Unit 3: Thermochemistry

the study of heat changes associated with physical and chemical reactions

“first word”First word: Heat

H Holy cow, this is hot.

E Exothermic and endothermic

A A thermometer

T Transfers between objects

Example of a “first word”First word: Motion

M Mass must be involved.

O Often the speed varies.

T Time is also involved.

I Initiated by a push or a pull.

O Our planet has its own type of motion.

N Newton had something to do with motion.

Mrs. Thompson’s Monday

As you listen to the story, jot down any key words you think have something to do with Thermochemistry.

What were some of the words you wrote down?

Key words Energy: the ability to do work or produce heat.

Heat: energy in the process of flowing from warmer objects to cooler ones.

Temperature: a measurement. (We will look at temperature in a couple of slides)

Let’s take a closer look at energy…

Energy: the ability to do work or produce heat.

2 Types of energy: KINETIC ENERGY AND POTENTIAL ENERGY

1) KINETIC ENERGY: energy of movement (molecular movement). It has everything to do with temperature.

example: flowing water out of dam gates

There are 3 kinds of KINETIC ENERGYa) Vibrational – molecular bonds vibrate

b) Rotational – molecule rotates

c) Translational – molecule moves through space from point to point

2) POTENTIAL ENERGY: stored energy due to position or composition (types of atoms, # of bonds, types of bonds, the particular way atoms are arranged).

example: water stored behind a dam (position)

Let’s look at some examples of kinetic and potential energy. Ask yourself, “Does this have the potential to do something? Or is it doing something?”

What about someone on a swing?

Is this an example of potential energy or kinetic energy? Be careful!

Measuring Heat Temperature: measures average kinetic energy of

molecules (how fast they move). Units in 0C or K

Heat: thermal energy “in transit” or “flowing”. Units in J (joules)

If you were to heat up water, the temperature change is a response to faster moving particles.

Heat is lost to the surroundings. example: Mrs. Thompson’s coffee cup losing heat the her hands

Measuring Heat The amount of heat required to raise the temperature

of 1g of pure water by 10C or 1K is defined as a CALORIE (cal).

This is not to be confused with the nutritional Calorie which is actually 1kcal=1000 calories.

The SI unit of heat = Joules (J), 1 calorie = 4.18J

What does the word ENTHALPY mean?The total heat content of a chemical system.

The amount of heat to raise the temperature of 1g of any substance by 10C/1K.

Each substance has its own specific heat because each has a different composition.

1. Specific Heat Capacity

Ex) H2O = 4.18 J

g • 0C

2. Molar Heat Capacity The amount of heat to raise the temperature of 1 mol of

any substance by 10/1 K.

Ex) H2O = 75.3 J

K • mol

3. Total Heat The amount of heat to raise the temperature of any

amount of substance by any change in temperature.

H = mc

Mass (g)

Specific heat capacity

Change in temperature

H = nc

mol

molar heat capacity

Change in temperature

Is everyone still on the bus??

Ex) Determine the heat required to raise the temperature of 100.0g of water from 250C to 100.00C.

**note: T(final) – T(initial)

H = m TCH = (100.0g)(100.00C-250C)(4.18J/g0C)H = 31350 J H = 31000J

Calorimetry

A calorimeter is an insulated device used for measuring the heat change during a chemical or physical process.

Ex) A mass of 100.0g of water is placed in a coffee cup calorimeter. The water temperature is measured to be 14.40C. A mass of 412g of calcium metal at a temp. of 40.60C is placed into the calorimeter. When the reaction is complete, the temp. of both the water and calcium is recorded as 24.60C.

A) What is the heat absorbed by the water?B) What is the heat released by the calcium?C) What is the specific heat of calcium?

A) H H2O = m TCH = (100.0g)(24.60C-14.40C)(4.18J/g0C)H = 4263.6 J H = 4260 J absorbed by H2O

B) If 4260 J was absorbed by water, then 4260 J must have been released by calcium! So the answer is -4260 J.

C) HCa = m TC

C = H = (-4260 J) m T (412 g)(24.60C-40.60C)

= 0.646 J/g0C