Thermochemistry

• Heat• a form of energy.• can be transferred between samples

• heat flows from matter at a higher temperature to matter at a lower temperature

• Temperature• a measure of the average kinetic energy of the

particles in a sample.

Thermochemistry

• Units of Heat• Joule (SI unit) • calorie

• cal• the amount of energy required to raise the

temperature of one gram of water one degree Celsius.• Calorie

• Cal • a dietary calorie.• kilocalorie, kcal (1,000 calories)

1 cal = 4.184 Joules

Thermochemistry

• Enthalpy

• the heat content of a system• represented by H• only changes in enthalpy can be measured

• ∴ ΔH is used

Thermochemistry

• Specific heat capacity, cp

• the amount of energy required to raise the temperature of one gram of a substance one degree Celsius

• used in equationq = m x cp x ΔT

Thermochemistryq = m x cp x ΔT

• note that in the Metric System, Joules are the unit of measure for heat.

Energy, or heat

(J)

mass(g)

Δ temp(°C)

specific heat(J/g•°C)

Heat capacity, cp

For water, C = 4.18 J/(g oC), and also C = 1.00 cal/(g oC) Thus, for water:

it takes a long time to heat up, andit takes a long time to cool off!

Water is used as a coolant!

q = m x cp x ΔT1. A 45.0-gram sample of iron is heated from 25.0°C to 50.0°C.

How much energy is required? (cp iron = 0.449 J/g°C)q = ?m = 45.0 gcp = 0.449 J/g°CΔT = 50.0°C – 25.0°C = 25.0°C

q = m cp ΔTq = 45.0g (0.449 J/g°C) (25.0°C) = 505.125 Jq = 505 J

2. What is the specific heat capacity of an object if a 12.5-gram sample is heated from 12.0°C to 28.0°C using 100.0 joules?

q = 100.0 Jm = 12.5 gcp = ?ΔT = 28.0 – 12.0 = 16.0°C

q = m cp ΔT

TΔmq

=cp )C0.16(g5.12J0.100

=cp

CgJ

500.0=cp

Heat - represented by “q”, is energy that transfers from one object to another, because of a temperature difference between them.

In studying heat changes, think of defining these two parts:

the system – the part of the universe you focus your attention onthe surroundings – everything else

If heat flows into a system from the surroundings, the system gains energy, and the change is said to be endothermic. Heat has a positive value.

If heat flows out of a system to the surroundings, the system loses heat, and the change is said to be exothermic. Heat has a negative value.

Every reaction has an energy change associated with it.

Exothermic reactions release energy, usually in the form of heat.Endothermic reactions absorb energy.

Energy is stored in bonds between atoms.

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

All the energy is accounted for as work, stored energy, or heat.

Calorimetry - the accurate and precise measurement of heat change for chemical and physical processes.

For systems at constant pressure, the heat content is the same as a property called Enthalpy (H) of the system.

Changes in enthalpy = Hq = H These terms will be used interchangeably.

Thus, q = H = m x C x T

H is negative for an exothermic reaction.

H is positive for an endothermic reaction.

16

C + O2 CO2E

nerg

y

Reactants Products

C + O2

C O2

395kJ

+ 395 kJ

THIS IS AN EXOTHERMIC REACTION.

THE CHEMICAL BONDS OF THE PRODUCTS CONTAIN LESS CHEMICAL POTENTIAL ENERGY THAN THE BONDS OF THE REACTANTS.

THE SYSTEM GIVES OFF ENERGY TO THE SURROUNDINGS.

H IS NEGATIVE.

ANOTHER WAY OF SHOWING THIS IS THE ENERGY CHANGE IS SHOWN AS A PRODUCT.

18

CaCO3 CaO + CO2E

nerg

y

Reactants Products

CaCO3

CaO + CO2

176 kJ

CaCO3 + 176 kJ CaO + CO2

THIS IS AN ENDOTHERMIC REACTIION. THE CHEMICAL BONDS IN THE PRODUCTS HAVE MORE CHEMICAL POTENTIAL ENERGY THAN THE CHEMICAL BONDS IN THE REACTANTS.

THE SYSTEM GAINS ENERGY FROM THE SURROUNDINGS.

H IS POSITIVE.

Chemistry Happens in MOLES An equation that includes energy is

called a thermochemical equation CH4 + 2O2 CO2 + 2H2O + 802.2

kJ 1 mole of CH4 releases 802.2 kJ of

energy. When you make 802.2 kJ you also

make 2 moles of water

21

Thermochemical Equations

A heat of reaction is the heat change for the equation, exactly as written•The physical state of reactants and products must also be given.

•Standard conditions for the reaction is 101.3 kPa (1 atm.) and 25 oC

22

CH4 + 2 O2 CO2 + 2 H2O + 802.2 kJ

If 10. 3 grams of CH4 are burned completely, how much heat will be produced?

10. 3 g CH4

16.05 g CH4

1 mol CH4

1 mol CH4

802.2 kJ

= 514 kJ

SPECIAL THANKS TO: WWW. pa016.k12.sd.us/Chapter%2011%20revised.ppt

THERMOCHEMISTRY

THE STUDY OF ENERGY CHANGES THAT ACCOMPANY

CHEMICAL AND PHYSICAL CHANGES