Bonds
When chemical reactions happen, bonds in the
reactants (starting materials) break, and new
bonds form to make products (ending
materials).
Bonds and Energy
-Breaking bonds requires energy input, so bondbreaking is an endothermic change.
-Forming bonds releases energy, so bondforming is an exothermic change.
Total Energy Change
The overall or net change can be endothermic or
exothermic, depending on whether the total
energy input (required to break bonds in
reactants) or the total energy output (given off
when bonds form in products) is greater.
Activation Energy
Bonds must always be broken before new bonds
can form in the products.
Some initial amount of energy must be supplied
to the system in order for the reaction to begin.
This is represented as an activation barrier, or an
initial “bump” in the curve, to get from
reactants to products. The height of this bump,
measured from the reactant energy, is called the
activation energy (Ea) and it is always positive.
Energy Diagrams
The relative locations of the reactant (starting)
energy and product (ending) energy indicate
whether the overall energy change is
endothermic or exothermic.
Endothermic Reaction
If the energy of the
reactants is lower than
that of the products,
then the system
absorbs heat energy
(endothermic ), so the
overall energy change
is positive.
Exothermic Diagram
• If the starting point
(reactants) is higher
than the ending
point (products),
then the system
releases heat energy
(exothermic ), so the
overall energy
change is negative.
Catalyst
To make a reaction happen either:
1) More quickly or
2) With less energy
We add a catalyst to the solution/reaction
The catalyst is something that makes the reaction happen
more quickly but is not itself used up in the reaction.
In Mg + H2O we can add NaCl to make the process
happen more quickly, but we can use Fe to make it
happen REALLY quickly.
Iron in this case is the catalyst.