Chemical Energetics, Energy changes in reaction ,Exothermic and Endothermic reaction Reaction profile ,Activation energy Bond and energy change bond energy calculation

2. 6.CHEMICAL
ENERGETICS
6. Energy changes in reaction
6.2 Exothermic Reaction
6.3 Endothermic Reaction
6.4 Reaction Profile

3. ENERGY CHANGES IN REACTION
• WHEN CHEMICAL REACTIONS TAKE PLACE, THERMAL ENERGY IS TRANSFERRED TO OR
• FROM THE SURROUNDINGS. SOME REACTIONS MAKE THEIR SURROUNDINGS HOTTER
• AND SOME MAKE THEM COLDER.

4. EXOTHERMIC REACTION
• IN EXOTHERMIC REACTIONS, THERMAL ENERGY IS TRANSFERRED FROM THE CHEMICALS TO THE
SURROUNDINGS. AS THERE IS MORE THERMAL ENERGY, THE TEMPERATUREINCREASES AND SO IT GETS
HOTTER.
• MOST CHEMICAL REACTIONS ARE EXOTHERMIC.
• IN SOME EXOTHERMIC REACTIONS, ONLY A SMALL AMOUNT OF THERMAL ENERGY IS TRANSFERRED AND
THE TEMPERATURE MAY ONLY RISE BY A FEW DEGREES.
• IN SOME REACTIONS A LOT OF THERMAL ENERGY IS TRANSFERRED AND THE SURROUNDINGS GET VERY
HOT.
• SOMETIMES THERE IS SO MUCH THERMAL ENERGY TRANSFERRED THAT THE REACTANTS CATCH FIRE

5. EXOTHERMIC REACTION
OXIDATION REACTIONS E.G.
RESPIRATION
OXIDATION REACTIONS TAKE PLACE WHEN
SUBSTANCES REACT WITH OXYGEN. MANY
OXIDATION REACTIONS ARE
EXOTHERMIC. RESPIRATION TAKES PLACE IN
THE CELLS OF LIVING CREATURES AND IS
THE REACTION OF GLUCOSE WITH
OXYGEN. THERMAL ENERGY IS RELEASED IN
THIS REACTION. FOR EXAMPLE:
GLUCOSE + OXYGEN CARBON DIOXIDE
+ WATER
C6H12O6 + 6O2 6CO2 + 6H2O
COMBUSTION REACTIONS E.G. BURNING
FUELS
COMBUSTION REACTIONS TAKE PLACE
WHEN SUBSTANCES REACT WITH OXYGEN
AND CATCH FIRE. THEY ARE A TYPE OF
OXIDATION REACTION. FUELS BURNING,
SUCH AS METHANE (CH4) IN NATURAL
GAS, ARE COMBUSTION REACTIONS. THESE
REACTIONS ARE VERY EXOTHERMIC, RELEASE
A LOT OF THERMAL ENERGY AND CATCH
FIRE. FOR EXAMPLE:
METHANE + OXYGEN CARBON DIOXIDE
+ WATER
CH4 + 2O2 CO2 + 2H2O
NEUTRALISATION REACTIONS E.G.
ACIDS REACTING WITH ALKALIS
NEUTRALISATION REACTIONS TAKE PLACE
WHEN ACIDS REACT WITH BASES. MANY
NEUTRALISATION REACTIONS ARE
EXOTHERMIC. FOR EXAMPLE, WHEN
HYDROCHLORIC ACID REACTS WITH SODIUM
HYDROXIDE SOME THERMAL ENERGY IS
TRANSFERRED TO THE SURROUNDINGS AND
THE TEMPERATURE RISES BY A FEW DEGREES.
FOR EXAMPLE:
HCL+NAOH NACL+H2O

6. ENDOTHERMIC REACTION
• IN ENDOTHERMIC REACTIONS, THERMAL ENERGY IS TRANSFERRED FROM THE SURROUNDINGS TO
THE CHEMICALS. AS THERE IS LESS THERMAL ENERGY, THE TEMPERATURE DECREASES AND SO IT
GETS COLDER .
• ENDOTHERMIC REACTIONS ARE LESS COMMON THAN EXOTHERMIC REACTIONS.

7. ENDOTHERMIC REACTION
DECOMPOSITION, E.G.
THERMAL DECOMPOSITION OF METAL CARBONATES
GREEN COPPER CARBONATE DECOMPOSES INTO BLACK COPPER OXIDE WHEN HEATED DECOMPOSITION REACTIONS
OCCUR WHEN SUBSTANCES BREAKDOWN INTO SIMPLER SUBSTANCES. FOR EXAMPLE WHEN METAL CARBONATES
ARE HEATED THEY BREAK DOWN INTO A METAL OXIDE AND CARBON DIOXIDE.
DECOMPOSITION REACTIONS ARE ALWAYS ENDOTHERMIC. FOR EXAMPLE:
COPPER CARBONATE COPPER OXIDE + CARBON DIOXIDE
CUCO3 CUO + CO2
REACTION OF ACIDS WITH METAL HYDROGEN CARBONATES CITRIC ACID AND SODIUM
HYDROGENCARBONATE ARE IN SHERBET SWEETS AND REACT IN YOUR MOUTH IN AN ENDOTHERMIC REACTION
WHEN METAL HYDROGENCARBONATES (E.G. NAHCO3) REACT WITH ACIDS (E.G. CITRIC ACID), THE TEMPERATURE
DROPS AS THIS
IS AN ENDOTHERMIC REACTION. FOR EXAMPLE:
CITRIC ACID + SODIUM HYDROGENCARBONATE SODIUM CITRATE + CARBON DIOXIDE + WATER

8. REACTION PROFILE
REACTION PROFILES (SOMETIMES CALLED ENERGY LEVEL DIAGRAMS) ARE
GRAPHICAL REPRESENTATIONS OF THE RELATIVE ENERGIES OF THE
REACTANTS AND PRODUCTS IN CHEMICAL REACTIONS
• THE ENERGY OF THE REACTANTS AND PRODUCTS ARE DISPLAYED ON
THE Y-AXIS AND THE REACTION PATHWAY IS SHOWN ON THE X-AXIS
• ARROWS ON THE DIAGRAMS INDICATE WHETHER THE REACTION IS
EXOTHERMIC (DOWNWARDS POINTING) OR ENDOTHERMIC (UPWARDS
POINTING)
• THE DIFFERENCE IN HEIGHT BETWEEN THE ENERGY OF REACTANTS
AND PRODUCTS REPRESENTS THE OVERALL ENERGY CHANGE OF A
REACTION
• THE INITIAL INCREASE IN ENERGY REPRESENTS THE ACTIVATION
ENERGY REQUIRED TO START THE REACTION
• THE GREATER THE INITIAL RISE THEN THE MORE ENERGY THAT IS
REQUIRED TO GET THE REACTION GOING E.G., MORE HEAT NEEDED

9. 6 REACTION PROFILE
• ENERGY IS GIVEN OUT IN EXOTHERMIC REACTIONS
• THE ENERGY OF THE PRODUCTS WILL BE LOWER THAN THE ENERGY OF THE REACTANTS, SO THE CHANGE
IN ENERGY IS NEGATIVE
• THIS IS REPRESENTED ON THE REACTION PROFILE WITH A DOWNWARDS-ARROW AS THE ENERGY OF THE
PRODUCTS IS LOWER THAN THE REACTANTS
• ENERGY IS TAKEN IN ENDOTHERMIC REACTIONS
• THE ENERGY OF THE PRODUCTS WILL BE HIGHER THAN THE ENERGY OF THE REACTANTS, SO THE
CHANGE IN ENERGY IS POSITIVE
• THIS IS REPRESENTED ON THE REACTION PROFILE WITH AN UPWARDS-ARROW AS THE ENERGY OF THE
PRODUCTS IS HIGHER THAN THE REACTANTS

10. 7 ACTIVATION ENERGY
• THE MINIMUM ENERGY THE COLLIDING PARTICLES NEED IN
ORDER TO REACT IS KNOWN AS THE ACTIVATION ENERGY
• THE ACTIVATION ENERGY CAN BE LOWERED BY THE ADDITION
OF A CATALYST.THIS MEANS THAT A HIGHER PERCENTAGE OF
THE PARTICLES WILL HAVE THE MINIMUM ENERGY REQUIRED
TO REACT, HENCE THERE ARE A HIGHER NUMBER OF
SUCCESSFUL COLLISION
• THEREFORE MORE PRODUCT MOLECULES ARE PRODUCED IN A
SHORTER TIME, THUS INCREASING THE RATE OF REACTION
• REACTION PROFILE GRAPHS SHOW THE RELATIVE ENERGY
LEVELS OF REACTANTS AND PRODUCTS ON A GRAPH
• THE REACTION PROFILE GRAPH SHOWS THE EFFECT OF
CATALYSTS ON REACTIONS
Transfer of thermal energy
during a
reaction is called the enthalpy
change, ΔH, of the
reaction. ΔH is negative for
exothermic reactions
and positive for endothermic
reactions
• Catalysts provide the reactants another pathway which has a
lower activation energy
• By lowering Ea, a greater proportion of molecules in the
reaction mixture have sufficient energy for an successful
collision
• As a result of this, the rate of the catalysed reaction is
increased compared to the uncatalyzed reaction

11. 7 BOND AND ENERGY CHANGE
• Energy is needed to break bonds which is absorbed
from the reaction surroundings, so bond breaking is an
endothermic process
• The opposite occurs for forming bonds as it releases
energy back to the surroundings in an exothermic
process
• Both processes occur in the same chemical reaction, for
example, in the production of ammonia:
• N2 + 3H2 ⟶ 2NH3
• The bonds in the N-N and H-H molecules must be
broken which requires energy while the bonds in the
NH3 molecule are formed which releases energy
• Most reactions occur in a number of steps including
steps that are exothermic and steps that are
endothermic
• Whether a reaction is overall endothermic or
exothermic depends on the difference between the sum
of the exothermic steps and the sum of the
endothermic steps

12. 7 BOND ENERGY CALCULATION
• Each chemical bond has a specific bond energy associated with it
• This is the amount of energy required to break the bond or the
amount of energy given out when the bond is formed
• This energy can be used to calculate how much heat would be
released or absorbed in a reaction
• To do this it is necessary to know the bonds present in both the
reactants and products
• We can calculate the total change in energy for a reaction if we
know the bond energies of all the species involved
• Add together all the bond energies for all the bonds in the reactants
– this is the ‘energy in’
• Add together the bond energies for all the bonds in the products –
this is the ‘energy out’
• Calculate the energy change using the equation:
• Energy change = Energy taken in - Energy given out
H2 + Cl2 ⟶ 2HCl

13. THANK
YOU
CHEMISTRY WITH ANUM