energy changes in chemical reactions chemistry teks 11

Energy Changesin

Chemical ReactionsChemistry 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 – transfer of energy by the movement of fluidsa. fluids are either a liquid or a

gas; Ex. water & airthese particles are free to

move aroundb. 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 to transfer heat by radiation

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

ConvectionConduction

Radiation

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

radiation

conduction

convection

YouTube Video

conductor

insulator

Science in the News• Find 3 articles each that describe the three types

of energy transfer (conduction, convection, radiation) – 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• Example –

– “Scientists Find Evidence of Global Warming on the Moon”

– Radiation - Satellite data shows the moon’s surface temperature has increased by 0.1 oC over the past 20 years. The moon receives the sun’s radiant heat as this type of energy can travel through the vacuum of space

TOMORROW….Specific Heat

I. Measuring HeatA. Heat is measured in units

called calories (cal)1. a calorie is the amount of heat

needed to raise 1 gram of water 1 oC

2. Note the two important factors: a) It's 1 gram of a substanceb) and it moves 1°C

3. Ex. To raise the temperature of 1 gram of water from 4oC

to 5oC, 1 calorie of heat is needed

4. The amount of heat needed to change the To depends on the mass

B. Specific Heat

1. The ability of a substance to absorb heat energy is called specific heat

2. Different substances absorb different amounts of heat

3. Water has a high specific heat

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

= Mass X ∆To X Specific heat

1. ∆To = change in Temperature(To final – To initial)

2. Raise in temperature would be positive∆To, lower temperature is negative (- ∆To)

3. Heat gained would be positive, heat lost isa negative

4. Specific heat is an absolute number

B. Ex. How much heat is needed to raise the To of 4 grams of

aluminum 5o C?

Heat gained = 4 g X 5 oC X 0.22 cal/goC

Heat gained = 4.4 cal

C. Ex. Calculate the heat lost by 10 g of copper if it is

cooled from 35 oC to 21 oC.

Heat lost =

10 g X (21o - 35o) X .09 cal/goC

= - 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 ∆To X Specific heat

500 g X 7 oC X 0.11 cal/goC

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 oC – 45 oC)

specific heat = .48 cal/goC

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 Heat200g X (45 oC – 20 oC) X 1 cal/goC

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