enthalpy. thermodynamics 101 first law of thermodynamics o energy is conserved in a reaction (it...
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Enthalpy
Thermodynamics 101
• First Law of Thermodynamics
o Energy is conserved in a reaction (it cannot be created or destroyed)---sound familiar???
o Math representation: ΔEtotal = ΔEsys + ΔEsurr = 0
• Δ= “change in” • ΔΕ= positive (+), energy gained by system• ΔΕ= negative (-), energy lost by system • Total energy = sum of the energy of each part in a chemical
reaction
Mg+ 2HCl MgCl2+ H2
Exothermic • Temperature increase (--isolated system)
• Heat is released to surroundings (--open/closed system)
• q = - value
• Chemical Thermal Energy
Endothermic • Temperature decrease (--isolated system)
o All energy going into reaction, not into surroundings
• Heat absorbed by system, surroundings have to put energy into reaction
• q = + value
• Thermal Chemical Energy
Heat of Reaction • Amount of heat exchange happening between the
system and its surroundings for a chemical reaction.
• Temperature remains constant
• Usually reactions happen at constant volume or constant pressure
How does work factor into heat of reaction? • W = -PΔV• If volume is constant (ΔV), PΔV = 0 and no other
work sooooo
• If pressure (P) is constant so volume can change, work is being done soooo
Enthalpy (H)• Measures 2 things in a chemical reaction:
1) Energy change 2) Amount of work done to or by chemical reaction
• 2 types of chemical reactions: 1) Exothermic—heat released to the surroundings, getting rid of heat,
-ΔΗ2) Endothermic—heat absorbed from surroundings, bringing heat in,
+ΔΗ **Enthalpy of reaction—heat from a chemical reaction which is given off or absorbed, units = kJ/mol
• Enthalpy of reaction o Heat from a chemical reaction which is given off or absorbedo At constant pressureo Units = kJ/mol
Enthalpy (H) cont. • Most chemical reactions happen at constant
pressure (atmospheric pressure)—open container
• Temperature and pressure are constanto Only work is through pressure/volume
• Sum of reaction’s internal energy + pressure/volume of systemo H = U + PVo ΔH = ΔU + PΔV
Properties of Enthalpy • Extensive Property
o Dependent on amount of substance used
• State Function o Only deals with current condition o Focus on initial and final states
• Enthalpy changes are unique o Each condition has specific enthalpy value SO enthalpy change (ΔH)
also has specific value
Example 1 • CH4 + 2O2 CO2 + 2H2O ΔH = -890.3
kJ
Example 2 • 2HgO 2Hg + O2 ΔH = + 181.66 kJ
• HgO Hg + ½ O2 ΔH = + 90.83 kJ
More Enthalpy • The reverse of a chemical reaction will have an
EQUAL but OPPOSITE enthalpy change
• HgO Hg + ½ O2 ΔH = + 90.83 kJ
• Hg + ½ O2 HgO ΔH = - 90.83 kJ
• SOOO-----total ΔH = 0
Example 1: • Based on the following: • 2Ag2S + 2H2O 4Ag + 2H2S + O2 ΔH =
+595.5 kJ
• Find the ΔH for the reaction below: • Ag + ½ H2S + ¼ O2 ½ Ag2S + ½ H2O ΔH = ?
Example 2: • Write a chemical equation for ice melting at 0°C
through heat absorption of 334 kJ per gram.
Stoichiometry Returns
Example 1: • H2 + Cl2 2HCl ΔH = -184.6
kJ
Example 2: • Calculate the ΔH for the following reaction when
12.8 grams of hydrogen gas combine with excess chlorine gas to produce hydrochloric acid.
• H2 + Cl2 2HCl ΔH = -184.6 kJ
Example 3: • Pentaborane (B5H9) burns to produce B2O3 and
water vapor. The ΔH for this reaction is -8686.6 kJ/mol at 298°K. What is the ΔH with the consumption of 0.600 mol B5H9 ?
• 2B5H9 + 12O2 5B2O3 + 9H2O
Homework • Study for intermolecular quiz-----Tuesday
• Problems p. 251 #27, 29-31, 33-35 due Wednesday