GED · Science · 6 min read
Energy Changes, Momentum, and Heat
- Distinguish between endothermic and exothermic chemical processes and understand activation energy.
- Apply the laws of conservation of energy and conservation of momentum to physical systems and collisions.
- Explain the concepts of temperature, thermal energy, and the three primary methods of heat transfer.
Energy in Chemical Reactions
Chemical reactions occur when chemical bonds in the reacting substances are altered. In many reactions, some bonds break and others form, sometimes simultaneously; not every bond in a reactant must be broken. Breaking chemical bonds always requires an input of energy. Forming chemical bonds releases energy, but whether the reaction as a whole releases energy to its surroundings depends on its overall energy balance. The overall energy balance of the reaction depends on the difference between the energy required to break bonds and the energy released when new bonds are formed.
When a reaction releases net thermal energy into its surroundings, it is classified as an exothermic reaction. In an exothermic process, heat is transferred to the surroundings, which may cause their temperature to rise. Because energy is released outward, the chemical potential energy of the products ends up lower than the chemical potential energy of the reactants. Combustion is a familiar everyday example of an exothermic chemical process, releasing heat and warming the area around it.
An endothermic reaction does the opposite by absorbing net thermal energy from its surroundings. Because the reaction absorbs heat, the temperature of the surroundings may decrease. In an endothermic process, the chemical potential energy of the products is higher than that of the reactants. Photosynthesis requires light energy and has a positive enthalpy change, so it may be described as endothermic; absorbing light is not the same thing as absorbing heat.
Many chemical reactions need an initial energy input to overcome an activation-energy barrier, but some have little or no barrier. Activation energy is defined as the minimum amount of energy required to initiate a chemical reaction. Substances known as catalysts can increase the rate of a chemical reaction. They achieve this specifically by lowering the reaction's activation energy, allowing the transformation from reactants to products to happen more readily.
| Property | Exothermic Reaction | Endothermic Reaction |
|---|---|---|
| Net Thermal Energy | Releases net thermal energy | Absorbs net thermal energy |
| Surroundings Temperature | Transfers heat; may raise temperature | Absorbs heat; may lower temperature |
| Chemical Potential Energy | Products are lower than reactants | Products are higher than reactants |
| Example / Note | Combustion is a classic example | Absorbs heat (unlike light absorption in photosynthesis) |
Common mistakes
- Believing that total momentum is only conserved in elastic collisions; in reality, total momentum is conserved in both elastic and inelastic collisions within an isolated system.
- Confusing heat with temperature; temperature measures average kinetic energy of particles, whereas heat is the spontaneous transfer of thermal energy from a warmer object to a cooler one.
- Thinking that thermal radiation requires air or another medium to travel; thermal radiation consists of electromagnetic waves and travels across completely empty space, such as sunlight reaching Earth.
- Assuming that absorbing light alone establishes whether a reaction is endothermic; photosynthesis requires light energy and has a positive enthalpy change, so it may be described as endothermic.
Key points
- Breaking chemical bonds requires energy, while forming bonds releases energy; exothermic reactions release net thermal energy, whereas endothermic reactions absorb net thermal energy.
- Activation energy is the minimum energy required to initiate a reaction, and catalysts speed up reactions by lowering this energy requirement.
- Mechanical energy combines kinetic energy (motion) and potential energy (stored, such as gravitational or elastic); the law of conservation of energy states that energy cannot be created or destroyed, only transformed.
- Momentum is mass times velocity, and the total momentum of an isolated system is always conserved before and after collisions.
- Elastic collisions conserve both momentum and kinetic energy, while inelastic collisions conserve momentum but convert some kinetic energy into thermal energy, deformation, and sound.
- Heat is the spontaneous transfer of thermal energy from warmer to cooler objects until thermal equilibrium is reached at an equal temperature.
- Thermal energy transfers through direct contact (conduction), fluid motion (convection), or electromagnetic waves across empty space (radiation).
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