1. Describe how a chemical reaction happens

2. 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

3. Enthalpy Change in Exothermic and Endothermic Reactions
Chemical reactions can either release energy to their surroundings, __________, or energy can be transferred to them from the surroundings, ____________.
EXOTHERMIC
ENDOTHERMIC
Video Clip
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).

4. Energy Level Profile Diagrams 1
EXOTHERMIC
The energy released on bond formation in the products is greater than that absorbed through breaking bonds in the reactants
Ooh look –
-ve
Energy of reactants > energy of products
These are the most common type of reaction eg. Combustion, neutralisation
Example

5. Examples of Exothermic Reactions
Combustion
Neutralisation
Freezing
Condensation
Displacement
Crystallising liquid salts

6. Energy Level Profile Diagrams 2
ENDOTHERMIC
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
Ooh look - +ve
Energy of reactants < energy of products
These are less common chemical reactions eg. Photosynthesis
Example

7. 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

8. Which of the following is true when chemical bonds are broken?
The process is exothermic
Energy is given out
A physical change is occurring
The reaction will have a H that is positive(+).

9. OCR AS Chemistry p79
Answer Questions 1, 2 10 Minute Exercise

10. 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)

11. ΔH = ΔH products - ΔH reactants
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

12. ΔH = ΔH products - ΔH reactants
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

13. Using this in everyday life
Heat Packs Video

14. 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 = kJmol-1

15. ANSWERS CH4(s) + 2O2(g) → CO2(g) + 2H2O(l) EXOTHERMIC
2HCl(g) → H2(g) + Cl2(g)
4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(l)
EXOTHERMIC
Methane
Oxygen
Carbon dioxide
Water
ENDOTHERMIC
Hydrogen chloride
Hydrogen
Chlorine
EXOTHERMIC
Ammonia
Oxygen
Nitrogen monoxide
Water
For each of the reactions above, construct a simple enthalpy level diagram showing the enthalpy change.

16. CH4(s) + 2O2(g)
ΔH =-890 kJmol-1
CO2(g) + 2H2O(l)

17. H2(g) + Cl2(g)
ΔH = +185 kJmol-1
HCl(g)

18. NH3(g) + O2(g)
ΔH =-1169 kJmol-1
NO(g) + H2O(l)