1. Energetics
Topic 5.1 – 5.2

2. Topic 5.1 Exothermic and Endothermic Reactions? ?

3. total energy of the universe is a constant
if a system loses energy, it must be gained by the surroundings, and vice versa

4. H Enthalpy a measure of the heat content (not temp)
you cannot measure the actual enthalpy of a substance
but you can measure an enthalpy CHANGE because of energy it takes in or releases
 = Greek letter ‘delta’ meaning change
H = heat.
so, H means ‘heat change’.

5. Why a standard? enthalpy values vary according to the conditions
a substance under these conditions is said to be in its standard state
pressure: 100 kPa (1 atmosphere)
temperature: usually 298K (25°C)
if these were not standardized, then energy could be leaving or entering the system
modify the symbol from
Enthalpy Change Standard Enthalpy Change
(at 298K)

6. Enthalpy (Heat) of Reaction
H = Hproducts − Hreactants
lower energy is more stable 6

7. Exothermic reactions
heat energy is given out by the reaction hence the surroundings increase in temperature (feels hot)
occurs when bonds are formed
new products are more stable and extra energy is given off
Hproducts < Hreactants
H is negative
examples
combustion of fuels
respiration
neutralization reactions (acid reacts with something)

8. energy given out, ∆H is negative
activation energy
energy necessary to get the reaction going
REACTION CO-ORDINATE
ENTHALPY
energy given out, ∆H is negative
reactants
products

9. H2 + Cl2  2HCl H, H, Cl, Cl (Atoms) H-H, Cl-Cl Reactants H-Cl, H-Cl
Energy taken in to break bonds.
H, H, Cl, Cl (Atoms)
Energy given out when bonds are made.
H-H, Cl-Cl
energy
Reactants
Overall energy change, H
H-Cl, H-Cl
Products

10. H2 + Cl2  2HCl H, H, Cl, Cl (Atoms) H-H, Cl-Cl Reactants H-Cl, H-Cl
Energy in
= +678kJ
H, H, Cl, Cl (Atoms)
Energy out = -862kJ
H-H, Cl-Cl
energy
Reactants
Overall energy change,
H = -184kJ
H-Cl, H-Cl
Products

11. Endothermic reactions
heat energy is taken in by the reaction mixture hence the surroundings decrease in temperature (feels cold)
occurs when bonds are broken
the reactants were more stable (bonds are stronger)
therefore, took energy from the surrounding to break bonds
Hreactants < Hproduct
H is positive
examples

12. energy taken in, ∆H is positive
activation energy
energy necessary to get the reaction going
REACTION CO-ORDINATE
ENTHALPY
energy taken in, ∆H is positive
products
reactants

14. 14

15. Endothermic reactions
Summary Table
Exothermic reactions
Endothermic reactions
Energy is given out to the surroundings
Energy is taken in from the surroundings
∆H is negative
∆H is positive
Products have less energy than reactants
Products have more energy than reactants

16. Topic 5.2 Calculation of enthalpy change
Notice less sources of error here compared to our lab…

17. calorimetry calorimeter heat temperature measurement of heat flow
device used to measure heat flow
heat
energy that is transferred from one object to another due to a difference in temperature
measures total energy in a given substance
temperature
a measure of the average kinetic energy of a substance regardless how much is there

18. 100ml of water contains twice the heat of 50 ml.
Temperature vs. Heat
50 ml water
100 C
100 ml water
100C
100ml of water contains twice the heat of 50 ml.

19. Heat Capacity/Specific Heat
the amount of energy a substance absorbs depends on:
mass of material
temperature
kind of material and its ability to absorb or retain heat.
heat capacity
the amount of heat required to raise the temperature of a substance 1 oC (or 1 Kelvin)
molar heat capacity
the amount of heat required to raise the temperature of one mole 1 oC (or 1 Kelvin)
specific heat
the amount of heat required to raise the temperature of 1 gram of a substance 1 oC (or 1 Kelvin) 19

20. Specific Heat (c) values for Some Common Substances
J g-1 K-1
Water (liquid)
4.184
Water (steam)
2.080
Water (ice)
2.050
Copper
0.385
Aluminum
0.897
Ethanol
2.44
Lead
0.127 or
kJ kg-1 K-1
if multiply by 1000 20

21. Heat energy change q = m c DT
q = change in heat (same as DH if pressure held constant)
m = mass in grams or kilograms
c = specific heat in J g-1 K-1 or kJ kg-1 K-1
(or Celsius which has same increments as Kelvin)
DT = temperature change

22. Measuring the temperature change in a calorimetry experiment can be difficult since the system is losing heat to the surroundings even as it is generating heat.
By plotting a graph of time vs. temperature it is possible to extrapolate back to what the maximum temperature would have been had the system not been losing heat to the surroundings.

23. Heat Transfer Problem 1
Calculate the heat that would be required to heat an aluminum cooking pan whose mass is grams, from 20.5oC to 201.5oC. The specific heat of aluminum is J g-1 oC-1.
q = mcDT
= (402.5 g) (0.902 J g-1 oC-1)(181.0oC)
= 65, J
= 65,710 J with correct sig. figs.
only 4 sig. figs.

24. Heat Transfer Problem 2 Let T = final temperature
What is the final temperature when grams of water at 20.5oC is added to grams water at 60.5oC? Assume that the loss of heat to the surroundings is negligible. The specific heat of water is J g-1 oC-1
Solution: Dq (Cold) = Dq (hot) so… mCDT = mCDT
Let T = final temperature
(50.15 g) (4.184 J g-1 oC-1)(T oC)
= (80.65 g) (4.184 J g-1 oC-1)(60.5oC- T)
(50.15 g)(T oC) = (80.65 g)(60.5oC- T)
50.15T = – 80.65T
130.80T = 5830
T = oC

25. Heat Transfer Problem 3
On complete combustion, 0.18g of hexane raised the temperature of 100.5g water from 22.5°C to 47.5°C. Calculate its enthalpy of combustion.
Heat absorbed by the water…
q = mcDT
q = (4.18) (25.0) = 10,500 J which is same as 10.5 kJ
Moles of hexane burned = mass / molar mass
= g / 86 g/mol
= moles of hexane
Enthalpy change means find heat energy / mole
= kJ/ mol
= kJ mol -1 or 5.0 x kJ mol -1
hexane is C6H14