Chemical Reactions Chapter Notes | Year 8 Science IGCSE (Cambridge) PDF Download

Exothermic Reactions

Burning

• Burning is a chemical reaction where a substance, known as a fuel, combines with oxygen.
• Fuels, such as charcoal, wood, coal, natural gas, and oil, store chemical energy.
• During burning, the chemical energy in the fuel is converted into thermal, light, and sound energy, which dissipate into the surroundings.
• Burning requires three components: fuel, oxygen, and heat (thermal energy).
• Combustion is another term for burning.
• In a burning reaction, atoms of the fuel and oxygen rearrange, converting chemical energy into thermal energy, causing a rise in temperature.
• A chemical reaction that releases thermal energy is called an exothermic reaction.

Burning Other Substances

• Hydrogen can be used as a fuel, and its combustion is an exothermic reaction where hydrogen combines with oxygen to form water.
• During hydrogen combustion, the rearrangement of hydrogen and oxygen atoms releases energy in the form of kinetic, thermal, sound, and light energy.
• In an experimental setup, a plastic bottle filled with a mixture of hydrogen and air can be propelled like a rocket when a spark ignites the mixture, releasing significant energy that propels the bottle.
• Burning magnesium in air is another exothermic reaction, producing magnesium oxide and releasing heat and light as magnesium and oxygen atoms rearrange.
• When a substance burns, it combines with oxygen to form an oxide, and such reactions are classified as oxidation reactions.

An Exothermic Reaction with Water

• The reaction between potassium and water is exothermic, producing potassium hydroxide and hydrogen.
• In this reaction, the bonds between hydrogen and oxygen atoms in water break, and the atoms rearrange to form new products.
• Stored chemical energy is converted into thermal energy, which dissipates into the environment.

An Exothermic Reaction with Acid

• Adding magnesium to dilute hydrochloric acid results in an exothermic reaction, producing magnesium chloride and hydrogen.
• The reaction releases thermal energy, causing the test tube to become hot.

Endothermic Reactions

Endothermic Reactions

• Endothermic reactions absorb thermal energy from the surroundings, converting it into chemical energy stored in chemical bonds.
• During an endothermic reaction, the temperature at the end is lower than at the start, making the surroundings feel cooler.
• An example is the reaction between sodium hydrogen carbonate and citric acid, which produces sodium citrate, water, and carbon dioxide.
• In this reaction, thermal energy is absorbed, lowering the temperature of the surroundings, such as a test tube, which feels cooler.
• This reaction occurs in sherbet sweets, where citric acid and sodium hydrogen carbonate dissolve in saliva, react, and create a cool, fizzy sensation in the mouth.

Another Way to Cool Down

• Dissolving potassium chloride in water is an endothermic process, not a chemical reaction, as no new substances are formed.
• When potassium chloride dissolves in water, thermal energy is absorbed from the surroundings, causing the beaker to feel cold.
• Ice melting is another endothermic process, where thermal energy is absorbed from the surroundings to change solid ice into liquid water.
• In solid ice, particles are fixed in a lattice, vibrating minimally due to strong intermolecular forces.
• As thermal energy is absorbed, particles vibrate more, gaining enough energy to overcome these forces, allowing them to move and form liquid water.

Endothermic or Exothermic?

• Exothermic reactions and processes release thermal energy to the surroundings, increasing the surrounding temperature.
• Endothermic reactions and processes absorb thermal energy from the surroundings, decreasing the surrounding temperature.
• For example, boiling water is an exothermic process (releases heat), while melting ice is an endothermic process (absorbs heat).

Using Exothermic Reactions

• Exothermic reactions are used in self-heating cans for food or drinks, which contain two chemicals (e.g., calcium oxide and water) in separate compartments.
• When the can is opened, the chemicals mix, react, and release heat, warming the contents.
• The reaction is: calcium oxide + water → calcium hydroxide, which is exothermic and transfers thermal energy to the food.
• These cans are useful in remote areas, emergencies, or camping where heating facilities are unavailable.
• Production challenges include ensuring sealed compartments to prevent contamination and achieving even heating of the food.

Using Endothermic Processes

• Ice packs are used to treat injuries by absorbing heat from the injured area, an endothermic process where ice melts, cooling the area and reducing swelling.
• Traditional ice packs are stored in a fridge or freezer and reused after refreezing.
• Chemical ice packs use an endothermic process, typically involving ammonium nitrate and water in separate compartments.
• When the pack is activated by breaking the internal compartment, ammonium nitrate dissolves in water, absorbing thermal energy and lowering the temperature.
• Chemical ice packs are convenient for use without refrigeration, ideal for immediate injury treatment.