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Enthalpy

Enthalpy. p302-306 section 5.1-5.2. Changes in chemical systems involve many forms of energy: 1) 2) 3) 4). Enthalpy is… the energy of a system – includes all kinetic and potential energies. It is a state function (not dependent on pathways ). Enthalpy is…

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Enthalpy

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  1. Enthalpy p302-306 section 5.1-5.2

  2. Changes in chemical systems involve many forms of energy: 1) 2) 3) 4) • Enthalpy is… • the energy of a system – includes all kinetic and potential energies. It is a state function (not dependent on pathways).

  3. Enthalpy is… • the energy of a system – includes all kinetic and potential energies. It is a state function (not dependent on pathways). • Denoted: H, mathematically How do we measure all of these energies? Can’t measure the sum of all these energies! ____________________!

  4. We can measure a change though! ΔHsystem= Hproducts - Hreactants – the energy absorbed or released to the surroundings when a system changes from reactants to products (ΔH) !note = always a change! So how do we measure???????????????????

  5. When a system is under constant pressure (THINK OF H = HEAT)

  6. Reactions ________________ – heat transferred from the system to the surroundings ________________ – heat transferred to the system from the surroundings

  7. Sample Problem 0.2 mol of sodium hydroxide is neutralized by 0.1mol of sulphuric acid in 250mL of water. The water’s temperature increases from 20.60C to 20.90C. How much energy is released from the neutralization reaction? (ANS 313.5J)

  8. Molar Enthalpy (section 5.2 p306- ) Consider the following reactions: 2H2(g) + O2(g) → 2 H2O(l) + 571.6kJ C (graphite) + H2 → CH4(g) + 74.9kJ Which one is a better fuel hydrogen or carbon?? Need to be able to consider equal quantities of stuff – like one mole!

  9. Molar Enthalpy is… • the enthalpy associated with a change of one mole of substance • ! must have one mole of your substance! • Symbol: ΔHxyz where xyz represents the change that is occurring

  10. So the molar enthalpy for the following reaction of hydrogen is… 2H2(g) + O2(g) → 2 H2O(l) + 571.6kJ

  11. Changes: see pg 306 table 1

  12. **use ½ in reactions to be able to balance one mole of the interested substance This way ΔH values can also be arranged according to the amount in a reaction. 2H2(g) + O2(g) → 2 H2O(l) + 571.6kJ OR H2(g) + ½ O2(g) → H2O(l) + 285.8kJ

  13. 0.2 mol of sodium hydroxide is neutralized by 0.1mol of sulphuric acid in 250mL of water. The water’s temperature increases from 20.60C to 20.90C. How much energy is released from the neutralization reaction? (ANS 313.5J)… SO WHAT IS THE ENTHALPY OF NEUTRALIZATION for Sodium Hyrdoxide?? (ANS 1567.5J/mol or 1.57kJ/mol)

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