Energy Changes in Chemical Reactions
Energy changes
Exothermic reactions
Examples
: respiration, combustion, oxidation, neutralisation
•
Energy ‘exits’ reaction – heats surroundings
•
Thermometer readings rises
Uses:
self heating can and hand warmers
Energy changes
Endothermic reaction
Examples
: photosynthesis, thermal decomposition, reaction of citric acid
and sodium hydrogencarbonate
•
Energy ‘enters’ reaction (takes in energy) – cools surroundings
•
Thermometer readings fall
Uses:
sports injury packs
1.
measure volume of hydrochloric acid in a
measuring cylinder
2.
pour hydrochloric acid into suitable
container
3.
measure temperature of hydrochloric acid
using a thermometer
4.
measure mass of magnesium using a
balance
5.
add magnesium to hydrochloric acid and
stir
6.
measure
maximum temperature reached
7.
calculate the change / rise in temperature
8.
repeat procedure for same mass of
magnesium
9.
find mean temperature rise
10.
repeat with different masses of magnesium
R
e
a
c
t
i
o
n
p
r
o
f
i
l
e
s
Bond Breaking and Bond making
•
For a reaction to happen bonds have to be broken to then form new ones
H
BENDOMEX
•
Bendo = Breaking is endo
•
Mex = Making is exo
BENDOMEX
•
Where the
energy from bond forming exceeds that needed for bond
breaking
the reaction is
exothermic
.
•
Where the
energy for bond breaking exceeds that from bond forming
the reaction is
endothermic
.
How does a battery operate?
•
Two metals of
different
reactivity
•
An electrolyte
(salt solution)
•
Wires
•
The
most reactive
metal always
forms the
negative
electrode.
•
The electrons
flow
from
the
most reactive
metal
to
the
least
reactive
metal
If magnesium is
losing
electrons and
copper is
gaining
electrons, which
metal is being ‘reduced’ and which
metal is being ‘oxidised’
Remember
OILRIG
(oxidation is loss,
reduction is gain)
The bigger the difference in
reactivity between the
metals, the higher the voltage
of the battery
2H
2
4H
+
+ 4e
-
O
2
+ 4H
+
+ 4e
-
2H
2
O
Hydrogen goes through a catalyst
and splits in protons and
electrons. The protons (H+) move
through the electrolyte and the
electrode move round the
electrode and meet up on the
other side
Oxygen gas reacts with (is reduced) by
the hydrogen and the electrons to
produce water.
Overall reaction =
H
2
+ O
2
H
2
O
Exothermic reactions release energy to the surroundings, exhibited in processes like respiration and combustion. On the other hand, endothermic reactions absorb energy, demonstrated in examples such as photosynthesis. By observing changes in temperature and reactions between various substances, one can explore the energy dynamics of different chemical reactions. Bond breaking and forming play a crucial role in the overall energy change during reactions, highlighting the principles of exothermic and endothermic processes.
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Presentation Transcript
Energy changes Exothermic reactions Examples: respiration, combustion, oxidation, neutralisation Energy exits reaction heats surroundings Thermometer readings rises Uses: self heating can and hand warmers
Energy changes Endothermic reaction Examples: photosynthesis, thermal decomposition, reaction of citric acid and sodium hydrogencarbonate Energy enters reaction (takes in energy) cools surroundings Thermometer readings fall Uses: sports injury packs
1. measure volume of hydrochloric acid in a measuring cylinder pour hydrochloric acid into suitable container measure temperature of hydrochloric acid using a thermometer measure mass of magnesium using a balance add magnesium to hydrochloric acid and stir measure maximum temperature reached calculate the change / rise in temperature repeat procedure for same mass of magnesium find mean temperature rise repeat with different masses of magnesium 2. 3. 4. 5. 6. 7. 8. 9. 10.
Reaction profiles Reaction profiles
Bond Breaking and Bond making For a reaction to happen bonds have to be broken to then form new ones H H H H C O O O O Bond Breaking Bond Forming Energy in chemicals H O O C O O O H H H H H H O O C O H H Progress of reaction
BENDOMEX Bendo = Breaking is endo Mex = Making is exo
BENDOMEX Where the energy from bond forming exceeds that needed for bond breaking the reaction is exothermic. Where the energy for bond breaking exceeds that from bond forming the reaction is endothermic. Exo Endo Bonds break Bond forming Energy in chemicals Energy in chemicals Bonds form Bonds break products reactants H H reactants products
How does a battery operate? If magnesium is losing electrons and copper is gaining electrons, which metal is being reduced and which metal is being oxidised Two metals of different reactivity The most reactive metal always forms the negative electrode. The electrons flow from the most reactive metal to the least reactive metal An electrolyte (salt solution) Wires The bigger the difference in reactivity between the metals, the higher the voltage of the battery Remember OILRIG (oxidation is loss, reduction is gain)
Hydrogen goes through a catalyst and splits in protons and electrons. The protons (H+) move through the electrolyte and the electrode move round the electrode and meet up on the other side Oxygen gas reacts with (is reduced) by the hydrogen and the electrons to produce water. 2H2 4H+ + 4e- O2 + 4H+ + 4e- 2H2O Overall reaction = H2 + O2 H2O
