1. 5.1 Measuring Enthalpy Changes

2. 5.1 Measuring energy changes
Understandings:
Heat is a form of energy.
Temperature is a measure of the average kinetic energy of the particles.
Total energy is conserved in chemical reactions.
Chemical reactions that involve transfer of heat between the system and the surroundings are described as endothermic or exothermic.
The enthalpy change (ΔH) for chemical reactions is indicated in kJ mol-1.
ΔH values are usually expressed under standard conditions, given by ΔH°, including standard states.

3. 5.1 Measuring energy changes
Applications and skills:
 Calculation of the heat change when the temperature of a pure substance is changed using q = mcΔT.
 A calorimetry experiment for an enthalpy of reaction should be covered and the results evaluated.

4. Calorimeter
An object used for measuring the heat of chemical reactions or physical changes.

5. Measuring Heat of Reaction in Solutions

6. c = specific heat capacity of water = 4.18 J g-1 K-1
q = heat gained by water
c = specific heat capacity of water = 4.18 J g-1 K-1
The heat energy needed to raise the temperature of 1g of water by 1K
Value given in databook
Other units used: 4.18 J g-1 oC-1; 4.18 kJ kg-1 K-1; 4.18 kJ kg-1 oC-1
Density of water = 1g/ml
ΔT = the rise in water temperature in K or oC
(the increments are the same)
Extrapolate the cooling section of the graph back to the starting point of the reaction

7. Measuring Heat of Reaction in Solutions

8. International Mindedness
What units do we use? Kelvin,K, or Celsius, °C, (same increments) or Fahrenheit, °F? (USA still use °F for all non-science communication)

9. C2H5OH + 3 O2  2 CO2 + 3 H2O
Weight the mass of the alcohol burner before and after combustion.
Calculate change in mass of the alcohol burner
= mass of alcohol combusted (g)
Molar mass of C2H5OH = ____46______ g mol-1

10. ΔHreaction = -q/n = -mcΔT/n
ΔHreaction calculated from bond enthalpy = kJ mol-1
ΔHcombustionθ from Databook = kJ mol-1
Not all heat produced by the combustion reaction is transferred to the water. Some is needed to heat the copper calorimeter; some is lost to the surrounding.
Combustion of ethanol might be incomplete due to limited oxygen.
Experiment condition differs from standard condition (298K, 100kPa)

11. Percent error
(Literature value – Experimental value) / Literature value x 100%

12. Setting up PASCO Capstone to record data
Plug in probe
Select display: Table + Graph
For both table and graph, choose “Time” and “Temperature” for the appropriate columns/ axes
At the bottom of the interface, adjust the sampling rate. The default is set at 2.00Hz (2 samples per second) which is rather frequent. It might be more sensible to change it one sample every 2 seconds.

14. - Click “Highlighter” button
- Move to highlight the straightest section of the cooling curve

15. Click the “Curve fit” button
Select “Linear: mt+ b”
Drag the y-axis to readjust the graph display

16. Click the “Coordinates tool” button
Select “Add Coordinates/ Delta tool”
Drag the square to the extrapolated point corresponding to the highest temperature potentially reached

17. 5.1 Exam questions It’s all about applying ΔH = mcΔT/n
… and the assumptions made when doing calculation using experimental data
Task: Working individually or in pairs, identify the “tricky maneuver ” in each MC question
Pay attention to units

18. Enthalpy of combustion
Why are experimental values lower than literature value?
Not all heat produced by the combustion reaction is transferred to the water.
Calorimeter
Surrounding
Limited oxygen  incomplete combustion of alcohol
Experiment not performed under standard condition
Temperature fluctuation in the room

19. Enthalpy of reaction in solutions
Assumptions when using ΔH = mcΔT/n
Specific heat capacity of solution = Specificity heat capacity of water = 4.18 J g-1 K-1
Density of solution = Density of water = 1.0 g/cm3
All heat produced in the reaction transferred to the solution, no heat loss

20. “Maximum temperature reached if the reaction had taken place instantaneously with no heat loss”
Assumption: Temperature decreases at uniform rate when above room temperature