Unit 09Thermochemistry

Unit 9 Vocabulary Thermochemistry - concerned with heat

changes that occur during chemical reactions

Energy- capacity of an object to do work or produce heat

Kinetic energy- the energy an object has due to its motion

Potential energy- stored energy Temperature- describes amount of motion

of particles

Unit 9 Vocabulary

Heat - describes amount of energy transferred from one object to another

Endothermic process- heat absorbing process

Exothermic process- heat releasing process

Specific heat- Amount of heat it takes to raise 1g of a substance by 1°C

The Flow of Energy

Heat is transferred from one object to another.

Cranking up the Heat: Basic Thermodynamics Energy is required in all

biological and chemical processes.

Kinetic Energythe energy an object has due

to its motion

Potential Energystored energy

Energy in the universe is constant, though it changes forms.

Law of Conservation of energyLaw of Conservation of energy – energy can neither be created nor destroyed

Heat vs. Temperature Temp. and heat are not the same thing.

Temp. describes amount of motion of particles

Heat describes amount of energy transferred from one object to another

Describing Energy Changes Heat flow is from the warmer to the

cooler object

Heat Flow

Heat Flow

Describing Energy ChangesHot day 50°C = 122°F

Beans lost energy, it was transferred to its surroundings. This change of energy would be negative change = ∆

Energy transferred from fire to beans. Change in energy for beans would be positive.

Endothermic reactions – absorbs heat, ∆E is positive, the surroundings cool down

Exothermic reactions – releases heat, ∆E is negative, the surroundings heat up

Heat

Surroundings

Heat

Surroundings

Describing Energy Changes

Units used to describe Energy:

calorie – called “little calorie” – (cal)

kilocalorie – called “Big calorie”- (CAL) 1000 calories – same as a food calorie

Joule – the SI unit of heat - (J)

CONVERSION FACTOR

1 cal = 4.184 joules

1 cal 4.184 J

4.184 J 1 cal

Heat Units: Conversion Problems1. Convert 32.7 cal to joules.

2. Convert 6.24 J to cal.

Specific Heat - Amount of heat it takes to raise 1g of

a substance by 1°C

Metals have a low specific heat Water has a high specific heat

Learning Check1. On a sunny day, the concrete deck around

an outdoor swimming pool becomes hot, while the water stays cool. This is because…

A. The deck has a higher specific heat than the water

B. The deck has a lower specific heat then the water

C. Both objects have the same specific heat

Learning Check2. Two objects are sitting next to each other

in the sunlight. Object A gets hotter than object B.

A. Object A has a lower specific heat than object B

B. Object A has a higher specific heat than object B

C. Both objects have the same specific heat

Specific Heat of Water

4.18 J/g × °C

or

1 cal/g × °C

Matches definition: Amount of heat it takes to raise 1g of a substance by 1°C

Formula for Specific Heat

q = amount of heat – measured in cal or J

m = mass – measured in g

C = specific heat – units are J/g x °C or cal/g x °C

ΔT = change in temp – measured in °C

q = mc ΔT

Specific Heat Problems:1. Aluminum has a specific heat of 0.9020 J/g × °C. How

much heat is lost when a piece of aluminum with a mass of 23.984 g cools from a temperature of 415.0°C to a temperature of 22.0°C?

2. 850 calories of heat are applied to a 250 g sample of liquid water with an initial temperature of 13.0 °C. 

Find: a) the change in temperature and

b) the final temperature. 

(remember, the specific heat of liquid water, in calories, is 1.00 cal/g x °C.)

3. The temperature of a sample of water increases by 69.5 °C when 24,500 J are applied.  The specific heat of liquid water is 4.18 J/g x °C.  What is the mass of the sample of water?

4. When 34,700 J of heat are applied to a 350 g sample of an unknown material the temperature rises from 22.0°C to 173.0°C.  What must be the specific heat of this material?

Specific Heat Problems:

Bell Work - Wed 1/23/08

When 34,700 J of heat are applied to a 350 g sample of an unknown material the temperature rises from 22.0°C to 173.0°C.  What must be the specific heat of this material?

Energy and Change of StateYou do not have to write this

When energy is added to a solid substance, its temperature increases until its melting point is reached. Upon the addition of more energy, the substance begins to melt.

The temperature remains the same until all the substance has melted. ****

Energy required to melt 1 gram of a substance at its melting point

Enthalpy of Fusion (Hfus)

Heating Curve: ( For Water)

Time

-20

-10

0

Heating Ice

(solid)

Melting point (°C)

Hfus

After the substance melts and if heating continues, the temperature will continue to rise until it reaches its boiling point.

۞The temperature remains the same until all the substance has boiled.***

Energy and Change of State

Energy required to vaporize 1 gram of a substance at its boiling point

Enthalpy of Vaporization (Hvap)

Time

-20

-10

0

Heating Ice

Melting Point

100

Heating Water

(liquid)

Boiling Point

Hvap

(°C)

Hfus

► ► Important Formulas to Important Formulas to Know ◄Know ◄When a substance is being heated, use the

following formula: (inclined line)

q = (m)(c)(T)

When a substance is meltingmelting or boilingboiling, use the following formula: (flat line)

q = (m) (Hfus or Hvap)

q = (m)(c)(q = (m)(c)(T)T)

(C) = Specific Heat:

- of ice = 2.06 J/g x °C

- of water = 4.18 J/g x °C

- of steam = 2.02 J/g x °C

q = (m) (q = (m) (HHfusfus or or HHvapvap))

Enthalpy Values for Water:

Hfus = 334 J/g

Hvap = 2260 J/g

Learning Check:

Sample Problem #1How much heat is required to melt

5.67g of ice?

Q = (m)(ΔH fus)

Q = (5.67g)(334 J/g)

Q =

Sample Problem #2

How much energy is required to boil 85g of water?

Sample Problem #3

How much energy is required to raise the temperature of 25g of water from 15°C to 75°C ?

Sample Problem #4

How much energy is required to convert 40g of ice at -10°C to steam at 150°C ?

Use Both Formulas Draw a graph

Sample Problem #5

What is the final temperature when 1250 J of energy is added to 15g of water at 25°C ?

Warm Up

1. How much energy is required to boil 50 grams of water?

2. What is the final temperature when 2500 J of heat is added to 35 grams of water at 15 degrees Celsius?

3. How much energy is required to convert 75 grams of ice at -20 degrees Celsius to steam at 125 degrees Celsius? (Hint: This problem has 5 steps!!)