thermochemistry the study and measurement of the heat involed or absorbed during chemical reactions....

Energy

Thermochemistry• The study and measurement of the heat involed

or absorbed during chemical reactions.

• Heat Temperature

– Temperature = average kinetic energy

– which does depend on heat, but is not the

same

Energy

Heat vs. Temperature• Heat = Energy that flows between two samples

of matter due to differences in temperature.

• ¿Which has more heat?

– a cup of coffee or a bathtub filled with coffee both at 170 oF? (which takes longer to heat up?)

¿Carpet or hard wood floor in the same room?

Calorimetry

Calorimetry

• Calorimetry:• calor (L) + metry (Gr)

• The measurement of heat changes.

Calorimetry

• Essential Terms:Calorimetry

• Specific Heat (s) amount of heat required to raise the temperature

of 1 gram of substance by 1oC. intensive property

• The higher the specific heat, the more energy it takes to heat up the sample. And, the slower it loses heat.

• (N.B., Temperature = average kinetic energy is affected by heat. But also includes potential energy

(e.g., the chemical bonds broke when wood burns

Calorimetry

• Essential Equations: Calorimetry

• Specific Heat (s) q = m s DT

• m = mass (g)s = specific heat ( amt. heat required to raise the

temperature of 1 gram of a substance by 1oC (J/g oC)DT = change in temperature (oC)q = heat associated with reaction (‘ – q’ = exothermic

‘+ q’ = endothermic)

‘

• SI Units: Joule(J) 1 J = 4.184 caloriesfood Calorie is 1,000

calories

Calorimetry

Two Types of Calorimeters:Calorimetry

• Constant-Volume Calorimeter• used for combustion reaction

(e.g., Calories in food)

• Constant-Pressure Calorimeter• used for other reactions

(e.g., heat pack: CaCl2 + H2O)

we use this in class

Calorimetry

Example 6.5

A 394-g sample of water is heated from 10.75°C to 83.20°C. Calculate the amount of heat absorbed (in kilojoules) by the water.

Solution

• Specific Heat & Temperature of a Single Substance

Tsmq

CheckSign (+) means heat was absorbed; water was heated; Reaction was endothermic so ‘q’ should be ‘+’

Calorimetry

• Measuring Heat Change:

qsys = qcal + qrxn = 0(conservation of energy)

qcal = heat capacity of calorimeter (assume all q is transferred to water)

qrxn = –qcal

mA sA DTA = –mB sB DTB

Calorimetry

Example 6.7A lead (Pb) pellet having a mass of 26.47 g at 89.98°C was placed in a constant-pressure calorimeter of negligible heat capacity containing 100.0 mL of water. The water temperature rose from 22.50°C to 23.17°C. What is the specific heat of the lead pellet? Strategy Draw the initial and conditions. (It really does help!)

• Constant-Pressure Calorimetry

Calorimetry

Strategy Treat the calorimeter as an isolated system. We know the masses of water and the lead pellet as well as the initial and final temperatures. Assuming no heat is lost to the surroundings, we can equate the heat lost by the lead pellet to the heat gained by the water. Knowing the specific heat of water, we can then calculate the specific heat of lead.

Summary

• Constant-Pressure Calorimetry

mPb sPb DTPb = – mH2O sH2O DTH2O

Calorimetry

• Constant-Pressure Calorimetry

Solution

Fin

Calorimetry