Describe how a chemical
reaction happens
Enthalpy Change and Exothermic
and Endothermic Reactions
OBJECTIVES
•Demonstrate an understanding of the
term enthalpy change, ΔH
•Construct simple enthalpy level
diagrams showing the enthalpy change
•Recall the sign of ΔH for exothermic
and endothermic reactions
OCR AS Chemistry p78-79
Enthalpy Change in Exothermic and
Endothermic Reactions
Chemical reactions can either release
energy to their surroundings,
EXOTHERMIC or energy can be
__________,
transferred to them from the
Video Clip
ENDOTHERMIC
surroundings, ____________.
KEY IDEA
Bonds contain energy. You need to put in energy to
break bonds apart in the REACTANTS in a chemical
reaction. Energy is released when new bonds form in
the PRODUCTS in a chemical reaction. When bonds
break, energy is absorbed (endothermic). When bonds
form, energy is released (exothermic).
Energy Level Profile Diagrams 1
Ooh look –
-ve
EXOTHERMIC
The energy released
on bond formation in
the products is
greater than that
absorbed through
breaking bonds in the
reactants
Energy of reactants > energy of products
These are the most common type of reaction eg.
Combustion, neutralisation
Example
Examples of Exothermic Reactions
•
•
•
•
•
•
Combustion
Neutralisation
Freezing
Condensation
Displacement
Crystallising liquid salts
Energy Level Profile Diagrams 2
ENDOTHERMIC
Ooh look +ve
The energy needed to
be absorbed to break
bonds in the reactants
is greater than the
energy transferred to
the surroundings as
bonds are made in the
products
Energy of reactants < energy of products
These are less common chemical reactions
eg. Photosynthesis
Example
Examples of Endothermic Reactions
Melting
Boiling and Evaporation
Photosynthesis
Making an anhydrous salt from a
hydrate
• Forming a cation (+ve ion) from an
atom in the gas phase
•
•
•
•
Which of the following is true
when chemical bonds are
broken?
A.The process is exothermic
B. Energy is given out
C. A physical change is occurring
D.The reaction will have a H that
is positive(+).
OCR AS Chemistry p79
Answer Questions 1, 2
10 Minute Exercise
KEY IDEAS
Enthalpy is the total energy content of the
reacting materials. It is given the symbol, H.
Enthalpy change is the term used to
describe the energy exchange that takes
place with the surroundings at a constant
pressure and is given the symbol ΔH.
ΔH = ΔH products - ΔH reactants
The units are kilojoules per mole (kJmol-1)
EXOTHERMIC
An exothermic
enthalpy change is
always given a
negative value, as
energy is lost to
the surroundings.
ΔH = -kJmol-1
ΔH = ΔH products - ΔH reactants
Small number
Bigger number
ENDOTHERMIC
An endothermic
enthalpy change is
always given a
positive value, as the
energy is gained by
the system from the
surroundings.
ΔH = + kJmol-1.
ΔH = ΔH products - ΔH reactants
Big number
Small number
Using this in everyday life
Heat Packs Video
Balance each of the equations below
Name the compounds or elements
present in each one
Identify each one as exothermic or
endothermic.
CH4(s) + O2(g) → CO2(g) + H2O(l) ΔH = -890 kJmol-1
HCl(g) → H2(g) + Cl2(g)
ΔH = 185 kJmol-1
NH3(g) + O2(g) → NO(g) + H2O(l) ΔH = -1169 kJmol-1
ANSWERS
CH4(s) + 2O2(g) → CO2(g) + 2H2O(l)
Methane
Oxygen
Carbon
dioxide
Water
2HCl(g) → H2(g) + Cl2(g)
Hydrogen
chloride
Hydrogen
EXOTHERMIC
ENDOTHERMIC
Chlorine
4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(l) EXOTHERMIC
Ammonia
Oxygen
Nitrogen
monoxide
Water
For each of the reactions above,
construct a simple enthalpy level diagram
showing the enthalpy change.
CH4(s) + 2O2(g)
ΔH =-890 kJmol-1
CO2(g) + 2H2O(l)
H2(g) + Cl2(g)
ΔH = +185 kJmol-1
HCl(g)
NH3(g) + O2(g)
ΔH =-1169 kJmol-1
NO(g) + H2O(l)