Enthalpy EQ: How do you predict the sign of delta H?

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Presentation transcript:

Enthalpy EQ: How do you predict the sign of delta H?

A. Enthalpy (ΔH) 1.Enthalpy is the amount of heat in a system 2.In order to determine the amount of heat change in a reaction we use… 3.Reactions can be… ΔH rxn = H products - H reactants

A. Enthalpy (ΔH) A) Endothermic 1.Products have more energy than the reactants 2.ΔH=+ (positive) 3.heat into system 4.Heat is being absorbed 5.Surroundings feel cool 6.Ex: 25 kJ + NH 4 NO 3  NH NO 3 - Heat is going into the arrow (energy on the left)

A. Enthalpy (ΔH) A) Exothermic 1.Reactants have more energy than products 2.ΔH= - (negative) 3.heat exits system 4.Heat is being released 5.Surroundings feel warm 6.Ex: 4Fe + 3O 2  2 Fe 2 O kJ Heat exits the arrow (energy on the right)

HEATHEATHEATHEATHEATHEATHEATHEAT

Endo or Exo?

Endo or Exo

Endo or Exo?

B. Enthalpy of Combustion (ΔH comb ) 1. change in enthalpy when 1 mol of a substance is completely burned

B. Enthalpy of Combustion (ΔH comb ) Ex: How much heat is evolved when 54.0 g C 6 H 12 O 6 is burned? C 6 H 12 O 6 + 6O 2  6 CO 2 + 6H 2 O (ΔH comb = kJ) 54.0 g C 6 H 12 O g C 6 H 12 O 6 1 mol C 6 H 12 O kJ 1 mol C 6 H 12 O 6 = -842 kJ Use coefficient in balanced equation x x

C. Molar Enthalpy (heat released per mole) 1.Molar Enthalpy of vaporization: Heat required to evaporate one mole of a substance. 2.Molar Enthalpy of condensation -ΔH Cond = + ΔH VAP

3.Molar Enthalpy of fusion: Heat required to melt one mole of a substance 4.Molar enthalpy of freezing (ΔH solid ) solidification -ΔH Solid = + ΔH Fusion C. Molar Enthalpy (heat released per mole)

5.If the ΔH fus is 6.01 kJ/mol, than the ΔH solid = kJ/mol C. Molar Enthalpy (heat released per mole)

6.Ex: Calculate the energy required to melt 250 g of ice. (ΔH fus = 6.01 kJ/mol) 250 g H 2 O g H 2 O 1 mol H 2 O 6.01 kJ 1 mol H 2 O =83 kJ C. Molar Enthalpy (heat released per mole) xx

Calculate how much energy is absorbed when 7.20 moles of O 2 reacted with an unlimited amount of chlorine. 2Cl 2 + 7O kcal 2Cl 2 O moles of O 2 7 moles O kcal = 134 kcal D. More Enthalpy Stoichiometry ΔH = +130 kcal (+ because it’s endo) x

Determine the mass of iron used to produce 250 kJ of heat according to the balanced reaction below. 4Fe + 3O 2  2Fe 2 O kJ -250 kJ D. More Enthalpy Stoichiometry ΔH = kJ (- because it’s exo) x kJ 4 mol Fe x 1 mol Fe g Fe = 34 g Fe Produce = - energy