5.1 Endothermic and Exothermic Reactions
Thermochemistry is the study of heat energy changes that take place during chemical reactions.
Classification of Chemical Reactions Based on Heat Change
Chemical reactions can be classified into two main types based on heat absorption or release:
| Parameter | Exothermic Reaction | Endothermic Reaction |
|---|---|---|
|
Definition | A chemical reaction that
releases heat energy to the surroundings. | A chemical reaction that
absorbs heat energy from the surroundings. |
|
Temperature Change | Temperature of the surroundings
increases (thermometer reading rises). | Temperature of the surroundings
decreases (thermometer reading drops). |
|
Energy Change | Total chemical energy of reactants > Total chemical energy of products. | Total chemical energy of reactants < Total chemical energy of products. |
|
Heat Flow | Heat flows from the reacting chemicals
out into the surroundings. | Heat flows from the surroundings
into the reacting chemicals. |
|
Feeling to Touch | Container feels
warm or hot. | Container feels
cold. |
Examples of Exothermic Reactions
- Combustion reactions: Burning of fuels like methane, ethanol, or wood in oxygen.
- Neutralisation reactions: Reaction between an acid and an alkali (e.g., $HCl + NaOH \rightarrow NaCl + H_2O$).
- Respiration: Cellular release of energy ($C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$).
- Reaction of reactive metals with water or acid: (e.g., Sodium reacting with water).
- Rusting of iron: Oxidation of iron over time.
- Dissolving sodium hydroxide ($NaOH$) or anhydrous calcium chloride ($CaCl_2$) in water.
Examples of Endothermic Reactions
- Photosynthesis: Green plants absorb light and heat energy ($6CO_2 + 6H_2O + \text{Light/Heat} \rightarrow C_6H_{12}O_6 + 6O_2$).
- Thermal decomposition: Heating of carbonates or nitrates (e.g., breakdown of $CaCO_3$ into $CaO$ and $CO_2$).
- Dissolving ammonium salts in water: Dissolving ammonium chloride ($NH_4Cl$) or ammonium nitrate ($NH_4NO_3$).
- Extraction of metals from their ores: Smelting process requiring heat.
- Reaction between baking soda (sodium hydrogen carbonate) and citric acid/ethanoic acid.
Thermochemical Energy Profile Diagrams
- Exothermic Reaction: The energy level of the reactants is higher than the energy level of the products ($\Delta H$ is negative).
$$\text{Reactants} \rightarrow \text{Products} + \text{Heat Energy}$$
- Endothermic Reaction: The energy level of the reactants is lower than the energy level of the products ($\Delta H$ is positive).
$$\text{Reactants} + \text{Heat Energy} \rightarrow \text{Products}$$
Everyday Applications of Thermochemical Reactions
- Instant Hot Packs: Contain chemicals like anhydrous calcium chloride ($CaCl_2$) or magnesium sulfate ($MgSO_4$) and water in separate compartments. When squeezed, they mix and undergo an exothermic reaction, releasing heat to warm hands or soothe muscle cramps.
- Instant Cold Packs: Contain chemicals like ammonium nitrate ($NH_4NO_3$) and water in separate compartments. Squeezing ruptures the inner pouch, causing an endothermic reaction that absorbs heat, cooling the pack down to treat sports injuries, reduce swelling, or lower fever.