Chemical reactions Revision Notes

Understanding Chemical Formulae

Chemical formulae use element symbols and numbers to show the composition of compounds. The symbol represents the type of element (e.g., Na for sodium, Cl for chlorine). Subscripts indicate the number of atoms of that element in one molecule or formula unit (e.g., H₂O means two hydrogen atoms and one oxygen atom). If no subscript is present, it means there is only one atom of that element. Common element symbols and the formulae for simple compounds like H₂O (water), CO₂ (carbon dioxide), O₂ (oxygen), N₂ (nitrogen), and CH₄ (methane) should be known.

The Principle of Conservation of Mass

This fundamental principle states that during any chemical reaction, atoms are not created or destroyed, only rearranged. This has crucial implications for chemical equations: the total mass of the reactants before the reaction must always equal the total mass of the products after the reaction. Consequently, the number of atoms of each element must be identical on both sides of a chemical equation. This is why equations must be balanced.

Balancing Chemical Equations

Balancing an equation ensures that the principle of conservation of mass is upheld. This involves placing whole numbers, called coefficients, in front of the chemical formulae. You cannot change the small subscript numbers within a formula, as that would alter the compound's identity. The goal is to make sure the count of each type of atom on the reactant side (left of the arrow) matches the count on the product side (right of the arrow). A systematic approach often involves balancing metals first, then non-metals (excluding hydrogen and oxygen), then hydrogen, and finally oxygen. For example, Fe + O₂ → Fe₂O₃ is unbalanced. Balancing it gives 4Fe + 3O₂ → 2Fe₂O₃.

Steps for Balancing an Equation

1. Write the correct chemical formulae for all reactants and products. 2. Count the number of atoms of each element on both sides of the equation. 3. Start balancing by picking an element that appears in only one reactant and one product. Avoid balancing hydrogen and oxygen until near the end unless they are the only elements unbalanced. 4. Add coefficients in front of the formulae to balance the atoms. Remember, you can only change coefficients, not the subscripts within formulae. 5. Recount the atoms after each change. 6. Continue until all elements are balanced on both sides. Check your work at the end to ensure all atoms are equal.

Carbonate Reactions with Acids

Carbonates react with acids to produce a salt, water, and carbon dioxide gas. This reaction is often observed as fizzing or bubbling due to the release of carbon dioxide. The general word equation for this type of reaction is: Carbonate + Acid → Salt + Water + Carbon Dioxide. For example, sodium carbonate (Na₂CO₃) reacts with hydrochloric acid (HCl) to form sodium chloride (NaCl), water (H₂O), and carbon dioxide (CO₂). It is important to know the chemical formulas of common carbonates and acids to write the correct balanced symbol equations. For instance, calcium carbonate (CaCO₃) reacting with sulfuric acid (H₂SO₄) produces calcium sulfate (CaSO₄), water (H₂O), and carbon dioxide (CO₂). Ensure all atoms are balanced on both sides of the equation.

Metal Reactions with Acids

Reactive metals (those above hydrogen in the reactivity series) react with acids to produce a salt and hydrogen gas. The unreactive metals below hydrogen in the reactivity series (e.g., copper, silver, gold) do not react with dilute acids to produce hydrogen. The general word equation for this reaction is: Metal + Acid → Salt + Hydrogen. For example, magnesium (Mg) reacts with hydrochloric acid (HCl) to form magnesium chloride (MgCl₂) and hydrogen (H₂). Similarly, zinc (Zn) reacts with sulfuric acid (H₂SO₄) to form zinc sulfate (ZnSO₄) and hydrogen (H₂). Balancing these equations requires careful counting of atoms for each element on both sides of the reaction arrow. Pay attention to the valency of the metal and the acid's anion to correctly determine the salt's formula.

Writing Balanced Chemical Equations

To write a balanced chemical equation, first identify the reactants and products and write their correct chemical formulas. For carbonates reacting with acids, the products are always a salt, water, and carbon dioxide. For reactive metals reacting with acids, the products are a salt and hydrogen gas. After writing the unbalanced equation, adjust the stoichiometric coefficients in front of each chemical formula until the number of atoms of each element is the same on both sides of the equation. For example, in the reaction of zinc with hydrochloric acid, Zn + HCl → ZnCl₂ + H₂, you need two HCl molecules to balance the two chlorine atoms in ZnCl₂ and two hydrogen atoms in H₂. So, the balanced equation is Zn + 2HCl → ZnCl₂ + H₂.

The Mole: A Counting Unit (Higher)

The mole (symbol: mol) is the standard international (SI) unit for the amount of substance. It's a way to count extremely large numbers of atoms, molecules, ions, or other particles that are too small to count individually. Just as 'a dozen' means 12, 'a mole' represents a specific, very large number of particles. This concept is vital for quantitative chemistry, allowing us to relate the mass of a substance to the number of particles it contains. For example, when you talk about 1 mole of water, you are referring to a specific number of water molecules, which also has a specific mass (its molar mass).

Avogadro Constant (N_A) (Higher)

The Avogadro constant (N_A) is the number of particles (atoms, molecules, ions, etc.) in one mole of any substance. Its approximate value is 6.02 x 10²³ mol^-1. This value is a fundamental constant in chemistry. It allows us to convert between the number of moles of a substance and the actual number of particles present. For instance, if you have 2 moles of carbon atoms, you have 2 multiplied by 6.02 x 10²³ atoms. Understanding this constant is key to performing calculations involving chemical quantities and understanding the microscopic scale of reactions.

Relating Moles, Mass, and Particles (Higher)

The mole provides a bridge between the macroscopic world (mass) and the microscopic world (number of particles). The molar mass of a substance is the mass of one mole of that substance, expressed in grams per mole (g/mol). For elements, the molar mass is numerically equal to its relative atomic mass (Ar). For compounds, it's numerically equal to its relative formula mass (Mr). The number of moles (n) can be calculated using the formula n = mass (g) / molar mass (g/mol). Using the Avogadro constant, the number of particles = number of moles × N_A. This means if you know the mass of a substance, you can determine how many individual particles are present.

The pH Scale: Acidity, Neutrality, and Alkalinity (Higher Tier)

The pH scale quantifies the acidity or alkalinity of an aqueous solution, ranging from 0 to 14. A pH of 7 signifies a neutral solution, such as pure water. Solutions with a pH value less than 7 are acidic, indicating a greater concentration of hydrogen ions (H⁺). Conversely, solutions with a pH value greater than 7 are alkaline (or basic), meaning they have a lower concentration of hydrogen ions and a relatively higher concentration of hydroxide ions (OH⁻). The scale is logarithmic, meaning that each whole number change in pH represents a tenfold change in hydrogen ion concentration.

Hydrogen Ion Concentration and pH Value (Higher Tier)

The numerical value of pH is inversely related to the concentration of hydrogen ions (H⁺) in a solution. A higher concentration of H⁺ ions corresponds to a lower pH value and a more acidic solution. For instance, a solution with a pH of 1 has a hydrogen ion concentration ten times greater than a solution with a pH of 2, and one hundred times greater than a solution with a pH of 3. Each unit decrease in pH signifies a tenfold increase in H⁺ concentration. Conversely, as H⁺ concentration decreases, the pH value increases, indicating a less acidic (more alkaline) solution. For example, a solution with pH 12 has ten times lower H⁺ concentration than a solution with pH 11.