Thermochemistry

Enthalpy: change

in heat

Energy and Change of State

You 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

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 Problems

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

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

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

Sample Problem #5

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

Practice Problems

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

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

8.) 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!!)