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Energy Changes (Exothermic and endothermic reactions (Energy changes in…
Energy Changes
Exothermic and endothermic reactions
Required Practical
Reaction Profiles
Endothermic
A reaction is endothermic if less heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants
It takes in energy from the surroundings so the temperature of the surroundings decreases
Product molecules must have more energy than reactants
Examples of endothermic reactions include:
thermal decomposition reactions
E.g. heating calcium carbonate causes it to decompose into calcium oxide (also called quicklime) and carbon dioxide.
the reaction of citric acid and sodium hydrogencarbonate
Endothermic reactions are used in some sports injury packs - the chemical reaction allows the pack to become instantly cooler without having to put it in the freezer.
Energy changes in reactions & Bond energies
Energy is transferred when bonds are broken or are formed.
During a chemical reaction:
bonds in the reactants are broken
Breaking bonds - Energy is taken in
new bonds are made in the products
Forming bonds - Energy is given out
Why is heat released or absorbed in a chemical reaction?
When reactant molecules have stronger bonds, more energy is absorbed from the surroundings to break them. When product molecules have weaker bonds than the reactant molecules, little energy is released to the surroundings when the product molecules form.
When reactant molecules have weaker bonds, little energy is absorbed from the surroundings to break them. When product molecules have stronger bonds than the reactant molecules, more energy is released to the surroundings when the product molecules form.
Molecules inherently want to stay together, so formation of chemical bonds between molecules requires less energy as compared to breaking bonds between molecules, which requires more energy and results in heat being absorbed from the surroundings.
A bond energy is the amount of energy needed to break one mole of a particular covalent bond.
The energy needed to break bonds and energy released when bonds are formed can both be calculated from bond energies
Sum of energy taken in to break bonds – sum of energy released to
form bonds = overall energy change
● Energy taken in to break > energy released when formed = ENDOTHERMIC (because overall energy has been taken in)
● Energy taken in to break < energy released when formed = EXOTHERMIC (because overall energy has been released)
If heat is absorbed during the reaction, ΔH is positive; if heat is released, then ΔH is negative.
Exothermic
A reaction is exothermic if more heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants
It transfers energy to the surroundings so the temperature of the surroundings increases.
Product molecules must have less energy than the reactants, by the amount transferred.
Examples of exothermic reactions include:
combustion reactions
many oxidation reactions
most neutralisation reactions
Some hand warmers use the exothermic oxidation of iron in the air ( with a salt solution catalyst) to release energy
Self heating cans also rely on exothermic reactions between chemicals in their bases.
Chemical cells and fuel cells
Cells and Batteries/Chemical Cells
Fuel Cells