Exothermic Reactions
Definition: Exothermic reactions are chemical changes that give off heat to the area around them. Because of this, things near the reaction get warmer, and the temperature around the reaction goes up. It feels hot when you touch something involved in an exothermic reaction.
Self-Heating Packs
Practical Use: Self-heating packs are special packs that are made to warm up by themselves. They use exothermic reactions to make heat without needing electricity or fire.
Main Chemicals: These packs usually have calcium oxide (CaO), a white powder, and water. When these two mix, they start a reaction.
Reaction Trigger: The heat starts when the user breaks a seal inside the pack, allowing the calcium oxide to mix with water. The chemical reaction begins and gives off heat quickly.
Reaction Equation: The chemical equation is: CaO + H₂O → Ca(OH)₂ + heat. This means that calcium oxide reacts with water to form calcium hydroxide and gives off heat in the process.
Applications: The heat from this reaction is used in things like pocket warmers, heating pads, or even food rations to keep warm in cold places or help soothe body aches.
Alternative Chemicals: Besides calcium oxide, other chemicals such as calcium chloride (CaCl₂) or magnesium sulfate (MgSO₄) can be used because they also produce heat when mixed with water.
Combustion of Fuels
Definition: Combustion is a type of exothermic reaction where something burns in oxygen and gives off heat and light. It’s one of the most common examples of exothermic reactions.
Heat Production: When fuels like wood, cooking gas, petrol, or coal burn, they react with oxygen in the air. This releases a lot of heat and often a bright flame.
Daily Uses: We use combustion in our everyday lives—for cooking food on stoves, warming our homes with heaters, and powering cars and generators to produce electricity.
Methane Combustion: An example is burning methane gas: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(ce) + heat. This reaction produces carbon dioxide, water, and a lot of heat.
Ethanol Combustion: Another fuel is ethanol, a type of alcohol. When it burns: C₂H₅OH(ce) + 3O₂(g) → 2CO₂(g) + 3H₂O(ce), it also releases heat. The amount of heat is very high (−1376 kJ for every mole of ethanol burned).
Heat of Combustion: The energy given off when a substance burns is called the heat of combustion. We can find this using the formula ΔH = mcθ, where m is mass, c is specific heat capacity, and θ is the temperature change.
Respiration
Biological Process: Respiration is a chemical process that happens inside the bodies of animals and humans. It is an exothermic reaction that releases energy from food.
Reaction Equation: The reaction is: C₆H₁₂O₆(aq) + 6O₂(g) → 6CO₂(g) + 6H₂O(ce) + energy. This means that glucose and oxygen react to form carbon dioxide and water, releasing energy.
Purpose: The energy released from this reaction is used by our cells to do important work like moving muscles, thinking, growing, and keeping organs working.
Body Heat: This process also gives off heat, which helps keep our body temperature at around 37°C, even when it’s cold outside.
Endothermic Reactions
Definition: Endothermic reactions are chemical processes that take in heat energy from the surroundings. Because heat is absorbed, the temperature around the reaction goes down, making the environment feel cooler.
Instant Cold Packs
Practical Use: Cold packs are used to treat injuries such as sprains or swelling. They contain substances that react in a way that absorbs heat, making the pack feel cold without needing to be placed in a freezer.
Main Chemicals: These cold packs usually contain ammonium nitrate (NH₄NO₃) or ammonium chloride (NH₄Cl), along with water. The water and salt are kept in separate compartments until needed.
Activation Mechanism: When the pack is squeezed, the barrier between the water and the chemical breaks. The water mixes with the ammonium salt, and an endothermic reaction starts.
Heat Absorption: As the ammonium nitrate dissolves in the water, it absorbs heat from the surroundings. This is what causes the temperature of the pack to drop.
Example Reaction: NH₄NO₃(s) → NH₄⁺(aq) + NO₃⁻(aq)
This equation shows that solid ammonium nitrate splits into its ions in water and takes in heat during the process.
Cooling Effect: The absorbed heat causes the temperature of the pack to fall, making it useful for reducing pain and swelling on injured body parts.
Photosynthesis
Definition: Photosynthesis is a very important endothermic process that happens in green plants. It allows plants to make their own food using sunlight.
Energy Source: Plants use energy from the sun (solar energy) to make glucose, a type of sugar that stores energy. This energy is needed to power the reaction.
Main Reaction: 6CO₂(g) + 6H₂O(l) + light energy → C₆H₁₂O₆(aq) + 6O₂(g)
This means carbon dioxide from the air and water from the ground combine using sunlight to produce glucose and oxygen.
Energy Storage: The energy from sunlight is not lost—it gets stored in the glucose. This glucose can be used by the plant later for energy.
Outcome: Plants use glucose as food to grow and stay alive, and they also release oxygen into the air, which animals and humans need to breathe.
Cooking
Heat Requirement: Many cooking processes are endothermic, meaning they need heat to work. These include boiling, baking, frying, and steaming.
Examples: When bread is baked or water is boiled, heat energy is constantly being absorbed to keep the reaction going.
Egg Cooking: When you cook an egg, the proteins inside it change shape and harden. This process is called denaturation, and it only happens because heat is absorbed.
Continuous Heating: Endothermic cooking reactions must keep receiving heat from a stove, oven, or another heat source. If the heat stops, the reaction cannot continue.