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CHEMICAL THERMODYNAMICS. NANIK DWI NURHAYATI,S.Si, M.Si. www.nanikdn.staff.uns.ac.id (0271) 821585, 081556431053. Enthalpy and Enthalpy Change of Chemical Reactions.

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nanik dwi nurhayati s si m si

CHEMICAL THERMODYNAMICS

NANIK DWI NURHAYATI,S.Si, M.Si

www.nanikdn.staff.uns.ac.id

(0271) 821585, 081556431053

enthalpy and enthalpy change of chemical reactions
Enthalpy and Enthalpy Change of Chemical Reactions
  • We have already investigated energy transfers during physical transformations, what about chemical transformations – reactions?

H = Hproducts – Hreactants

  • An endothermic reaction (H > 0) is a reaction in which heat is absorbed from the surroundings.
  • An exothermic reaction (H < 0) is a reaction in which heat is given off to the surroundings.
thermochemical equations
Thermochemical Equations
  • Thermochemical Equation: a balanced chemical equation (including phase labels) with the molar enthalpy of reaction written directly after the equation

N2 (g) + 3 H2 (g)  2 NH3 (g); H = -91.8 kJ

a) Molar Interpretation: When 1 mol of nitrogen gas reacts with 3 mol of hydrogen gas to form 2 mol of ammonia gas, 91.8 kJ of energy is given off.

heat transfer
Heat Transfer

Heat Capacity (C): quantity of energy required to increase the temperature of a sample by one degree

C = q/T

The magnitude of the heat capacity depends on:

  • Mass of the sample
  • Composition of the sample
slide5

Calculate the heat capacity of an aluminum block that must absorb 629 J of heat from its surroundings in order for its temperature to rise from 22 C to 145 C.

Heat Capacity (C): C =q/T

C = 629 J / (145 - 22 C)

= 629 J / 123 C

= 5.11 J/ C

heat capacity
Heat Capacity

When comparing the heat capacities of different substances with different masses, it is more useful to compare specific heat capacities.

Specific Heat capacities (c): quantity of energy needed to increase the temperature of one gram of a substance by one degree Celsius

  • Molar Heat Capacity (cm): related to specific heat, but for one mole of substance
heat capacity1
Heat Capacity

When dealing with specific heat capacities (c):

Given: c = q/mT

Derive: q = cmT

T = Tfinal – Tinitial = q/cm

m = q/cT

q = thermal heat

c = specific heat

m = mass

T = change in temperature

slide8

What will be the final temperature of a 5.00 g silver ring at 37.0 C that gives off 25.0 J of heat to its surroundings

(c = 0.235 J/g C)?

T = Tfinal – Tinitial = q/cm

Tfinal – 37.0 C = -25.0 J / (0.235 J/g C)(5.00 g)

Tfinal – 37.0 C = -21.3 C

Tfinal = 37.0 C - 21.3 C

Tfinal = 15.7 C

slide9

148 J of heat are transferred to a a piece of glass (c = 0.84 J/gC), raising the temperature from 25.0 C to 49.4 C. What is the mass of the glass?

m = q/cT

m = (148 J)/(0.84 J/gC)(24.4 C)

m = 7.2 g

phase changes
Phase Changes
  • We just saw that energy transfers ALWAYS accompany temperature changes.
  • Energy transfers also accompany physical and chemical changes, even when there is no change in temperature.
  • eg. Energy is always transferred into or out of a system during a phase change.
melting freezing
Melting/Freezing

Heat of Fusion: quantity of thermal energy that must be transferred to a solid as it melts (qfusion = - qfreezing)

Water:

Heat of fusion =

+333 J/g at 0 C.

Specific Heat (l) =

1.00 cal/gC

Specific Heat (s) depends on T:

 0.5 cal/gC

near 0 C