Energy Changes in Chemical Reactions Chemistry TEKS 11

Learning Objective • TLW understand energy and its forms – kinetic, potential, chemical, and thermal (TEKS 11.A) • TLW understand the law of conservation of energy and processes of heat transfer (TEKS 11.B) • TLW use thermochemical equations to calculate energy changes in chemical reactions (enthalpy and entropy) and classify reactions as exothermic and endothermic (TEKS 11.C) • TLW perform calculations involving heat, mass, temperature change, and specific heat (TEKS 11.D) • TLW perform calorimetry to calculate heat of a chemical process (TEKS 11.E)

Forms of Energy

• Potential – stored energy – Mechanical - energy of position, such as effect of gravity or a spring – Chemical potential - energy is a form of potential energy related to the structural arrangement of atoms or molecules – Also electrical, electrostatic, nuclear, and magnetic • Kinetic – energy of motion • Chemical – energy from chemical reactions where substances are transformed into new substances as bonds break and reform • Thermal – portion of the thermodynamic or internal energy of a system that is responsible for the temperature of the system

How is Energy Transferred???

• Conduction • Convection • Radiation

I. Methods Of Energy Transfer A. Three ways energy can

transfer

1.

Conduction

– transfer of energy as heat between particles that

collide

a.

two

objects that are in

contact

with each other at

unequal

temperatures b. particles

within

an object

c. objects must be

direct

contact d. Ex. In case of a wire in a campfire, the rapidly moving

air

molecules close to the flame

collide

with the atoms at the end of the wire. The energy

transferred

to the atoms in the wire causes them to

vibrate

rapidly .

NO HEAT HEATED

•

As a group think of 5 other examples of conduction

2.

Convection

energy – transfer of by the

movement

of

fluids

a. fluids are either a

liquid gas

; Ex. water & air or a these particles are

free

move around to b. particles in a

solid

are

not

free to

move

, so convection can’t take place

c. heated substances follow a convection

current

1)

heated

air

rises

, then

cools

and

falls

back down 2) when something heats up – it

expands

3) when something cools down – it

contracts

d. This

cycle

of a

warm

fluid that rises and then cools and falls is called a

convection current

e. Example While roasting marshmallows, you may notice that tiny glowing embers from the fire

rise

and begin to swirl. They are following the movement of air

away

from the fire. The air

close

to the fire becomes hot and

expands

so that there is more

space

between the air

particles

As a result the air becomes less

dense

and moves upward, carrying its

energy

with it The rising warm air is

replaced

by cooler,

denser

air. Eventually, the rising hot air cools and

contracts

becomes denser, and sinks.

•

As a group think of 5 other examples of convection

3.

Radiation

– the transfer of energy by

electromagnetic

waves a. does

not

involve the movement of

matter

b.

only

method of energy transfer that can take place in a

vacuum

c. includes

infrared

radiation, visible

light

and ultraviolet rays

d. a

hot

object

radiates

more energy than a cooler object e. much of the energy we receive from the

sun

is transferred by

radiation

f. objects

do not

have to

touch

transfer heat by radiation to g. Ex. Warmth from the fire without standing

IN

the fire

As the

molecules

in your skin

absorb

the energy from the fire, the average

kinetic

energy of these molecules and the

temperature

of your skin –

increases

.

•

As a group think of 5 other examples of radiation

Which energy transfer is indicated by each arrow?????

Convection Conduction Radiation

convection YouTube Video conduction conductor insulator radiation

Science in the News • Find 3 articles each that describe the three types of energy transfer (conduction, convection, radiation) • Example – Moon” space – that will be a total of 9 articles • Write down the title/headline of each article • Under this briefly describe which type of energy transfer it is and why – “Scientists Find Evidence of Global Warming on the – Radiation - Satellite data shows the moon’s surface temperature has increased by 0.1 o C over the past 20 years. The moon receives the sun’s radiant heat as this type of energy can travel through the vacuum of

TOMORROW….

Specific Heat

I. Measuring Heat A.

Heat

is

measured

called

calories

in units (cal) 1. a calorie is the

amount

of heat needed to raise

1 gram

of water

1 o C

2. Note the two important factors: a) It's

1 gram

of a substance b) and it moves

1 °C

3. Ex. To raise the temperature of 1 gram of water from 4 o C to 5 o C, 1

calorie

of heat is needed 4. The amount of

heat change

the T o needed to depends on the

mass

B. Specific Heat 1. The ability of a substance to

absorb

heat

energy

called

specific heat

is 2. Different substances absorb different amounts of heat 3.

Water

has a

high

heat specific

4.

Water

has a specific heat of

1

this is one of the

highest

specific heats of any substance

Wood

is

0.42

Aluminum is 0.22

Mercury is 0.03

5. Specific heat can be used to

calculate

the amount of heat

gained

or

lost

II. Calculating Heat Energy A.

Formula: Heat gained or lost (Q = ∆H ) =

Mass X ∆T o X Specific heat

1 .

∆T o

=

change

in Temperature (T o final – T o initial) 2.

Raise in temperature would be positive

∆T o

, lower temperature is negative (-

∆T o )

3. Heat gained would be positive, heat lost is a negative 4.

Specific heat is an absolute number

B. Ex. How much heat is needed to raise the T o of 4 grams of aluminum 5 o C?

Heat gained =

4 g X 5 o C X 0.22 cal/g o C

Heat gained =

4.4 cal

C. Ex. Calculate the heat lost by 10 g of copper if it is cooled from 35 o C to 21 o C.

Heat lost =

10 g X (21 o - 35 o ) X .09 cal/g o C = - 12.6

Heat lost

12.6 calories

1. A 500 g piece of iron changes 7 ° C when heat is added. How much heat energy produced this change in temperature? Mass X ∆T o X Specific heat

500 g X 7 o C X 0.11 cal/g o C

Heat gained =

385 cal

2. When 300 cal of energy is lost from a 125 g object, the temperature decreases from 45 °C to 40 °C. What is the specific heat of this object? Specific heat = cal/ (mass X ΔT)

300 cal 125 g X (40 o C – 45 o C)

specific heat =

.48 cal/g o C

3. A piece of food is burned in a calorimeter that contains 200 g of water. If the temperature of the water rose from 20 °C to 45 °C, how much heat energy was contained in the food?

Heat gained = Mass X ∆T X Specific Heat

200g X (45 o C – 20 o C) X 1 cal/g o C 5000 cal

• Independent practice set – calculating specific heat

Enthalpy & Entropy • Enthalpy – • Entropy -

Calculating Heat of Reaction

Chemical Reactions & Energy Changes

•

Classified as:

–

Exothermic – chemical reactions releasing energy . Indicated by temperature increase .

•

List examples

–

Endothermic -- chemical reactions using energy . Indicated by temperature decrease

•

List examples

What’s Next • Lab – Calorimetry (and/or specific heat) – In Periodic Groups read all procedures – Determine potential hazards, precautions to take, any PPE needed – Set up lab according to scientific method – Complete any pre-work