemical formulae. For example, H₂ + O₂ → H₂O is unbalanced because there are two oxygen atoms on the left and only one on the right. By adding coefficients, we get 2H₂ + O₂ → 2H₂O. Now there are four hydrogen atoms and two oxygen atoms on both sides, making it balanced.

What is Electrolysis?

Electrolysis is a decomposition reaction that uses electrical energy to break down an ionic compound. This process occurs in an electrolyte, which is a molten ionic compound or an ionic compound dissolved in a solvent, allowing ions to move freely. The electrical current is passed through two electrodes: the cathode (negative electrode) and the anode (positive electrode). Ions carry the charge through the electrolyte.

Cathode: Negative Electrode Products

At the cathode (negative electrode), positively charged ions, known as cations, are attracted. These can be metal ions or hydrogen ions (H+). If the metal is more reactive than hydrogen, hydrogen gas (H₂) will be produced. If the metal is less reactive than hydrogen, the metal itself will be deposited. Therefore, metals or hydrogen gas are always produced at the cathode during electrolysis.

Anode: Positive Electrode Products

At the anode (positive electrode), negatively charged ions, known as anions, are attracted. These are typically non-metal ions, such as halide ions (e.g., Cl⁻, Br⁻, I⁻) or hydroxide ions (OH⁻). Non-metals are produced at the anode. For example, if chloride ions are present, chlorine gas (Cl₂) will be produced. If hydroxide ions are discharged, oxygen gas (O₂) and water will be produced. Water is in the product because hydroxide ions are discharged from it.

Inert Electrodes

Inert electrodes are crucial for ensuring that the products of electrolysis come solely from the electrolyte and not from the electrodes themselves. These electrodes do not chemically react with the electrolyte or the products formed. Common examples of inert electrodes include graphite (a form of carbon) and platinum. Using reactive electrodes would complicate the process by adding unwanted reactions or consuming the electrodes, making it difficult to determine the intended products.

The Law of Conservation of Mass

The Law of Conservation of Mass dictates that during any chemical reaction, the total mass of the reactants is always equal to the total mass of the products. This fundamental principle implies that no atoms are created or destroyed during a chemical reaction; they are simply rearranged to form new chemical substances. This law holds true for all chemical reactions, whether conducted in open or closed systems. The appearance of a mass change in an open system does not contradict this law, but rather highlights that some reactants or products have entered or left the measured system.

Apparent Mass Loss in Open Systems

In a non-enclosed (open) system, an apparent decrease in mass can occur if one or more of the products of the chemical reaction are gases that are allowed to escape into the surroundings. For instance, if calcium carbonate reacts with an acid to produce carbon dioxide gas, and this gas dissipates into the air, the measured mass of the remaining substances in the reaction vessel will be less than the initial mass of the reactants. This is because the mass of the escaped carbon dioxide is no longer included in the measurement, but the atoms that formed it still exist, just outside the system.

Apparent Mass Gain in Open Systems

Conversely, in a non-enclosed system, an apparent increase in mass can be observed if one of the reactants is a gas from the atmosphere surrounding the reaction, and it reacts to form a solid or liquid product that remains within the system. A common example is the rusting of iron. When iron (a solid) reacts with oxygen (a gas) from the air to form iron oxide (rust, a solid), the mass of the solid product will be greater than the initial mass of the iron. This is because the mass of the oxygen particles, which were initially outside the system, has now become part of the measured system's mass.

Using the Particle Model to Explain Mass Changes

The particle model is instrumental in understanding these mass changes. In reactions where mass appears to decrease, particles of gaseous products move away from the reaction vessel and are no longer weighed. The total number and mass of atoms remain constant in the universe, but the atoms specifically involved in the gaseous product have left the measurable system. In reactions where mass appears to increase, particles of gaseous reactants (like oxygen from the air) enter the reaction vessel, combine with other reactants, and their mass becomes part of the heavier product, thus increasing the measured mass of the system. The key is to distinguish between the total mass of atoms in the entire universe (which is conserved) and the measured mass within a specific, often open, reaction system.