Energy changes are at the heart of chemical reactions. When...
Chemistry174 views·Updated 19 Jul 2026·2 pagesUnderstanding Energy Changes
Evelyn Ridley@ev_alice
1
of 2
Exothermic and Endothermic Reactions
Ever noticed how some chemical reactions get hot? These are exothermic reactions which release thermal energy into their surroundings, causing temperature increases. Common examples include combustion (burning fuels), respiration in your cells, neutralisation reactions, and when metals react with acids.
In contrast, endothermic reactions absorb energy from their surroundings, making them feel cold. These include thermal decomposition (like limestone in a blast furnace), photosynthesis in plants, and certain types of electrolysis. The fizzing sensation in sherbet occurs due to an endothermic reaction!
The energy changes in reactions can be understood through bond energies - the energy needed to break bonds between atoms. When bonds break, energy is absorbed (endothermic process), and when new bonds form, energy is released (exothermic process). The overall energy change depends on which process requires more energy.
Remember this! In an exothermic reaction, more energy is released when forming new bonds than is absorbed when breaking existing bonds. The opposite is true for endothermic reactions.
Chemists visualize these energy changes using reaction profiles, which show the energy pathway from reactants to products. All reactions require some initial energy to get started - the activation energy - even if they ultimately release energy overall. This explains why even highly exothermic reactions like combustion need a spark to begin.
2
of 2
Chemical Cells and Fuel Cells
Have you ever wondered how batteries work? They're based on redox reactions where one metal loses electrons (oxidation) and another gains them (reduction). When two different metals are placed in a salt solution and connected by a wire, the more reactive metal donates electrons to the less reactive one, creating a flow of electric current.
In a simple cell, the voltage produced depends on the difference in reactivity between the metals. The greater this difference, the higher the voltage. This is why combining reactive magnesium with less reactive copper produces a higher voltage than pairing magnesium with zinc, which is closer to it in reactivity.
Fuel cells represent an exciting application of energy changes, particularly hydrogen fuel cells. Unlike traditional batteries, fuel cells can generate electricity continuously as long as fuel (hydrogen) is supplied. They work by oxidising hydrogen at the negative electrode while oxygen is reduced at the positive electrode, with water as the only waste product.
Think about this: Hydrogen fuel cells could revolutionize transportation with zero harmful emissions, but challenges remain in hydrogen production and storage.
Hydrogen fuel cells offer significant advantages - they don't need electrical recharging, produce no pollutants, and can be sized for different applications. However, they also present challenges: hydrogen is highly flammable, often produced using non-renewable sources, and difficult to store safely. Despite these challenges, fuel cell technology continues to advance as we seek cleaner energy solutions.
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Chemistry174 views·Updated 19 Jul 2026·2 pagesUnderstanding Energy Changes
Evelyn Ridley@ev_alice
Energy changes are at the heart of chemical reactions. When substances react, bonds break and form, leading to either the release or absorption of energy. Understanding these energy transfers helps explain why some reactions feel hot, others cold, and how...
1
of 2
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Exothermic and Endothermic Reactions
Ever noticed how some chemical reactions get hot? These are exothermic reactions which release thermal energy into their surroundings, causing temperature increases. Common examples include combustion (burning fuels), respiration in your cells, neutralisation reactions, and when metals react with acids.
In contrast, endothermic reactions absorb energy from their surroundings, making them feel cold. These include thermal decomposition (like limestone in a blast furnace), photosynthesis in plants, and certain types of electrolysis. The fizzing sensation in sherbet occurs due to an endothermic reaction!
The energy changes in reactions can be understood through bond energies - the energy needed to break bonds between atoms. When bonds break, energy is absorbed (endothermic process), and when new bonds form, energy is released (exothermic process). The overall energy change depends on which process requires more energy.
Remember this! In an exothermic reaction, more energy is released when forming new bonds than is absorbed when breaking existing bonds. The opposite is true for endothermic reactions.
Chemists visualize these energy changes using reaction profiles, which show the energy pathway from reactants to products. All reactions require some initial energy to get started - the activation energy - even if they ultimately release energy overall. This explains why even highly exothermic reactions like combustion need a spark to begin.
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Chemical Cells and Fuel Cells
Have you ever wondered how batteries work? They're based on redox reactions where one metal loses electrons (oxidation) and another gains them (reduction). When two different metals are placed in a salt solution and connected by a wire, the more reactive metal donates electrons to the less reactive one, creating a flow of electric current.
In a simple cell, the voltage produced depends on the difference in reactivity between the metals. The greater this difference, the higher the voltage. This is why combining reactive magnesium with less reactive copper produces a higher voltage than pairing magnesium with zinc, which is closer to it in reactivity.
Fuel cells represent an exciting application of energy changes, particularly hydrogen fuel cells. Unlike traditional batteries, fuel cells can generate electricity continuously as long as fuel (hydrogen) is supplied. They work by oxidising hydrogen at the negative electrode while oxygen is reduced at the positive electrode, with water as the only waste product.
Think about this: Hydrogen fuel cells could revolutionize transportation with zero harmful emissions, but challenges remain in hydrogen production and storage.
Hydrogen fuel cells offer significant advantages - they don't need electrical recharging, produce no pollutants, and can be sized for different applications. However, they also present challenges: hydrogen is highly flammable, often produced using non-renewable sources, and difficult to store safely. Despite these challenges, fuel cell technology continues to advance as we seek cleaner energy solutions.
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