Energy changes in chemical reactions chemistry teks 11
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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)

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Energy Changes in Chemical Reactions Chemistry TEKS 11

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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 transfer of energy by the movement of fluids

a. fluids are either a liquid or a gas; Ex. water & air

these particles are free to move around

b. particles in a solid are not free to move, so convection cant 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 convectioncurrent


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 donot 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


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

Conduction

Convection

Radiation


YouTube Video

conduction

convection

insulator

conductor

radiation


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 moons surface temperature has increased by 0.1 oC over the past 20 years. The moon receives the suns radiant heat as this type of energy can travel through the vacuum of space


TOMORROW.

Specific Heat


I. Measuring Heat

A. Heat is measured in units called calories (cal)

1. a calorie is the amount of heat needed to raise 1gram of

water 1 oC

2. Note the two important factors:

a) It's 1 gram of a substance

b) and it moves 1C


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 specificheat

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 Energy

  • 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 is

    a 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 T)

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 Heat

200g 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


Whats 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


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