Heat and Energy

1
Heat and Energy

• Thermodynamic reactions

2
Heat

• Heat
• The transfer of kinetic energy from hotter object
  to cooler object.
• Analogy

3
Types of Reactions

• Composition
• Decomposition
• Replacement
• Combustion

• Endothermic
• Exothermic

4
Exothermic

• Release heat
• Bonds formed are at a lower energy level than the
  atoms making the bonds
• Often violent
• Any combustion

5
Endothermic

• Take heat
• Bonds formed are at a higher energy level than
  the atoms forming the bonds
• Rarely violent
• Cold packs

6
Hesss Law

• Total enthalpy change equals the sum of the
  reactions enthalpies.
• Reaction 1?H 40 J
• Reaction 2?H -20 J
• Total ?H for both Reaction 1 2 is 20 J

7
Enthalpy

• Heat absorbed or released
• Greekenthalpein to warm
• NOTE We cannot measure actual enthalpy. We can
  only measure the change in enthalpy (?H).

8
Quick question

• Why can we only measure the change in enthalpy of
  a substance and not the substances enthalpy
  itself?
• Once we measure a substances energy, we change
  it. Heisenberg's uncertainty principle addresses
  such phenomena.

9
Systems

• OpenMaterials and/or energy move freely between
  the system and the object.

Surroundings
10
Systems

• ClosedMaterials and/or energy cannot move
  between the system and the object.

Surroundings
11
Quick Question

• Which box represents an endothermic reaction?
• Which box represents a closed system?

2
1
12
Solving ?H Problems

• Three reactions will be given.
• Flip equations if necessary to match the last
  equation.
• Change ?H for any equations you flipped.
• Add the equations. Dont forget ?H.
• Fractions are OK!
• Cancel reactants with products.
• Multiply if necessary to make the final two
  equations identical.

13
Solving ?H Problems1

• Three reactions will be given.
• SOCl2 (g) NiO (s) ? SO2 (g) NiCl2 (s)
• ?H -150. kJ
• SOCl2 (g) H2O (g) ? SO2 (g) 2HCl (g)
• ?H -27 kJ
• Calculate ?H for the reaction NiO (s) 2HCl (g)
  ? H2O (g) NiCl2 (s)

14
Solving ?H Problems2 3

• Flip equations if necessary to match the last
  equation.
• Change ?H for any equations you flipped.
• SOCl2 (g) NiO (s) ? SO2 (g) NiCl2 (s)
• ?H -150. kJ
• SO2 (g) 2HCl (g) ? SOCl2 (g) H2O (g)
• ?H - 27kJ

15
Quick Question

• Why would I change the ?H for an equation I
  flipped?
• Because I reversed the reaction.
• The reactants became the products and the
  products became the reactants.
• If the reaction gave off heat, it now takes heat.
• If it was endo one way, it is exo the other.

16
Solving ?H Problems4

• Add the equations. Dont forget ?H.
• Fractions are OK!
• SOCl2 (g) NiO (s) SO2 (g) 2HCl (g) ?
• SO2 (g) NiCl2 (s) SOCl2 (g) H2O (g)
• ?H -150. kJ ?H 27kJ -123 kJ

17
Solving ?H Problems5

• Cancel reactants with products.
• SOCl2 (g) NiO (s) SO2 (g) 2HCl (g) ?
• SO2 (g) NiCl2 (s) SOCl2 (g) H2O (g)
• ?H -123 kJ

18
Solving ?H Problems6

• 6. Multiply if necessary to make the equations
  identical (dont forget to multiply ?H)
  Fractions are OK!
• Calculate ?H for the reaction
• NiO (s) 2HCl (g) ? H2O (g) NiCl2 (s)
• ?H -123 kJ

19
Solving ?H Problems

• Three reactions will be given.
• Flip equations if necessary to match the last
  equation.
• Change ?H for any equations you flipped.
• Add the equations. Dont forget ?H.
• Fractions are OK!
• Cancel reactants with products.
• Multiply if necessary to make the final two
  equations identical.

20
Solving ?H Problems1

• Three reactions will be given.
• Cu2O (s) H2O2 (l) ? 2CuO (s) H2O (l)
• ?H -239 kJ
• 2H2O2 (l) ? 2H2O (l) O2 (g)
• ?H -196 kJ
• Calculate ?H for the reaction
• Cu2O (s) ½O2 (g) ? 2CuO (s)

21
Solving ?H Problems2 3

• Flip equations if necessary to match the last
  equation.
• Change ?H for any equations you flipped.
• Cu2O (s) H2O2 (l) ? 2CuO (s) H2O (l)
• ?H -239 kJ
• 2H2O (l) O2 (g) ? 2H2O2 (l)
• ?H - 196 kJ
• Multiply the 1st equation by 2
• 2Cu2O (s) 2H2O2 (l) ? 4CuO (s) 2H2O (l)
• ?H -478 kJ

22
Quick Question

• Why would I change the ?H for an equation I
  flipped?
• Because I reversed the reaction.
• The reactants became the products and the
  products became the reactants.
• If the reaction gave off heat, it now takes heat.
• If it was endo one way, it is exo the other.

23
Solving ?H Problems4

• Add the equations. Dont forget ?H.
• Fractions are OK!
• 2Cu2O (s) 2H2O2 (l) 2H2O (l) O2 (g) ?
• 4CuO (s) 2H2O (l) 2H2O2 (l)
• ?H -478 kJ 196 kJ -282 kJ

24
Solving ?H Problems5

• Cancel reactants with products.
• 2Cu2O (s) 2H2O2 (l) 2H2O (l) O2 (g) ?
• 4CuO (s) 2H2O (l) 2H2O2 (l)
• ?H -282 kJ

25
Solving ?H Problems6

• 6. Multiply if necessary to make the equations
  identical (dont forget to multiply ?H)
  Fractions are OK!
• 2Cu2O (s) O2 (g) ? 4CuO (s) ?H -282 kJ
• Cu2O (s) ½O2 (g) ? 2CuO (s)
• Multiply by ½.
• -282 kJ ½ -141 kJ ?H

26
Calorimetry?H Measurements

• System
• Open Flow of materials and/or energy
• Closed Materials and/or energy do not flow in
  or out

27
Calorimetry?H Measurements

• Heat capacity
• Energy required to raise an objects temperature
  1C
• Specific heat
• Heat capacity of 1 g of a substance

28
CalorimetrySI Units

• Joule
• Energy required to move a 1 kg object 1 meter (on
  the planet earth). kg-m3/s2
• Calorie
• Energy required to raise 1 gram of water 1 C.
• 1 cal. 4.184 J

29
CalorimetryLaw of Conservation of Energy

• Heat gained by a rxn equals the heat lost by the
  surroundings.
• qrxn -qsur

30
CalorimetryCalorimeter

• Heat gained by a rxn equals the heat lost by the
  surroundings.
• qrxn -qsur

31
CalorimetryCalorimeter

• qsur m C (Tf Ti)

32
Quick Question

• If -qsur indicates the surroundings lost kinetic
  energy, what kind of reaction took place?
• The reaction took in
• heat which means it
• must have been
• endothermic

33
Quick Review

• -?H means exothermic
• ?H means endothermic
• -qsur means endothermic
• qsur means exothermic

34
Calculating ?H

• ?H
• qrxn/moles used in experiment
• Multiply by the mole number in the balanced
  chemical equation (coefficient).