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Topic: Enthalpy and Table I
Do Now: packet p.1
Chemistry is the study of matter and energy. So far we have learned about the types of matter and how they can physically combined to make mixutures or
chemically combine in chemical reactions to from new compounds.Every physical or chemical change is accompanied by energy change
Energy released = _________________
Energy absorbed = _______________
exothermic
endothermic
G
L
S
exot
herm
ic
endothermic
The universe wouldn’t exist without energy. It provides all our work, our heat, can be stored to use
later and it never goes away (conservation of energy) The total amount of energy a substance contains depends on many factors, some of which are not
totally understood…it’s impossible to know the total heat content of a substance.
So scientists measure scientists can measure the amount of heat lost or gain by a system
H = Enthalpy = heatΔH = Heat of Reaction
The heat gained or lost by the system (at constant pressure)
ΔH = Hproducts – Hreactants
ΔH=Q
Energy is stored in chemical bonds
For any reaction occurring at constant pressure ΔH = Q
• Q = mCT– Q = Energy change
– m = mass of water
– C = specific heat of water
– T = temperature change = Tf – Ti
4Fe(s) + 3O2(g) 2Fe2O3(s) + 1625 kJOR
4Fe(s) + 3O2(g) 2Fe2O3(s) H = -1625 kJ (heat is lost)
NH4NO3(s) + 27 kJ NH4+(aq) + NO3
-(aq)OR
NH4NO3(s) NH4+(aq) + NO3
-(aq) H = 27 kJ (heat is gained)
If ΔH is negative…- ΔH = exothermic – heat is lost
- Hproducts < Hreactants
- PE of Products < PE of Reactant- Example: 4Fe(s) + 3O2(g) 2Fe2O3(s) + 1625 kJ
- ΔH = -1625kJ
If ΔH is positive…+ ΔH = endothermic – heat is gained
- Hproducts > Hreactants
- PE of Products > PE of Reactant
Example: NH4NO3(s) + 27 kJ NH4+(aq) + NO3
-(aq)- ΔH = +27kJ
Universe
Environment
System A
B
Since energy is conversed…the system changes in one direction and the surrounding have to change in the opposite direction
A. Reaction is Exothermic, environment gets _________________
B. Reaction is Endothermic, environment gets _________________
warmer
colder
Since there are different types of reactions, you have various ΔH’s
• ΔHcomb = enthalpy (heat) of combustion – the enthalpy change for the complete bunring of one mole of the substance
• ΔHformation = enthalpy (heat) of formation – the enthalpy change for the formation of a compound from its constituent elements
• ΔHsolution = enthalpy (heat) of solution – the enthalpy change when 1 mole of an ionic substance is dissolved in water.
Look at Table I: Heats of ReactionLets label the various types from above
• Rxns 1-6: combustion rxns
-H = heat of combustion
• Rxns 7-18: formation (synthesis) rxns
– Substance is formed from its elements
– H = heat of formation
• Rxns 19-24: dissolving equations
– H = heat of solution
Table I
Many other processes other than chemical reactions absorb or release energy
like, Changes of state
Hvaporization = molar heat of vaporization
= amount of heat required to vaporize one mole of a liquid
Hfusion = molar heat of fusion
= amount of heat required to melt one moles of a solid
Energy depends on amount
• Remember – it takes more energy to heat up water in the ocean than to make a cup of tea
CH4(g) + 2O2(g) CO2(g) + 2H2O (l) H = -890.4 kJ
1 mole of methane + 2 mole of oxygen →
1 mole of carbon dioxide gas & 2 moles of liquid water
reaction is ____________ (negative sign for ΔH)
890.4 kJ energy released per mole of CH4(g) burned
exothermic
What would happen if we had 2 moles of methane?
Twice as much energy would be released
2 x 890.4 kJ = 1780.8 kJ will be released
Reactions: Energy depends on direction too!
• N2(g) + 3H2(g) 2NH3(g) H = -91.8 kJ
• 2NH3(g) N2(g) + 3H2(g) H = _______
If reverse equation, reverse sign of H
91.8 kJ
Table I
• Exothermic: ΔH = -
• Pay attention to number of moles, may need to divide or multiply ΔH
• Pay attention to direction, if in reverse direction, switch the sign
• Check POINT
• 1. According to Ref. Table I, the decomposition of which compound is exothermic?
• A) C2H6 B)NO
• C) CO2 D) NH3
• 2. According to Ref. table I, what is the heat of reaction for the formation 1 mole of H2O (l) from hydrogen and oxygen gas at STP?
• A) -571.6kJ B) -483.6 kJ
• C)-55.8kJ D) -285.8kJ
If we can’t calc. H for one individual substance how are we
able to calc. ΔH???!
FYI: Hess’s Law(not on regents)
Can add 2 or more equations by adding the H’s
Enables you to calculate H for # of rxns
Say you’re interested in
2S(s) + 3O2(g) 2SO3(g)
2S(s) + 3O2(g) 2SO3(g)Have H’s for the following:
a) S(s) + O2(g) SO2(g) H = -297 kJ
b) 2SO3(g) 2SO2(g) + O2(g) H = 198 kJ
2 2 2
2SO2(g) + O2(g) 2SO3(g) H = -198 kJ
H = -594KJ + (- 198kJ) H = -792 kJ
x (2)