Chapter 11

Heat and temperatureHeat is a thermal energy. The Universe is made up of matter and

energy. Matter is made up of atoms and molecules and energy causes the atoms and molecules to always be in motion - either bumping into each other or vibrating back and forth.

The motion of atoms and molecules creates a form of energy called heat or thermal energy which is present in all matter.

Temperature is a measure of the average heat or thermal energy of the particles in a substance. Since it is an average measurement, it does not depend on the number of particles in an object. It does not depend on the size of it. For example, the temperature of a small cup of boiling water is the same as the temperature of a large pot of boiling water even if the large pot is much bigger than the cup and has millions and millions more water molecules.

We experience temperature every day. When it is very hot outside or when we have a fever we feel hot and when it is snowing outside we feel cold. When we are boiling water, we wait for the water temperature to increase and when we make popsicles we wait for the liquid to become very cold and freeze.

Heat TemperatureThe unit for heat is

Joules.It is the total random

kinetic energy of its atoms.

The unit for temperature is Kelvin.

It is the average random kinetic energy of the atoms.

When water is heated in a beaker its temperature gradually changes. If the beaker contains 360.3 g of H₂O (20.0 mol) initially at 15⁰C and the heater is switched on , the water temperature begins to rise . If the temperature is recorded every 15 seconds till it reaches a particular temperature( degree celcius) you can record the data by converting the temperature to kelvin. After an initial lag the temperature slowly rises with time. We can calculate the slope=(y2-y1)/(x2-x1)=rise/run=∆T/∆t

This is the change in temperature, ∆T, that occurs while time is changing by an interval,∆t.

Molar heat capacityThe heat required to increase the

temperature of 1 mol of a substance through 1K. It is the same as specific heat capacity. Given by the formula q=nC∆T

C is the specific heat capacity of a substanceq=heat needed to increase the temperature

of n moles of a substance by ∆T.

Class PracticeA 100 W heater is used to heat 20.0mol of

water. The temperature is recorded every 15 sec and data is recorded and graph is plotted. The slope of the graph is calculated to be 0.066K/s. Calculate the molar heat capacity for water.

Molar heat capacity depends on the number of atoms. A mole of tungsten has a mass of 184 g and a mole of aluminium has a mass of about 27 g. As predicted the more heat would be needed to increase the temperature of 1 mol of tungsten than 1 mol of aluminium by 1K; but they have the same molar heat capacity.Heat energy is taken up increasing the kinetic energies of the atoms there are same number of atoms in 1 mol of each metal.

ThermodynamicsThermodynamics is the study of the

connection between heat and work and the conversion of one into the other. This study is important because many machines and modern devices change heat into work (such as an automobile engine) or turn work into heat (or cooling, as in a refrigerator).

The science of thermodynamics is founded on two principles, both of which involve the concept of energy.  The first principle asserts that energy is conserved, i.e., energy can neither be created nor destroyed.  The second principle asserts that the overall distribution of energy tends to become more uniform, never less uniform.  These two principles are called the first and second laws of thermodynamics

Entropy and enthalpyEntropy is a measure of the disorder of a

system. Example ice has a less entropy than liquid water because it has more order.The entropy of a substance increase with temperature.The gaseous form of water has a greater entropy than liquid form which in turn has a greater entropy that solid form.

Molar entropies are always positive except at 0.0K. The entropy for pure substances are o at absolute o Kelvin. No disorder no entropy.

Entropy ∆S=Q/T

EnthalpyThe first law of thermodynamics is the

application of the conservation of energy principle to heat and thermodynamic processes:

The change in internal energy of a system is equal to (the heat added to the system)- (the work done by the system)= ∆U=Q-W

When something is heated at constant pressure, its enthalpy increase equals the thermal energy it receives. The symbol for molar enthalpy is H because enthalpy was also called as “heat content”.

Temperature affects molar enthalpies.The total enthalpy increase is equal to q ( the heat input), so the molar enthalpy increase ∆H, can be found by dividing by n the moles of any substance: ∆H=q/n

From slide # 6 we saw that q=nC∆T; substitute in the above equation we get ∆H=nC∆T/n=C∆T

Class Practice Page 393 sample problem 11B

HomeworkPage #395 do # 8,9,10,11,12

Changing states for waterWater has 3 states of matter:Thermal energy is needed to melt ice and to

boil water. Thermal energy increases the kinetic energy of molecules, allowing them to break free of intermolecular forces.

solid

liquid

Molar enthalpy of fusion

gas

Molar enthalpy of vaporization

The molar enthalpy changes that occurs during melting is called the molar enthalpy of fusion or heat of fusion ∆Hfus.

The molar enthalpy of vaporization , (∆H vap ) or heat of vaporization is the difference between the molar enthalpies of 1 mol of a substance in its gaseous and its liquid states.

Gibbs energyA thermodynamic property incorporating

both enthalpy and entropy. It is expressed as G=H-TS T is the temperature (SI unit: kelvins) S is the entropy (SI unit: joules per kelvin) H is the enthalpy (SI Units: joules)Change in Gibbs energy can be used to

compare the change in the enthalpy and entropy using the following equation

∆G=∆H-T∆S

A process is spontaneous if ∆G is negative. All spontaneous process occurs with a decrease in Gibbs energy.

Spontaneous process takes place ‘naturally’ with no apparent cause or stimulus.

Non spontaneous process requires that something be done in order for it to occur.

HomeworkPage 401Q.4,q.8,q.12,q.17

Hess’s LawThis law states that that the overall enthalpy

change in a reaction is equal to the sum of the enthalpy changes of the individual steps in the process.

Class Practice1. Calculate the change in enthalpy ,∆H , for

the following reactionH2(g) + CO2(g) -H2O(g) + CO(g)

Is the reaction exothermic or endothermic?

2. Calculate the entropy change for the following reaction H2(g) +CO2(g) H2O(g) + CO(g)

Does the reaction proceed towards a more ordered or disordered state?

HomeworkPage 419Q.34,35Test prep all