Download Potential energy is energy of position or the energy possessed by

Chemical Reactions and Potential Energy (PE)
Chemical reactions involve the breaking and forming of chemical bonds.
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Endothermic reactions require energy (ex: bond breaking)
Exothermic reactions release energy (ex: bond forming)
Potential energy is energy of position or the energy possessed by something due to its position
relative to something else. Potential energy is abbreviated EP or PE. It is measured in Joules (J) or
kiloJoules (kJ).
For any chemical reaction, there is always a net change in chemical potential energy - it is either lost
or gained as a result of bond breaking and bond forming.
Kinetic energy is energy of motion. In chemistry, we are interested in the motion of atoms, ions and
molecules and the relationship between kinetic energy and potential energy. Kinetic energy is
abbreviated Ek or KE.
Temperature is an indicator of kinetic energy. The higher the temperature, the higher the KE.
In many circumstances, only some of the particles in a sample have enough kinetic energy to collide hard
enough to cause bond breaking. This critical kinetic energy level is called the
threshold energy.
The high energy molecules are the ones that
possess enough kinetic energy to result in the
breaking of old bonds when they collide. The
other molecules will just bounce off each other
when they collide.
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Heat of Reaction ( ∆H)
Can be written 2 ways:
1) Written next to the equation
> Positive number for endothermic reactions (heat is gained)
ex:
>
Negative number for exothermic reactions (heat is lost)
ex:
2) Written as part of the equation
> Written as a reactant for endothermic reactions (heat is absorbed)
ex:
>
Written as a product for exothermic reactions ( heat is produced/given off)
ex:
Potential Energy Diagrams
Potential Energy (PE) diagrams illustrate the potential energy changes that occur as reactants become
products during a chemical change.
It has five distinct regions:
1. The PE of the reactants
2. The PE gain that must take place in order for
old bonds to be stretched to the breaking point.
This is called the activation energy.
3. The PE of the transition state.
This is the activated complex. An activated
complex is a species formed at the transition
point of a collision. The species is highly
unstable - it can become reactants again or
turn into products.
4. The PE released as new bonds form during a
chemical change. This is the∆H.
5. The PE of the products
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Important Notes:
1.
Reactions with low activation energy are fast, while those with high activation energy are slow.
2.
The higher the activation energy, the slower the rate of the reaction.
If the potential energy of the reactants is greater than that of the products, the reaction is
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exothermic - it results in the net release of potential energy as heat.
If the potential energy of the reactants is less than that of the products, the reaction is
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endothermic - it results in the net gain of energy from some external source (e.g. the sun) which is then
stored in the products.
For exothermic reactions, the potential energy of the reactants is always greater than the
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potential energy of the products. The potential energy difference between the reactants and products is
the heat of reaction (∆H - negative).
For endothermic reactions, the potential energy of the reactants is always less than the potential
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energy of the products. The potential energy difference between the reactants and products is the heat
of reaction (∆H - postive).
For exothermic reactions, the larger the value of ∆H, the more stable the products are and the
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lower the likelihood of the reverse reaction occurring. This is because greater ∆H for the forward
reaction means a larger activation energy for the reverse reaction.
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Reversibility of Chemical Reactions
Many chemical reactions are reversible. For example, hydrogen and oxygen gas can react to form water
The formation of water is
exothermic, 286 kJ of heat is
released per mole of water formed.
By convention, the sign of this value
is negative, -286 kJ.
The decomposition of water is
endothermic, 286 kJ of heat is
absorbed per mole of water broken
down into hydrogen and oxygen gas.
By convention, the sign of this value
is positive, +286 kJ.
If the sign of the forward reaction is negative, then the sign of the reverse reaction is positive (and vice
versa).
Given two of the three labelled values in the diagram above, you can calculate the third.
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Sample Exercise 3
The heat of reaction for the formation of water is -286 kJ. Assume that the activation energy for the formation
of water from hydrogen and oxygen gas is 112 kJ.
Calculate the activation energy for the decomposition of water (i.e. the reverse reaction) using the information
provided in this diagram.
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