1. Chapter 5 Notes: Energetics Thermochemistry
Chapter 5.1: The enthalpy changes from chemical reactions can be calculated from their effect on the temperature of their surroundings.

2. Important terms for this section
Energy
Work
System
Surroundings
Open and closed systems
Enthalpy
Endothermic
Standard enthalpy changes
Absolute zero
Kelvin scale
Heat capacity

3. Energy and heat transfer energy
Energy: measured in Joules = kg*m2/s2 = N*m
Ability to do work
Ability to move an object against an opposing force
Some forms
Heat, Light, Sound, Electricity, Chemical energy
Focus is heat in this unit = E transfer with Temp diff.
Inc. heat = inc. ave. K.E. of molecules in a disordered fashion

4. System and surroundings
System: area of interest
Surroundings: everything else
Open system
Can exchange energy AND matter with surroundings
Closed system
Can exchange energy ONLY with surroundings
Total Energy cannot change during process only exchange

5. Enthalpy Enthalpy “heat inside”: heat content of a system
Changes in enthalpy is noted: ΔH
If heat is added to a system, enthalpy increases (is positive)
If heat is released from system, enthalpy decreases (negative)

6. Exothermic & endothermic
Exo = release (external) exothermic releases heat
Might feel hot when reacting
Giving out heat means a negative enthalpy change
Ereactants > Eproducts
Endo = take in endothermic needs heat
Might feel cold when reacting
Heat added to the system means a positive enthalpy change
Ereactants < Eproducts

7. Standard enthalpy changes
Symbol for standard enthalpy change: ΔHѳ
Conditions
Pressure of 100 kPa
Concentration = 1 mol dm-3 for all solutions
All substances in their standard states
Temp is not part of definition, but is typically 298K

8. Thermochemical equations
Combustion of methane:
Releases 890 kj mol-1 of heat energy
Photosynthesis:
Absorbs kj mol-1 of energy

9. Kelvin and kinetic energy
Average kinetic energy of molecules related to Kelvin
Absolute zero
Absolute temp directly proportional to average KE particles
If same amt of heat is added to 2 different objects, will temp change be the same?
Heat changes depend on
Mass of object
Heat added
Nature of substance

10. Heat changes continued
Specific heat capacity
Amount of heat needed to increase the temp of unit mass by 1K
Depends on
Number of particles present in the sample
And therefore the mass of individual particles
Heat capacity
Amount of heat needed to increase temp of an object 1K

11. Heat capacities
So, to recap:

12. Enthalpy and direction of change
Energy moves from higher to lower stored energy
And might look different than you assume 

13. Measuring/calculating Enthalpy changes
Measuring enthalpy changes of Combustion, ΔHcѳ
The enthalpy change for the complete combustion of 1 mole of a substance in its standard state in excess oxygen under STP.
Calorimetry

14. Measuring/calculating Enthalpy changes
Calculating ΔH from Temperature changes
For Exothermic RXN
Temp of H2O increases
ΔH is negative
For Endothermic RXN
Temp of H2O decreases
ΔH is postitive

15. Measuring/calculating Enthalpy changes
ΔHsol of RXN in solution
The enthalpy change when 1 mole of solute is dissolved in excess solvent to form a solution of ‘infinite dilution’ under std. conditions
The results are not always what you expect
Loss of heat to environment/
materials/etc. vs water

16. Definitions ΔHѳ standard enthalpy change
Change in enthalpy under std. conditions 100kPa, 298K
ΔHѳr standard enthalpy change of reaction
Enthalpy change when molar amounts of reactants, as shown in a stoichiometric equation react together under std. conditions to give products
ΔHѳc standard enthalpy change of combustion
Enthalpy change when 1 mole of a substance is completely burnt in oxygen under std. conditions

17. definitions ΔHn enthalpy change of neutralization
Enthalpy change when 1 mole of water molecules are formed when an acid (H+) reacts with an alkali (OH-) under std. conditions
ΔHsol of RXN in solution
The enthalpy change when 1 mole of solute is dissolved in excess solvent to form a solution of ‘infinite dilution’ under std. conditions
c specific heat capacity
The energy required to raise the temperature of 1kg (or 1g) of a substance by 1K (1ºC)
q heat energy
How much heat energy must be supplied to raise the temp of mass m by ΔTºC
q=mc ΔT

18. Problem 1
Using the data, determine the ΔHѳc of ethanol (C2H5OH) given that the cH2O = 4.18 J g-1 ºC-1 (remember c is specific heat capacity)
Mass of water = g
Initial temp of water = 19.5 ºC
Maximum temp of water = 45.7 ºC
Initial mass of spirit burner = g
Final mass of spirit burner = g
REMEMBER: ΔHѳc is the enthalpy change when 1 mole of a substance is completely burnt in oxygen under std. conditions

19. Problem 1 solution
So if ΔHѳc is the enthalpy change when 1 mole of a substance is completely burnt in oxygen under std. conditions
We need to find:
The heat energy change (how many Joules are being produced)
How many moles are being burned of ethanol

20. Problem 1 solution Plug in the values:
Amount of energy supplied to the water:
Use q =mcΔT
c: given as 4.18 J g-1 ºC-1
ΔT: = 26.2 ºC
m: g water
Plug in the values:
q = x 4.18 x 26.2 = J
And is the heat energy supplied by the burning of ethanol

21. Problem 1 solution How many moles of ethanol are burned
Initial mass of spirit burner = g
Final mass of spirit burner = g
So, Mass burned = – = 1.05 g
Convert to moles ethanol (46.08 g mol-1)
1.05 g (1 mol/46.08 g) = mol ethanol
Therefore, when mol ethanol are burned, it produces J heat
Convert to 1 mol: J/ mol = J mol-1
Thus, ΔH = -721 kJ mol-1

22. Problem 1 solution
Now, let’s compare the calculated value with the literature value
ΔH = -721 kJ mol-1
ΔHѳc = kJ mol-1
Why are they so different?
Heat loss to surroundings
Some heat goes to heating copper can and air
Incomplete combustion of ethanol
Evaporation of ethanol and water
Improvements:
Taken into account the c of the can in calculations
Insulating the can
Use other kind of calorimeter

23. Problem 2
Consider the experiment:100.0 cm3 of water are measured out and poured into a polystyrene cup and the temp of the water measured. Then 5.20 g ammonium chloride are measured out. The ammonium chloride was added to the water and the solution stirred vigorously until all the solute dissolved. The minimum temp was recorded.
Results:
Initial temp of water = 18.3 ºC
Minimum temp = 15.1 ºC
Find the enthalpy change of solution for ammonium chloride, ΔHsol
Remember: The enthalpy change when 1 mole of solute is dissolved in excess solvent to form a solution of ‘infinite dilution’ under std. conditions

24. The specific heat capacity of the solution is the same as water
Problem 2 hints
What do you need to do?
Find the enthalpy change of solution for ammonium chloride, ΔHsol
The enthalpy change when 1 mole of solute is dissolved in excess solvent to form a solution of ‘infinite dilution’ under std. conditions
Actual data you have:
100.0 cm3 water (so actually g water)
Initial temp of water = 18.3 ºC
Minimum temp = 15.1 ºC
5.20 g ammonium chloride
You will make the following assumption:
The specific heat capacity of the solution is the same as water

25. Problem 3
The following experiment may be used to determine the enthalpy of reactions for:
Zn(s) + CuSO4(aq)  ZnSO4(aq) + Cu(s)
50.0 cm3 of mol dm-3 copper(II) sulfate solution are placed in a polystyrene cup. The temperature was recorded every 30s for 2 min. At 2 min
every 30s for 2 min. At 2 min 30s, 1.20 g of powdered zinc are added. The mixture was stirred vigorously and the temp recorded every 30s for several minutes. The results obtained were then plotted to give the graph shown
Use these data to determine the enthalpy change for this reaction.

26. Problem 3 hints Determine the enthalpy change for this reaction.
Remember: ΔHѳr standard enthalpy change of reaction
Enthalpy change when molar amounts of reactants, as shown in a stoichiometric equation react together under std. conditions to give products
Data you actually have: Zn(s) + CuSO4(aq)  ZnSO4(aq) + Cu(s)
50.0 cm3 of mol dm-3 CuSO4(aq)
The graph to the right 
By extrapolating back to the area above the start of
the reaction,
Temp change, ΔT = 10.3 ºC
1.20 g Zn(s)
Assumption: density of CuSO4 solution = water
therefore the m and c are the same as water
50.0 g and 4.18