Warm-up… EXPLAIN HOW THE ICE CREAM LAB WORKS

What does the salt do (collig. Pro and temp gap)

How does the ice melt– explain the heat movement

Calorimetry

CP Unit 9Chapter 17

CALORIMETRY The enthalpy change associated with a

chemical reaction or process can be determined experimentally.

Measure the heat gained or lost during a reaction at CONSTANT pressure

Device used to measure the heat absorbed or released during a chemical or physical process Styrofoam

cup

Calorimeter

Example If you leave your keys and your chemistry

book sitting in the sun on a hot summer day, which one is hotter?

Why is there a difference in temperature between the two objects?

Because… Different substances have different

specific heats (amount of energy needed to raise the temperature of 1 g of a substance by 1 degree Celsius).

What happens in a calorimeter

One object will LOSE heat, and the other will ABSORB the heat System loses heat to surroundings = EXO = -q System absorbs heat from surroundings = ENDO = +q

When a hot chunk of metal is dropped in a cool glass of water, the metal cools off. Where did the heat from the metal go?Did the metal lose more heat then the water gained?HEAT GAINED = HEAT LOST (ALWAYS!)

To do calorimetry problems… Make a Chart:

Water Object/Reaction

Heat

Mass

Specific Heat 4.184

Final Temp

Initial Temp

The numbers in these two boxes are always the same, but with different signs (+/-). What heat one lost, the other gained.

EXAMPLE: A small pebble is heated and placed in a foam cup calorimeter containing 25.0 g of water at 25.0 C. The water reaches a maximum temperature of 26.4 C. How many joules of heat were released by the pebble? The specific heat of water is

4.184 J/g C. (#13)

Water Pebble

Heat

Mass

Specific Heat 4.184

Final Temp

Initial Temp 25.0 oC

25.0 g

26.4 oC

The numbers in these two boxes are always the same, but with different signs (+/-). What heat one lost, the other gained.

The pebble because the water heated up from 25.0 C to 26.4 C.

Pebble loses heat (-q, exothermic) while water gains heat (+q, endothermic)

Do you calculation based on water (since the problem gave all the water’s information)

Water Pebble

Heat

Mass

Specific Heat 4.184

Final Temp

Initial Temp 25.0 oC

25.0 g

26.4 oC

qwater = mwatercwaterTwater

qwater = (25.0g)(4.184J/goC)(26.4oC-25.0oC)

qwater = 150 J

If the water ABSORBED 150 J of heat,

then the pebble _______ ______ J of heat.

qpebble = - 150 J

Example 2 (LAB type of CALC) Suppose that 100.00 g of water at 22.4 °C

is placed in a calorimeter. A 75.25 g sample of Al is removed from boiling water at a temperature of 99.3 °C and quickly placed in a calorimeter. The substances reach a final temperature of 32.9 °C . Determine the SPECIFIC HEAT of the metal.The specific heat of water is 4.184 J/g C.

MAKE YOUR CHART

Suppose that 100.00 g of water at 22.4 °C is placed in a calorimeter. A 75.25 g sample of Al is removed from boiling water at a temperature of 99.3 °C and quickly placed in a calorimeter. The substances reach a final temperature of 32.9 °C . Determine the SPECIFIC HEAT of the metal.The specific heat of water is 4.184 J/g C.

Water Pebble

Heat

Mass

Specific Heat 4.184

Final Temp

Initial Temp 22.4 oC

100.00 g

32.9 oC

75.25 g

99.3 oC

32.9 oC

1. Make chart

2. Calculate q for water

3. Q for water is the same (but with different sign) as q for metal.

4. Using q metal, calculate c metal

0.879 J/g °C

Example 3 A lead mass is heated and placed in a

foam cup calorimeter containing 40.0 g of water at 17C. The water reaches a temperature of 20.0 C.How many joules of heat were released by the lead?The specific heat of water is 4.184 J/g C.

502 J = 5.0 x 102 J for 2 sig figs