Energy Changes
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Energy transfer in reactions
Energy is conserved in chemical reactions — it is transferred between the reacting substances and their surroundings, not created or destroyed.
This principle underpins all energy change calculations at GCSE.
Energy transfer in reactions — Key Knowledge
- Conservation of energy energy cannot be created or destroyed
- Energy transfer between chemicals and surroundings
Exothermic reactions
Exothermic reactions transfer energy to the surroundings, causing a temperature increase.
Hand warmers and self-heating cans rely on exothermic reactions.
Exothermic reactions — Key Knowledge
- Exothermic energy transferred to surroundings
- Combustion burning fuels
- Neutralisation acid + alkali
- Oxidation e.g. rusting
Endothermic reactions
Endothermic reactions take in energy from the surroundings, causing a temperature decrease.
Sports cold packs use an endothermic dissolving reaction to cool an injury.
Endothermic reactions — Key Knowledge
- Endothermic energy absorbed from surroundings
- Thermal decomposition breaking down by heating
- Citric acid + sodium hydrogencarbonate classic endothermic example
Reaction profiles
A reaction profile diagram shows the relative energies of reactants and products, with the overall energy change visible as the difference between them.
The vertical axis represents energy; the horizontal axis represents the progress of the reaction.
Reaction profiles — Key Knowledge
- Exothermic profile products lower than reactants
- Endothermic profile products higher than reactants
- Overall energy change difference between reactant and product energy levels
Activation energy
Activation energy is the minimum energy needed for a reaction to start. It is shown as the height of the "hump" on a reaction profile.
A spark lighting a fuel is providing the activation energy — the reaction then releases far more energy than was put in.
Activation energy — Key Knowledge
- Activation energy minimum energy to start a reaction
- Energy barrier must be overcome even in exothermic reactions
Bond energy calculations (4.5.1) [HT]
Breaking bonds requires energy (endothermic). Forming bonds releases energy (exothermic). The overall energy change equals the total energy to break bonds minus the total energy released forming bonds.
Energy change = sum of bonds broken − sum of bonds formed
If more energy is released forming new bonds than is needed to break old bonds, the reaction is exothermic overall.
Bond energy calculations (4.5.1) [HT] — Key Knowledge
- Bond breaking requires energy input
- Bond forming releases energy
- Overall energy change energy in minus energy out
Chemical cells
A chemical cell produces a potential difference (voltage) by placing two different metals in an electrolyte.
The metals must be different — two identical electrodes produce zero potential difference.
Chemical cells — Key Knowledge
- Chemical cell two different metals + electrolyte → voltage
- Reactivity difference greater difference between metals → greater voltage
- Same metals no voltage produced
Batteries
A battery consists of two or more chemical cells connected in series to provide a higher voltage.
Everyday "batteries" (e.g. AA) are technically single cells; a 9V block contains multiple cells.
Batteries — Key Knowledge
- Battery two or more cells in series
Hydrogen fuel cells
A hydrogen fuel cell reacts hydrogen with oxygen to produce electricity, with water as the only by-product.
Fuel cells need a continuous supply of fuel, unlike batteries which store a fixed amount of energy.
Hydrogen fuel cells — Key Knowledge
- Hydrogen fuel cell hydrogen + oxygen → water + electricity
- By-product water only — no pollutant gases at point of use
Fuel cells — advantages and disadvantages
Fuel cells offer high efficiency and no pollutant gases at point of use, but face practical challenges around hydrogen production and storage.
The "clean" label only applies at point of use — the full life cycle depends on how the hydrogen is produced.
Fuel cells — advantages and disadvantages — Key Knowledge
- Advantages no pollutant gases, high efficiency, continuous operation
- Disadvantages hydrogen difficult to store, most hydrogen from fossil fuels, expensive to manufacture
- Comparison with rechargeable batteries fuel cells don't need recharging but need constant fuel supply