Chapter 2 Heat Capacity and Specific Heat

Rice

Some Basic Concepts

Kinds of Energy

      Heat Light Chemical Nuclear Electrical Mechanical

Any one of these can be converted into any of the other forms.

Types of Energy

1) Kinetic Energy – energy of motion KE = ½ mv 2 m= mass in kg v = velocity in m/s 2) Potential Energy – stored energy 1. Gravitational Potential Energy 2. Electrostatic Attraction Potential Energy 3) Total Energy of a Substance is equal to: E

Total

= PE + KE

How does KE & PE relate to Chemistry?

 KE = the motion of the molecules in chemical substances.

 PE = the sum of all the attractions in a chemical substance, including all the covalent bonds, ionic bonds, and electrostatic attractions.

Temperature

    A measure of the average kinetic energy of the particles in a sample of matter.

It is a description of one characteristic of the individual particles in an object.

How hot or cold something is.

 K.E  Temp

Heat or Heat Energy

 The sum total of the kinetic energies of the particles in a sample of matter. The quantity of energy in an object.

 Heat always flows spontaneously from matter at a higher temperature to matter at a lower temperature. When the temperature of the water is constant…heat will no longer flow in the system.

Is there a difference between Heat and Temperature?

Temperature relates to the average K.E. of the individual particles.

Heat relates to the total quantity of energy (K.E.) in the object.

Latent Heat of Phase Change

Heat Transfer

 Heat flows from regions of high heat content to regions of low heat content.

–

Conduction:

molecules.

heat spread by collisions between –

Convection:

heat spread by movement of air –

Radiation:

energy moving through space in the form of electromagnetic radiation

Heat vs Temperature

 http://www.youtube.com

/watch?v=rU sPzshVnM 50 °C

Which would make the water warmer?

100 °C

The Measurement of Energy

    Energy cannot be measured directly.

Heat is measured by determining temperature changes caused by the release or absorbtion of heat in a chemical process.

Heat energy was originally defined in terms of the calorie (the amount of heat needed to raise the temperature of 1g of water 1 °C). 1 cal = 4.184 J

Heat Capacity

 The amount of heat energy needed to raise the temperature of a given sample of matter by 1 °C

Specific Heat

The amount of energy needed to raise the temperature of 1g of water 1 °C.

 The specific heat of water is 4.184 J/g °C or 1 cal/g °C.

Once the specific heat of water is defined, the specific heat of any other substance can be determined.

 One method is to immerse a hot object in a known quantity of H 2 O and then measure the temperature change.

q (gained by water) = q (lost by metal)

Units of Temperature vs Units of Heat

 Temperature °C, °F, K  Heat J (joule)  cal (calorie) or Cal (food calorie) 1 Cal = 1000 cal

1 cal = 4.184 J

Convert the following:

1) 110 cal to J 2) 22 kJ to cal

Solutions

1) 110 cal to J 110 cal x 1 cal = 26 J 1 4.184 J 2) 22 kJ to cal 22kJ x 1000 J x 4.184 J = 92000 cal 1 1 kJ 1 cal

Calorimetry



Calorimetry

is the science of measuring the heat of chemical reactions or physical changes . Calorimetry involves the use of a calorimeter . The word calorimetry is derived from the Latin word

calor

, meaning heat.

Coffee Cup Calorimeter

http://www.chem.iastate.edu/group/Greenbowe/sections/projectf older/flashfiles/thermochem/heat_metal.html

Coffee Cup Calorimeter

 Constant pressure calorimeter is called a coffee cup calorimeter.

Water is put in the cup and the temperature is taken, the temperature of heated metal is taken. The heated metal is then placed in the calorimeter and the cover is placed on the calorimeter. When the temperature no longer changes, it is recorded. q lost = q gained

Example An insulated cup contains 75.0g of water at 24.00

°C. A 26.00g sample of a metal at 85.25

°C is added. The final temperature of the water and metal is 28.34

°C.

–

What is the specific heat of the metal?

Determining the specific heat

 What is the specific heat of the metal?



q = c x m x ΔT

q = heat c = specific heat capacity m = mass in grams ΔT = T f – T i

Determine the specific heat of the metal

An insulated cup contains 75.0g of water at 24.00

°C. A 26.00g sample of a metal at 85.25

°C is added. The final temperature of the water and metal is 28.34

°C.

q lost = q gained H 2 O gained heat, Metal lost heat + q water = - q metal c H2O x m H2O x ΔT H2O = - (c metal x m metal x ΔT metal )

( 4.184) (75.0g) (28.34-24.00) = - c (26.00) (28.34-85.25)

- 0.920 J/g °C = c metal

Your Turn

An insulated cup contains 250g of water at 25.0

°C. A 5.0g sample of a metal at 100.0°C is added. The final temperature of the water and metal is 25.2

°C.

– What is the specific heat of the metal?

Solution

1)

c H2O x m H2O x ΔT H2O = - (c metal x m metal x ΔT metal )

(4.184)(250g)(25.2-25.0) = - c(5.00)(25.2-100.0)

0.559 J/g °C = c metal

Your Turn

The specific heat of graphite is 0.71 J/gºC. Calculate the energy needed to raise the temperature of 75kg of graphite from 294 K to 348 K.

Solution

First convert: 75kg x 1000g = 75,000g  T f T i = 348K 1 1 kg 273 = 75ºC = 294K 273 = 21ºC q = cm ΔT

Need ºC for Units

q = ( 0.71 J/gºC) (75,000g)(75-21) q = 2,875,500 J or 2,875.5 kJ

Your Turn

A 46.2 g sample of copper is heated to 95.4ºC and then placed in a calorimeter containing 75.0 g of water at 19.6ºC. The final temperature of both the water and the copper is 21.8ºC. What is the specific heat of copper?

Solution

+q gained = -q lost  c H2O x m H2O x ΔT H2O = - (c metal x m metal x ΔT metal ) ( 4.184)(75.0g)(21.8 -19.6) = - c (46.2)(21.8 - 95.4) 0.203 J/ gºC = c metal