Cambridge Science Mastery: Stage 8 Properties of Materials and Chemical Reactions

Principles and Mechanics of Paper Chromatography

Paper chromatography is a laboratory technique utilized to separate a mixture of dissolved substances, known as solutes, based on their relative solubility in a solvent and their attraction to a stationary material.

• The Separation Process: Separation occurs as the substances move through a stationary material at different rates.

• Stationary Phase: In this specific method, the chromatography paper serves as the stationary phase.

• Mobile Phase: This is the solvent that moves up the paper, typically consisting of water or ethanol.

• Components of a Chromatography Setup:

  • Beaker: The container holding the solvent and the paper.

  • Solvent: The liquid (mobile phase) used to transport the solutes.

  • Origin: The starting point, marked with a pencil baseline, where the ink spot or mixture is initially placed.

  • Ink Spot: The sample of the mixture to be separated.

  • Solvent Front: The furthest point reached by the solvent as it travels up the paper.

• Identification of Substances:

  • Pure Substances: These will yield only one distinct spot on the chromatogram.

  • Mixtures: These will separate into multiple spots at different heights.

Calculating and Utilizing $R_f$ Values

The Retention Factor, or $R_f$ value, is a numerical constant used to identify specific chemical substances with accuracy.

• Measurement Requirement: The distance must always be measured starting from the pencil baseline (the origin).

• The mathematical formula for $R_f$ is:          $R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$     

• The Crucial Rule of $R_f$ Values: The calculated value can never be greater than 1. This is because a dissolved substance cannot physically travel further than the solvent front that is carrying it.

Thermal Energy Changes in Chemical Reactions

Chemical reactions involve changes in energy, specifically the movement of thermal energy between the reaction and its surroundings.

• Exothermic Reactions:

  • Definition: Reactions that release thermal energy into the surroundings.

  • Observable Effect: The temperature of the surroundings rises.

  • Molecular Concept: Energy is released when new chemical bonds are formed.

  • Common Examples: Combustion, neutralization reactions, and commercial hand warmers.

• Endothermic Reactions:

  • Definition: Reactions that absorb thermal energy from the surroundings.

  • Observable Effect: The temperature of the surroundings falls.

  • Molecular Concept: Energy is required and consumed when existing chemical bonds are broken.

  • Common Examples: Photosynthesis, thermal decomposition, and commercial cold packs.

Energy Profile Diagrams

Energy profile diagrams visualize the energy levels of reactants and products over the course of a reaction.

• Exothermic Profile:

  • The reactants possess a higher energy level than the final products.

  • The "gap" or difference between the reactant energy and product energy represents the energy released as heat.

  • Includes an "Activation Energy" peak representing the initial energy required to start the reaction.

• Endothermic Profile:

  • The reactants possess a lower energy level than the products.

  • Energy must be absorbed from the environment to reach the higher energy state of the products.

  • Includes an "Activation Energy" peak that must be overcome for the reaction to proceed.

Conservation of Mass, Balancing, and State Symbols

• Law of Conservation of Mass: This fundamental principle states that the total mass of the products in a chemical reaction must always equal the total mass of the reactants. Within a closed system, no atoms are lost or created.

• Rules for Balancing Equations:

  • When balancing a chemical equation, you may only change the large coefficients placed in front of the chemical symbols.

  • Prohibition: You must never change the small subscript numbers within a chemical formula, as this would change the identity of the substance itself.

• Essential State Symbols: These symbols indicate the physical state of the substances involved and are necessary for a complete chemical description:

  • $(s)$: Solid

  • $(l)$: Liquid

  • $(g)$: Gas

  • $(aq)$: Aqueous (substance dissolved in water)

The Reactivity Series of Metals

The reactivity of metals varies significantly when exposed to water or acids, as detailed in the following hierarchy:

• Potassium and Sodium:

  • Reactivity with Water: Very vigorous.

  • Reactivity with Acid: Explosive and extremely dangerous.

  • Observations: Produces purple or orange flames.

• Calcium and Magnesium:

  • Reactivity with Water: Slow reaction with cold water.

  • Reactivity with Acid: Exhibits rapid fizzing.

  • Observations: Produces visible bubbles of Hydrogen gas.

• Zinc and Iron:

  • Reactivity with Water: Reacts only when exposed to steam.

  • Reactivity with Acid: Exhibits a slow reaction.

  • Observations: These metals glow when heated.

• Copper and Gold:

  • Reactivity with Water: No reaction occurs.

  • Reactivity with Acid: No reaction occurs.

  • Observations: These metals maintain their shiny appearance.

Specific Chemical Reactions of Metals

Metals undergo distinct reactions with oxygen, water, and acid to form new compounds.

• Reaction with Oxygen:

  • This process forms metal oxides.

  • Specific Example: Magnesium burns with a blinding white light when reacting with oxygen.

  • Equation: $2Mg + O_2 \rightarrow 2MgO$

• Reaction with Water:

  • This process forms metal hydroxides and hydrogen gas.

  • Specific Example: Highly reactive metals like sodium will fizz and float on the water's surface.

  • Equation: $2Na + 2H_2O \rightarrow 2NaOH + H_2$

• Reaction with Acid:

  • This process forms a salt and hydrogen gas.

  • Indicator: The rate of fizzing (effervescence) is a direct indicator of the metal's level of reactivity.

  • Equation: $Mg + 2HCl \rightarrow MgCl_2 + H_2$

Displacement Reactions

A displacement reaction is interpreted as a form of "chemical competition" between metals.

• Definition: A displacement reaction occurs when a more reactive metal takes the place of a less reactive metal within its compound.

• Standard Laboratory Example: Placing an iron nail into a blue copper sulfate solution.

  • The Chemical Shift: Iron, being more reactive than copper, displaces it from the sulfate compound.

  • Equation: $Fe + CuSO_4 \rightarrow FeSO_4 + Cu$

  • Results: A brown deposit of solid copper forms on the surface of the iron nail, and the solution changes as iron sulfate is formed.