1. PDP Chemistry Energetics
Thermochemistry: energy changes in chemical reactions

2. Topic 5: Energetics energy exothermic and endothermic reactions
calorimetry
enthalpy change
bond enthalpy
temperature and heat
absorption and emission of radiation
burning fuels

3. PDP moles tests comments
time was short…
show working for quantitative questions
13.b) graph scales- we need to do more work with these…
Chemical equations- balancing, subscripts and coefficients, charges
— 1. work out reactants and products (a word equation may help here)
— 2. write the formula for each compound (use the charge on ions to find the formula, but the charges are not written in the formula)
— 3. balance. change only the coefficients, NOT the formulas
13.d) it is hard to pick out the relevant information and ignore what you don’t need

4. Energy

5. Energy: the ‘capacity to do work’
the unit of energy is the Joule (J)
forms of energy:
kinetic
gravitational potential
elastic potential
chemical potential
electric potential
nuclear potential
thermal (internal energy)
radiant
explosion

6. Law of conservation of energy
The total energy of an isolated system remains constant- it is said to be conserved over time. Energy can be neither created nor be destroyed, but it can change form.

7. Exothermic and endothermic reactions

8. Why are reactions endothermic or exothermic?
In chemical reactions bonds break and new bonds form.
Different bonds have different amounts of chemical energy.
Exothermic reaction: less energy in the bonds after the reaction  energy released as heat
Endothermic reaction: energy is needed as there is more energy in the bonds after the reaction  energy taken in

9. Graphs sharp pencil and ruler graph paper- use the whole sheet
choose a linear scale for each axis
plot each point with an x or a + (or error bars)
titles and units on axes
title for graph
trendline (or maybe join points with straight lines)- no snakes

10. Description and explanation of results

11. Conclusion
Description and explanation of results from one exothermic and reaction and upload to MB

12. Temperature

13. Temperature and energy
Which gas is at the higher temperature?
Which gas contains more energy?
Show 2 gas property simulations

14. Temperature- average KE per particle
higher average speed  higher temperature
more particles at same average speed  same temperature

15. Measuring temperature
Absolute scale of temperature: zero Kelvin = zero kinetic energy zero K = -273°C

16. Convert to Celcius melting point of ice 273K room temperature 300K
temperature of deep space 4K
boiling point of oxygen at standard pressure 90K
surface of the sun 6000K
temperature of an exploding nuclear bomb 106K

17. Distribution of speeds of particles
number of particles
show phet sim and students draw graph on whiteboard or paper
speed

18. Distribution of speeds

19. Calorimetry

20. How much energy does it take to heat water?
4.18Joules of energy are needed to heat up 1 gram of water (approx 1mL) by 1°C. (4.18 is the specific heat capacity of water, and aqueous solutions are almost the same)

21. Calculating how much energy is released
Calculate how many Joules are released in each of your experiments from last week
Calculate how many kJ are released by each mole of reactant in the experiments.
4.18Joules of energy are needed to heat up 1 gram of water (approx 1mL) by 1°C.

22. Exothermic and endothermic reactions
Ammonium nitrate and water
Iron and oxygen

23. Hot pack (more than one is true)
The temperature of the hot pack decreases during the reaction
The temperature of the hot pack increases during the reaction
The hot pack transfers heat to the person touching it
Heat is taken in from the person to the hot pack
The hot pack has less energy stored in chemical bonds after it is used
The hot pack has more energy stored in chemical bonds after it is used

24. Energy changes

25. Exothermic reactions Stored chemical energy, J reactants ∆H products
time

26. Endothermic reactions
Stored chemical energy, J
products ∆H
reactants
time

27. Enthalpy change

28. Enthalpy, H Energy stored in chemical bonds of reactants (in Joules)
PE and KE of particles + energy to make space for substance
Image:

29. Burning fuels

30. Fuels combine with oxygen in combustion
most chemical fuels are organic compounds, often hydrocarbons (contain C, H) or carbohydrates (contain C, H, O)
carbon oxidized to CO2
hydrogen oxidized to H2O
C=O 890kJ/mol
H-O 463kJ/mol

31. Calculate a theoretical ∆H in kJ/mol for the reactions from yesterday’s lab
C2H6O + 3O2  2CO2 + 3H2O

32. Energy density

33. Energy density used to compare fuels
Energy density, kJ/g
hydrogen
142
natural gas (methane) 56
ethanol 26
coal 24
wood 16
data from
energy released by when 1g of fuel is burned

34. Standard enthalpy

35. Standard enthalpy change of a reaction
∆H⊖ to compare energy changes in reactions in kJ/mol (measured at STP 298K and 1atm)

36. Calculating standard enthalpy
0.2g of magnesium reacts with 10mL excess hydrochloric acid to increase the temperature by 50°C. Calculate the standard enthalpy of reaction.

37. Bond enthalpy

38. Bond enthalpy: the enthalpy change when one mole of bonds is formed in the gaseous state
X (g) + Y(g)  X-Y(g) Forming bonds is exothermic (negative ∆H) Breaking bonds is endothermic (positive ∆H)
units? kJmol-1

39. Exothermic reaction
combustion of methane CH4 + 2 O2  CO2 + 2H2O + heat Reactants Products DH = (Products) – (Reactants)  negative value Exothermic reactions: DH < O Endothermic reactions DH > O

40. Enthalpy of combustion
Bond
Average bond enthalpy kJ/mol
C-C
347
C=O
746
C-H
413
O=O
498
O-H
464
C-O
358
Image:

41. Calculating standard enthalpy change
What is the enthalpy change in kJ per mole if 45kJ are given out when 0.8g of methane is burned? What is the enthalpy change in kJ per mole if 1.6g of methanol is used to heat 200mL water from 20C to 38C?

42. Using temperature to calculate ∆Hᶱ
Heat energy = mass x specific heat capacity x temperature change Q = mc∆T

43. Pollution from combustion

44. Acid rain

45. Greenhouse gases

46. Carbon monoxide

47. Particulate pollution

48. Image: http://en.wikipedia.org/wiki/File:Periodic_table.svg