T HE U NIVERSITY O F Q UEENSLAND Foundation Year THERMOCHEMISTRY I

THE UNIVERSITY

OF QUEENSLAND

Foundation Year

THERMOCHEMISTRY I

Lesson Overview Thermochemistry

Specific heat capacity

Endothermic and Exothermic Equations

CalorimetryThermochemical

EquationsHeats of Changes

of State

Heat Capacity

Hess’s Law

Standard Heats of Formation

Introduction

• Thermochemistry is the study of the heat changes that occur during chemical reactions and physical changes of state.

Energy -(1) Types

• Energy is defined as the ability to do work or supply heat. There are different forms of energy:-

• Potential energy - is the energy stored within a body or within the chemistry of a substance.

• Chemical energy - is the energy that is bound up in the chemistry of a substance.

• Chemical Potential energy - is the energy that is stored in chemicals.

• Kinetic energy - is energy of movement.• Electrical energy - is energy that is found in the

movement of an electric current.• Heat energy - is energy that always flows form the

warmer object to the cooler object.

Energy -(2) Measurement

• The Law of the Conservation of Energy states that in any chemical or physical process, energy is neither created nor destroyed.

• Energy is measured in the SI unit of Joules (J) • It is defined as the amount of energy needed to

raise the temperature of 1g of pure water by 0.239C.

• Another unit of measurement is the calorie, where one calorie is the quantity of heat that raises the temperature of lg of pure water by 1C.

Energy -(3) Measurement

• 1 cal = 4.186J• 1J = 0.239cal• 1 Cal = 1000 cal

(Nutrional)

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Heat Capacity (c)

• Heat capacity. The heat capacity of an object is the amount of heat it takes to change that object's temperature by 1oC.

• Specific heat capacity/specific heat. The quantity of heat (q) needed to raise the temperature of 1g of that substance by 1oC is known as the specific heat of that substance. It is measured in J / g C.

• Water's specific heat capacity (or simply specific heat) is around 4.18 J/oC g.

Specific Heat Capacity (C)

• specific heat (C) = q

m T

q = heat (J)

m = mass (g)

T = change in temperature (C)

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Heat of Reaction(1)

• A process that loses heat to the surroundings is called an exothermic process.

• A process that absorbs heat from the surroundings is called an endothermic process.

Enthalpy (1)

• Enthalpy is equal to the total energy of the system, plus the pressure of the system times the volume of the system.

• Instead of enthalpy (H) itself, you will usually deal with a change of enthalpy (H ).

• If pressure is constant, and the only work allowed to work on the system is through volume, then:

H = q

Enthalpy(2)

• If the H is negative, then there was heat given off and the reaction was exothermic.

• If the H is positive, then there was heat absorbed from the surroundings and the reaction was endothermic.

• Exothermic Reaction H = -ve

• Endothermic Reaction H = +ve

H= H(products) - H(reactants) 

Exothermic

Endothermic

Exothermic Reaction

H = -ve

Reaction

Products

Reactants

Enthalpy

Endothermic Reaction

H = +ve

Reaction

Products

Reactants

Enthalpy

Exothermic Endothermic

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References

1. http://www.chem.vt.edu/RVGS/ACT/notes/Chap_8_Triptik.html

2. http://apchem.virtualave.net/concepts/thermochem.html

3. http://chemed.chem.purdue.edu/demos/movies/small_movies/5.2small.mov

4. http://www.wou.edu/las/physci/ch412/heatsoln.gif

5. http://wine1.sb.fsu.edu/chm1046/notes/SolnProp/SolnProc/Hsolv1gif.gif

6. http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/

End of Lecture