NANIK DWI NURHAYATI,S.Si, M.Siwww.nanikdn.staff.uns.ac.id

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CHEMICAL THERMODYNAMICS

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 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 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

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

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 CapacityWhen dealing with specific heat capacities (c):

Given:c = q/mT

Derive: q = cmTT = Tfinal – Tinitial = q/cmm = q/cT

q = thermal heat c = specific heat m = mass T = change in temperature

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

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

m = q/cT

m = (148 J)/(0.84 J/gC)(24.4 C)m = 7.2 g

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/FreezingHeat 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/gC

Specific Heat (s) depends on T: 0.5 cal/gCnear 0 C