Chemical Changes, Bonding, and the Periodic Table: A Comprehensive Study Guide

Reactivity of Metals and the Chemical Properties of Elements

The reactivity of a metal is defined by how chemically reactive it is, which is determined by how readily it forms a positive ion by losing electrons. Metals that react very vigorously when added to water or acid are classified as having high reactivity. Conversely, metals that barely react or do not react at all with these substances are classified as having low reactivity. In the reactivity series, metals are arranged in order of their reactivity: Potassium (highest reactivity), Sodium, Lithium, Calcium, Magnesium, Aluminium, Zinc, Iron, Tin, Lead, Copper, Silver, and Gold (lowest reactivity). Although they are non-metals, carbon and hydrogen are often included in this series to facilitate comparisons used in metal extraction. Specifically, carbon is placed between aluminium and zinc, while hydrogen is placed between lead and copper.

Metal Extraction and Redox Reactions

The method used to extract a metal depends on its position in the reactivity series. Unreactive metals, such as gold, are found as pure elements in the Earth's crust and can be mined directly. However, most metals exist as compounds within rocks known as ores, provided there is enough metal compound to make extraction economically viable. Metals that are less reactive than carbon, such as iron and zinc, can be extracted through a process of reduction with carbon. For example, iron oxide reacts with carbon to produce iron and carbon dioxide: $2Fe_2O_3 + 3C \rightarrow 4Fe + 3CO_2$. Metals more reactive than carbon, like aluminium and magnesium, must be extracted using electrolysis, a process requiring significant electrical energy.

Oxidation and reduction can be defined in terms of oxygen transfer or electron transfer. Oxidation is the gain of oxygen or the loss of electrons (OIL), while reduction is the loss of oxygen or the gain of electrons (RIG). In a reaction where iron oxide is heated with carbon, the iron oxide is reduced because it loses oxygen, and the carbon is oxidised because it gains oxygen. In terms of electrons, oxidation-reduction (redox) reactions involve the absolute transfer of charge. For instance, in the displacement reaction involving copper sulfate and iron, the iron atoms are oxidised as they lose two electrons to form iron ions ($Fe \rightarrow Fe^{2+} + 2e^-$), and the copper ions are reduced as they gain two electrons to form copper atoms ($Cu^{2+} + 2e^- \rightarrow Cu$).

Displacement Reactions and Ionic Equations

A displacement reaction occurs when a more reactive element takes the place of a less reactive element in a compound. For example, when solid iron is added to an aqueous solution of copper sulfate, iron replaces the copper because iron is more reactive. The chemical equation is represented as: $Fe_{(s)} + CuSO_{4(aq)} \rightarrow FeSO_{4(aq)} + Cu_{(s)}$. For Higher Tier (HT) students, these reactions can be expressed as ionic equations that show only the species that change in the reaction. When an ionic compound is dissolved in water, the ions separate. The sulfate ion ($SO_4^{2-}$) is called a spectator ion because it remains unchanged in the solution. Removing the spectator ions results in the simplified ionic equation: $Fe_{(s)} + Cu^{2+}_{(aq)} \rightarrow Fe^{2+}_{(aq)} + Cu_{(s)}$. In this specific reaction, the resulting solution contains iron ions rather than copper ions.

Acids, Alkalis, and the pH Scale

Acids are compounds that release hydrogen ions ($H^+$) when dissolved in water. Common laboratory acids include hydrochloric acid ($HCl$), sulfuric acid ($H_2SO_4$), and nitric acid ($HNO_3$). Alkalis are a subset of bases that are soluble in water and release hydroxide ions ($OH^-$). The pH scale, which ranges from 1 to 14, measures the acidity or alkalinity of an aqueous solution. Solutions with a pH less than 7 are acidic, those with a pH greater than 7 are alkaline, and a solution with a pH of exactly 7 is considered neutral. Indicators such as Universal Indicator can provide an approximate pH value by changing color, whereas electronic pH probes can provide exact numerical pH values.

Chemical Reactions of Acids and Salt Production

When acids react with metals or metal compounds, they form salts. A salt is an ionic compound where the hydrogen from an acid has been replaced by a metal or ammonium ion. The name of the salt depends on the acid used: hydrochloric acid produces chlorides, sulfuric acid produces sulfates, and nitric acid produces nitrates. For example, hydrochloric acid reacting with sodium hydroxide forms sodium chloride and water ($HCl + NaOH \rightarrow NaCl + H_2O$). When acids react with metal carbonates, the products include a salt, water, and carbon dioxide gas ($2HCl + Na_2CO_3 \rightarrow 2NaCl + H_2O + CO_2$). Acids also react with metals to form a salt and hydrogen gas ($Mg + 2HCl \rightarrow MgCl_2 + H_2$).

Neutralisation is the reaction between an acid and a base (or alkali) to form a salt and water. The universal ionic equation for neutralisation between an acid and an alkali is: $H^+_{(aq)} + OH^-_{(aq)} \rightarrow H_2O_{(l)}$. To produce a solid salt from an insoluble base (like a metal oxide), a process called crystallisation is used. This involve choosing the correct acid and base, warming the acid, adding the base until it is in excess, filtering the mixture to remove unreacted base, and then evaporating the water from the resulting solution until crystals begin to form. Finally, a water bath is used to evaporate the remaining water to leave pure salt crystals.

Acid Strength and Concentration

It is important to distinguish between the strength of an acid and its concentration. Strength refers to the degree of ionisation; strong acids like sulfuric and nitric acid are completely ionised in aqueous solutions ($HCl \rightarrow H^+ + Cl^-$). Weak acids, such as ethanoic or citric acid, only partially ionise in water. For a given concentration, the stronger the acid, the lower the pH will be. Concentration, on the other hand, measures the total amount of acid dissolved in a specific volume of water ($dm^3$). A concentrated acid has a large amount of acid in a small volume of water, while a dilute acid has a small amount of acid in a large volume of water. If the concentration of hydrogen ions increases by a factor of 10, the pH decreases by 1 unit.

Atomic Bonding and Structure: Ionic and Covalent

There are three primary types of chemical bonding: ionic, covalent, and metallic. Ionic bonding occurs between metal and non-metal atoms when electrons are transferred. Metal atoms lose electrons to become positive ions, while non-metal atoms gain electrons to become negative ions. These oppositely charged ions are held together by a strong electrostatic force of attraction in a giant ionic lattice. Ionic substances have high melting and boiling points due to the energy required to break these strong bonds. They conduct electricity only when molten or dissolved in water, as the ions are then free to move and carry charge. Covalent bonding occurs when non-metals share pairs of electrons to achieve full outer shells. These can form small molecules (like oxygen or water), large molecules (like polymers), or giant covalent structures.

Carbon Allotropes: Diamond, Graphite, and Fullerenes

Carbon can form various allotropes with distinct properties. Diamond is a giant covalent structure where each carbon atom is bonded to four others, making it extremely hard with a high melting point. Graphite, though also a giant covalent structure, has each carbon atom bonded to only three others, forming hexagonal layers. These layers are held together by weak intermolecular forces, allowing them to slide over each other, making graphite soft and useful as a lubricant. Since each carbon in graphite has one delocalised electron, it can conduct electricity. Graphene is a single layer of graphite, just one atom thick, known for its strength and electrical conductivity.

Fullerenes are molecules of carbon atoms with hollow shapes, such as spheres or tubes. Buckminsterfullerene ($C_{60}$) was the first discovered and consists of 60 carbon atoms in a spherical arrangement. Carbon nanotubes are cylindrical fullerenes with high tensile strength, making them useful in electronics and nanotechnology. These structures can be used as catalysts or for drug delivery in the body.

Metallic Bonding and Nanoparticles

Metals consist of a giant structure of positive ions arranged in a regular pattern, surrounded by a "sea" of delocalised electrons. The strong electrostatic attraction between the positive ions and the negative delocalised electrons holds the structure together. Because the atoms are arranged in layers that can slide over each other, pure metals are often soft and malleable. Alloys are mixtures of metals with other elements; the different-sized atoms of the added element distort the regular layers, making it harder for them to slide, thereby making the alloy stronger than the pure metal.

Nanotechnology involves the study of nanoparticles, which range in size from $1\,nm$ to $100\,nm$ ($1 \times 10^{-9}\,m$ to $1 \times 10^{-7}\,m$). They have a very high surface area to volume ratio compared to bulk materials, which gives them unique properties. Particulate matter is categorized by size: coarse particles ($PM_{10}$) are approximately $10\,\mu m$ or $1 \times 10^{-5}\,m$, and fine particles ($PM_{2.5}$) range from $100\,nm$ to $2500\,nm$. Nanoparticles are used in medicine, electronics, and sunscreens, although their long-term health and environmental impacts are still being researched.

Questions & Discussion

How is the modern Periodic Table ordered? It is ordered by increasing atomic number (the number of protons).

How were the early lists of elements ordered? They were arranged by atomic mass.

Why did Mendeleev swap the order of some elements? He swapped them to ensure that elements with similar chemical properties were grouped together, even if it meant they were not in strict order of atomic mass.

Why did Mendeleev leave gaps in his Periodic Table? He left gaps for elements that he predicted had not yet been discovered.

Why do elements in a group have similar chemical properties? They have the same number of electrons in their outer shell.

Where are metals and non-metals located on the Periodic Table? Metals are located on the left and center, while non-metals are located on the right.

What name is given to the Group 1 elements and why? They are called alkali metals because they react with water to form alkaline solutions.

How do you balance a chemical equation? You must ensure the number of each type of atom is equal on both sides of the equation. For example, $2H_2 + O_2 \rightarrow 2H_2O$ is balanced because there are 4 hydrogen atoms and 2 oxygen atoms on each side.