1. Title: Revision of Enthalpy
Learning Objectives:
By the end of the lesson I will be able……..
To understand and explain enthalpy
To describe the difference between endo and exothermic reactions
Be able to draw and understand Activation Energy and enthalpy profile diagrams
And be able to do enthalpy calculations

2. http://youtu.be/XgiCn1IpvzM Endo Exo thermic youtube song
Endo exothermic Song
Endo Exo thermic youtube song

4. Pub quiz answers
An exothermic reaction is when a chemical reaction releases or gives out heat. TRUE
Breaking a bond needs energy. TRUE
In enthalpy calculations which Greek letter symbolises standard states and measurements under standard conditions.
Θ, Δ, or Ω
The standard conditions in a reaction are 100kPa (1 atm) pressure and a temp of 273K (0οC) False
Enthalpy change ΔH is measured in KJ mol-1 TRUE
All combustion reactions are endothermic. FALSE
Is this a balanced equation for combustion of methane?
CH4(g) + 2O2(g) - CO2(g) + 2H20(l) TRUE

5. Write down an example of bond enthalpy using H­2 Answer H2(g) 2H (g)
If heat is lost by reacting chemicals is less than the amount and gained by the surroundings then ΔH is negative. TRUE
The quantity of energy needed to break a particular bond in a molecule is called bond enthalpy. TRUE
Write down an example of bond enthalpy using H­2
Answer H2(g) 2H (g)
If you need more energy to break the bonds than is released when bonds are formed then ΔH is negative ? FALSE
This is an enthalpy profile diagram for an endothermic reaction. FALSE
The activation energy is the largest amount of energy needed to break bonds and start a chemical reaction. FALSE it’s the smallest
Standard enthalpy change of formation ΔHf Θ is the enthalpy change when 1 mole of an element is formed from its compounds in their standard states under standard conditions. False

6. Demo
Endothermic and Exothermic
Endo: requires heat energy
Exo: releases heat energy

7. Activity 1: Ice melting: What is causing this ice cube to melt
Activity 1: Ice melting: What is causing this ice cube to melt? Is this an endothermic or exothermic reaction?
Is heat energy being absorbed from the surroundings (endo) causing the ice to melt? Or
is heat energy given out from the ice cube (exo) to the surroundings causing the ice to melt?

8. So to break a bond…….

10. Mini whiteboard activity To break a bond………….
We need energy to break a bond!
So is this an endo or exothermic reaction?
…………………………………………………………….

11. When a bond is made…………. Energy is released or given out!
1 Nitrogen molecule
Energy is released or given out!
Use your data book to calculate the bond energies of the above; (H-H), (N-N), (N-H)
So is this an endo or exothermic reaction?
…………………………………………………………….

12. What is enthalpy (H)?
Heat content stored in a chemical system

13. What do these symbols mean?
Δ means ……………
H means ……………
Ѳ means …………..
Enthalpy is measured in …………..

14. AFL: To break a bond………….
So is (∆H) POSITIVE OR NEGATIVE?
…………………………………………………………….

15. Bond energies =The energy needed to break a chemical bond
Different chemical bonds have different bond energies
Do activity 1 15

16. Activity 1
Work out the enthalpy change (∆H) for this reaction
2H2 + O2 = 2H2O
Hint: Enthalpy change of reaction = products - reactants
Is it endothermic or exothermic reaction?
See if you can sketch an energy profile diagram

17. Examples of Exothermic reactions
Oxidation and neutralisation reactions are all exothermic reactions.
-they give out heat energy
so ∆H is positive or negative?

18. AFL: When a bond is formed in a reaction………….
1 Nitrogen molecule
Energy is released or given out!
So is (∆H) POSITIVE OR NEGATIVE?
………………………………….
activity 2

19. Activity 2 REACTANTS -> PRODUCTS
HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l) Energy of the PRODUCTS = kJ
Energy of the REACTANTS = kJ
What is the enthalpy change ΔH of this reaction?
Give the units…..
Is this reaction endothermic or exothermic?
Draw an energy profile diagram
Hint: Enthalpy change of reaction = products - reactants

20. Look at the figures on the Y-axis to explain why this is
Energy Profile Diagram
Look at the figures on the Y-axis to explain why this is
an endothermic reaction.
Write down your explanation using words: bond breaking,
bond forming, energy

21. Endo or Exo

22. Endo or Exo
activity 3

23. Activity 3: Fill in the following words on the diagram
Enthalpy
reactants, products, activation energy arrow, ΔH arrow
+ΔH/ – ΔH, correct units for enthalpy

24. What’s the difference between these two diagrams?

25. Activity 4 Sketch an enthalpy profile diagram for this reaction
CO (g) + NO2 -> CO2(g) + NO(g) ΔH = -226kJ mol-1 Ea = 134kJ mol-1 Show activation energy arrow and ΔH arrow State whether it’s an endo or exothermic reaction.

26. 2H2 + O2 = 2H2O Activity 1 answers
Work out the enthalpy change (∆H) for this reaction
2H2 + O2 = 2H2O
2X (436)  2 X (464) ∆H = +442 kj mol -1
Is it an endothermic reaction

27. REACTANTS -> PRODUCTS
Activity 2 answers
REACTANTS > PRODUCTS
HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l) Energy of the PRODUCTS = kJ
Energy of the REACTANTS = kJ
What is the enthalpy change ΔH of this reaction? kJ mol -1
It is an exothermic reaction
energy profile diagram

28. Activity 3 answer: Fill in the following words on the diagram
Enthalpy kJ mol -1
reactants
– ΔH
products
reactants, products, activation energy arrow, ΔH arrow
+ΔH/ – ΔH, correct units for enthalpy

29. Activity 4 answers Sketch an enthalpy profile diagram for this reaction
CO (g) + NO2 -> CO2(g) + NO(g) ΔH = -226kJ mol-1 Ea = 134kJ mol-1 Show activation energy arrow and ΔH arrow State whether it’s an endo or exothermic reaction.

30. AFL: thumbs up/thumbs down
How confident do I feel about…………….
I can explain enthalpy to the person beside me
I understand the difference between endo and exothermic reactions
I can draw Activation Energy arrows and ∆H arrows on energy profile diagrams

31. Enthalpy Change of Combustion ΔHcѳ
Fill in the blanks CH4(g) + …….. (g) -> …… (g) + …… (l) Do you think combustion reactions are endothermic or exothermic? ……………
ΔHcѳ - the enthalpy change of combustion when one mole of substance is completely combusted.
CH4(g) + 2O2 (g) -> …CO2… (g) + 2H2O (l)
ΔHcѳ is exothermic -890kJ mol-1 31

32. Standard Enthalpy Change of Reaction ΔHѳr
Zn(s) + CuSO4(aq) -> ZnSO4(aq) + Cu(s) ΔHѳr is the enthalpy change when one mole of solid zinc reacts with 1 mole of copper sulphate (of concentration 1 mol dm-3) at 298K and 100kPa (1atm)

33. Standard Enthalpy Change of Combustion ΔHѳc
Is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.