Chemistry Core Concepts: Reactions, Calculations, and Analysis

Posted on Nov 23, 2025 in Chemistry

Temperature Effects on Reaction Rate

As temperature increases, the rate of reaction increases. This is explained by collision theory:

• Particles gain more kinetic energy, causing them to move faster and collide more frequently.
• Crucially, a higher proportion of particles have energy equal to or greater than the activation energy.
• This leads to more successful collisions per second, increasing the rate of reaction.

Example: When hydrochloric acid reacts with magnesium, increasing the temperature causes faster bubbling and quicker disappearance of the magnesium.

Energy Profile Diagrams for Exothermic Reactions

In an exothermic reaction, energy is released to the surroundings, meaning the energy of the products is lower than the energy of the reactants.

• The diagram shows energy decreasing from reactants to products.
• The initial curved hump represents the activation energy (E_a)—the minimum energy needed for a successful collision, required to break bonds in the reactants.
• The difference in energy between reactants and products represents the overall energy change (enthalpy change, ΔH), which is negative for exothermic reactions.
• After the activation energy is overcome, new bonds form in the products, releasing energy.

Comparing Lithium and Potassium Reactivity (Group 1)

Potassium is significantly more reactive than lithium. Group 1 reactivity increases as you move down the group due to:

• Increased Atomic Radius: Potassium atoms have more electron shells than lithium.
• Increased Shielding: The outer electron is further from the nucleus and is shielded by more inner shells.
• Weaker Attraction: This makes the outer electron less strongly attracted to the nucleus, making it easier to lose.

Therefore, potassium reacts more violently with water than lithium, producing hydrogen gas and potassium hydroxide.

Testing for Sulfate and Halide Ions

Testing for Sulfate Ions (SO₄²⁻)

1. Add a few drops of dilute hydrochloric acid to the solution to remove any carbonate ions.
2. Then add barium chloride solution.
3. If sulfate ions are present, a white precipitate of barium sulfate will form.

Testing for Halide Ions

To test for halide ions, add dilute nitric acid followed by silver nitrate solution:

• Chloride (Cl⁻): White precipitate
• Bromide (Br⁻): Cream precipitate
• Iodide (I⁻): Yellow precipitate

Chromatography: Comparing Mixtures and Calculating Rf Values

1. Draw a pencil line near the bottom of the chromatography paper (the baseline).
2. Place small spots of each mixture on the line.
3. Place the paper in a beaker with the solvent level below the baseline and allow the solvent to rise.
4. Measure how far each dye travels and calculate the R_f values.

Dyes with the same R_f value and position on the chromatogram are likely to be the same substance.

Cracking Hydrocarbons to Meet Fuel Demand

Crude oil contains many long-chain hydrocarbons, which are less useful as fuels. Cracking is a process that breaks these long chains into shorter-chain alkanes and alkenes.

Short-chain alkanes are more volatile and flammable, making them better fuels, such as petrol. Cracking helps meet the high demand for these useful fuels from longer, less useful molecules.

Pressure Effects on Ammonia Yield (Haber Process)

The forward reaction in the Haber process (N₂ + 3H₂ ⇌ 2NH₃) produces fewer gas molecules (4 molecules on the left, 2 on the right).

According to Le Chatelier’s Principle, increasing pressure shifts the equilibrium to the side with fewer gas molecules. Therefore, increasing pressure increases the yield of ammonia.

Note: Using excessively high pressure is expensive and dangerous, so a compromise pressure is typically used in industrial settings.

Hydrocarbon Reactions and Reactivity

Complete Combustion of Methane

Word equation:
Methane + Oxygen → Carbon Dioxide + Water

Symbol equation:
CH₄ + 2O₂ → CO₂ + 2H₂O

Why Alkenes Are More Reactive Than Alkanes

Alkenes contain a double carbon-carbon bond (C=C), which is more reactive than the single bonds found in alkanes.

• The double bond is an area of high electron density.
• This allows it to be easily attacked by other atoms or molecules in addition reactions.
• Alkanes have only single bonds, making them saturated and significantly less reactive.

Calculating Percentage Atom Economy

Atom economy measures the efficiency of a reaction by calculating the proportion of reactant atoms that end up in the desired product.

Formula:
Atom Economy = (Mass of desired product ÷ Total mass of reactants) × 100

Example Calculation: If 88 g of product is made from 100 g of reactants:

Atom Economy = (88 g ÷ 100 g) × 100 = 88%

Greenhouse Gases and the Mechanism of Global Warming

Greenhouse gases, such as carbon dioxide and methane, cause global warming by trapping heat in the atmosphere.

These gases absorb infrared radiation emitted from the Earth’s surface. Instead of allowing this heat to escape into space, they re-radiate it back towards the Earth. This leads to an increase in average global temperatures, known as global warming. Human activities, such as burning fossil fuels and farming, contribute significantly to the rise in these gases.

Chemical Tests and Combustion

Testing for Carbon Dioxide Gas

Bubble the gas through limewater (calcium hydroxide solution). If carbon dioxide is present, the limewater turns cloudy due to the formation of insoluble calcium carbonate.

Testing for Hydrogen Gas

Insert a lit splint into the test tube. If hydrogen is present, it will make a characteristic squeaky pop sound as it reacts rapidly with oxygen.

Incomplete Combustion of Hydrocarbons

Incomplete combustion occurs when there is insufficient oxygen supply. Instead of forming carbon dioxide and water, it produces carbon monoxide (CO) or carbon (soot).

• This happens because the carbon in the fuel is not fully oxidized.
• Incomplete combustion is dangerous because carbon monoxide is a toxic, colourless, and odourless gas.

Addition vs. Condensation Polymerisation

Production of Ethanol via Fermentation

Ethanol can be produced biologically by the fermentation of glucose using yeast.

• The reaction is carried out in anaerobic conditions (without oxygen).
• The optimal temperature is around 30°C.
• The enzyme in the yeast converts glucose into ethanol and carbon dioxide.

Calculating Solution Concentration (mol/dm³)

Concentration is calculated by dividing the moles of solute by the volume of the solution in cubic decimetres (dm³).

Formula:
Concentration (mol/dm³) = Moles (mol) ÷ Volume (dm³)

Example: 2.5 mol of NaOH is dissolved in 500 cm³ of solution.

1. Convert volume to dm³: 500 cm³ ÷ 1000 = 0.5 dm³
2. Calculate concentration: 2.5 mol ÷ 0.5 dm³ = 5.0 mol/dm³

Stages of a Life Cycle Assessment (LCA)

A Life Cycle Assessment (LCA) evaluates the total environmental impact of a product throughout its existence. The four main stages analyzed are:

1. Extracting and processing raw materials.
2. Manufacturing and packaging the product.
3. Using the product (including maintenance and transport).
4. Disposing of the product at the end of its useful life.

Each stage is analyzed for energy use, pollution, and waste generation.

Comparing Energy Released by Different Fuels

To compare the energy released by two fuels, a calorimetry experiment is performed:

1. Measure a fixed volume of water in a calorimeter (or metal can). Record the starting temperature.
2. Burn a known mass of the first fuel under the can until the temperature rises by a set amount (e.g., ~20°C).
3. Measure the final temperature.
4. Repeat the procedure for the second fuel.

The energy transferred (J) is calculated using the equation: Energy transferred = mass of water × 4.2 × temperature change. The results are then compared based on the energy released per gram of fuel.

Flame Tests for Identifying Metal Ions

Flame tests are used to identify certain metal ions based on the characteristic colour they emit when heated.

Procedure

1. Clean a nichrome wire loop by dipping it in concentrated hydrochloric acid and then heating it in a roaring Bunsen flame until no colour is observed.
2. Dip the loop into the sample and place it back into the flame.

Results

• Lithium (Li⁺): Crimson red
• Sodium (Na⁺): Yellow
• Potassium (K⁺): Lilac
• Calcium (Ca²⁺): Orange-red
• Copper (Cu²⁺): Green

Identifying Metal Ions Using Sodium Hydroxide

Adding a few drops of sodium hydroxide solution to an unknown solution can identify specific metal ions based on the colour and solubility of the precipitate formed:

• Al³⁺: White precipitate (dissolves in excess NaOH)
• Ca²⁺: White precipitate (does not dissolve in excess NaOH)
• Mg²⁺: White precipitate (does not dissolve in excess NaOH)
• Fe²⁺: Green precipitate
• Fe³⁺: Brown precipitate
• Cu²⁺: Blue precipitate

Testing for Ammonium Ions (NH₄⁺)

To test for ammonium ions, add sodium hydroxide solution to the sample and gently heat the mixture.

If ammonium ions are present, ammonia gas (NH₃) is produced. This gas can be detected by holding damp red litmus paper near the mouth of the test tube—the paper will turn blue, as ammonia is alkaline.

Esterification: Alcohols and Carboxylic Acids

An alcohol reacts with a carboxylic acid in the presence of a strong acid catalyst, such as sulfuric acid, to form an ester and water.

This reaction is known as esterification and is a type of condensation reaction.

Example:
Ethanol + Ethanoic Acid → Ethyl Ethanoate + Water

Comparing Properties of Alcohols and Carboxylic Acids

Nanoparticles vs. Bulk Materials: Size and Reactivity

Nanoparticles are extremely small particles, typically 1–100 nanometres in size. They differ significantly from bulk materials primarily due to their size-related properties.

• Nanoparticles have a very high surface area to volume ratio compared to bulk materials.
• This high ratio makes them much more reactive.
• Consequently, smaller amounts of nanoparticles are needed for the same effect (e.g., when used in catalysts or sun creams).

Key Properties and Risks of Nanoparticles

Due to their unique size, nanoparticles:

• Are highly effective as catalysts due to their large surface area.
• May exhibit different chemical or physical behaviour due to quantum effects.
• Are highly efficient, requiring less material.

However, their long-term effects on health and the environment are still uncertain, and they may be toxic or cause harm if inhaled.