Chemical reactions Revision Notes

What is Electrolysis?

Electrolysis is the process of using direct electrical current to drive an otherwise non-spontaneous chemical reaction. For ionic compounds, this involves decomposing the compound into its constituent elements. It requires the ions to be mobile, which happens when the ionic compound is molten or dissolved in solution. In the molten state, the high temperature provides enough energy to overcome the strong electrostatic forces holding the ions in a lattice structure, allowing them to move freely.

Components of an Electrolytic Cell

An electrolytic cell consists of a power supply, two electrodes (conductors), and an electrolyte. The electrolyte is the molten ionic compound containing the free-moving ions. The anode is the positive electrode, and the cathode is the negative electrode. The power supply pushes electrons around the external circuit, causing reactions to occur at the electrodes.

Predicting Products at the Cathode (Negative Electrode)

The cathode is the negative electrode, so it attracts positively charged ions (cations). At the cathode, cations gain electrons (reduction) and are discharged as neutral metal atoms. For example, if molten potassium iodide (KI) is electrolysed, the K⁺ ions move to the cathode and gain an electron to form solid potassium metal: K⁺(l) + e⁻ → K(s). The general rule for binary ionic compounds is that the metal will be produced at the cathode.

Predicting Products at the Anode (Positive Electrode)

The anode is the positive electrode, so it attracts negatively charged ions (anions). At the anode, anions lose electrons (oxidation) and are discharged as neutral non-metal atoms or molecules. For example, if molten potassium iodide (KI) is electrolysed, the I⁻ ions move to the anode and lose electrons to form iodine gas: 2I⁻(l) → I₂(g) + 2e⁻. The general rule for binary ionic compounds is that the non-metal will be produced at the anode.

What is the Law of Conservation of Mass?

The Law of Conservation of Mass states that in any chemical reaction, the total mass of the reactants is equal to the total mass of the products. This means that mass is neither created nor destroyed during a chemical change. Atoms are rearranged to form new substances, but the total number and type of atoms remain constant throughout the reaction.

Importance in Chemical Reactions

This law is essential for understanding and predicting the outcomes of chemical reactions. When writing chemical equations, the law of conservation of mass dictates that the equation must be balanced, meaning that the number of atoms of each element on the reactant side must be equal to the number of atoms of that same element on the product side. This reflects that atoms are conserved.

Mass Changes in Reactions

Even if a reaction appears to involve a change in mass, such as a gas being produced and seemingly 'disappearing', the total mass is conserved. If a gas is produced and escapes an open system, the mass of the remaining solid or liquid in the container will decrease. Conversely, if a gas from the surroundings reacts with a solid/liquid, the mass of the contents in the container might increase. However, in a closed system, the total mass will always remain constant.

Chemical Symbols from the Periodic Table

Every element has a unique chemical symbol, found on the Periodic Table. These symbols are usually one or two letters. The first letter is always a capital letter, and if there is a second letter, it is always a lowercase letter. For example, 'C' for Carbon, 'Ca' for Calcium, 'Na' for Sodium. It is essential to use the correct capitalisation as 'CO' (carbon monoxide) is different from 'Co' (cobalt). The Periodic Table lists these symbols along with other element information.

Writing Chemical Formulas

Chemical formulas use element symbols and subscripts to show the number of atoms of each element in a compound. For instance, the formula for carbon dioxide is CO₂, indicating one carbon atom and two oxygen atoms. If there is only one atom of an element, the subscript '1' is usually omitted. For example, in H₂O (water), there are two hydrogen atoms and one oxygen atom. Formulas provide a concise way to represent the composition of substances.

Understanding Chemical Equations

Chemical equations represent chemical reactions. Reactants are shown on the left side of an arrow, and products are shown on the right. For example, in the reaction A + B → C + D, A and B are reactants, and C and D are products. The arrow indicates the direction of the reaction. Chemical equations use the symbols and formulas derived from the Periodic Table to represent the substances involved.

Balancing Chemical Equations

A balanced chemical equation shows that the number of atoms of each element is conserved during a chemical reaction, adhering to the Law of Conservation of Mass. Coefficients (large numbers placed in front of chemical formulas) are used to balance equations. For example, H₂ + O₂ → H₂O is unbalanced. To balance it, we write 2H₂ + O₂ → 2H₂O. Now, there are four hydrogen atoms and two oxygen atoms on both sides of the equation. This balancing act ensures that atoms are neither created nor destroyed during the reaction.

What is Electrolysis?

Electrolysis is the process where electrical energy is used to break down an ionic compound. This compound must be in a molten state or dissolved in an aqueous solution for its ions to be mobile and conduct electricity. Solid ionic compounds cannot undergo electrolysis because their ions are fixed in a lattice structure. The process is carried out in an electrolytic cell, which includes electrodes and an electrolyte connected to a power supply.

Ions and their Movement

In electrolysis, ions are the charge carriers. Cations are positively charged ions and are attracted to the negative electrode, called the cathode. Anions are negatively charged ions and are attracted to the positive electrode, called the anode. This movement of ions towards oppositely charged electrodes is fundamental to the electrolytic process, allowing for the flow of charge through the electrolyte.

Reactions at the Electrodes

At the cathode (negative electrode), reduction occurs. Cations gain electrons to become neutral atoms or molecules. For example, metal ions gain electrons to form metal atoms. At the anode (positive electrode), oxidation occurs. Anions lose electrons to become neutral atoms or molecules. For example, halide ions lose electrons to form halogen molecules. The specific products formed depend on the ions present in the electrolyte.

Key Terms in Electrolysis

Electrolyte: The molten or dissolved ionic substance that conducts electricity. Electrodes: Conductors through which current enters and leaves the electrolyte. Anode: The positive electrode where oxidation (loss of electrons) occurs. Cathode: The negative electrode where reduction (gain of electrons) occurs. Anion: A negatively charged ion attracted to the anode. Cation: A positively charged ion attracted to the cathode.