Unit 9 ThermochemistryUnit 9 Thermochemistry

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Lesson 1Heat and Chemical Change

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Energy and Heat Thermochemistry - concerned with

heat changes that occur during chemical reactions

Energy - capacity for doing work or supplying heat

• weightless, odorless, tasteless

• if within the chemical substances- called chemical potential energy

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Energy and Heat Heat - represented by “q”, is energy

that transfers from one object to another, because of a temperature difference between them.

• only changes can be detected!

• flows from warmer cooler object

Temperature is:

how hot (or cold) something is; specifically, a measure of the average kinetic energy of the particles in an object.

When samples of different temperatures are in contact, energy is transferred from the sample that has the higher temperature to the sample that has the lower temperature.

Temperature is an intensive property, which means that the temperature of a sample does not depend on the amount of the sample.

Heat is an extensive property, which means that the amount of energy transferred as heat by a sample depends on the amount of the sample.

In studying heat changes, think of defining these two parts:

• the system - the part of the universe on which you focus your attention

• the surroundings - includes everything else in the universe

Together, the system and it’s surroundings constitute the universe

Thermochemistry is concerned with the flow of heat from the system to it’s surroundings, and vice-versa.

Essentially all chemical reactions, and changes in physical state, involve either:

• release of heat - exothermic

• absorption of heat - endothermic

Exothermic reactions release energy to the surroundings.

Endothermic reactions absorb energy from the surroundings.

Exothermic and Endothermic Processes

The Law of Conservation of Energy states that in any chemical or physical process, energy is neither created nor destroyed.

• All the energy is accounted for as work, stored energy, or heat.

Exothermic and Endothermic Processes

heat flowing into a system from it’s surroundings:

• defined as positive

• q has a positive value

• called endothermic

–system gains heat as the surroundings cool down

Endothermic Reactions

Exothermic and Endothermic Processes

heat flowing out of a system into it’s surroundings:

• defined as negative

• q has a negative value

• called exothermic

–system loses heat as the surroundings heat up

Exothermic Reactions

Every energy measurement has three parts.

1. A unit ( Joules or calories).

2. A number how many.

3. and a sign to tell direction. negative - exothermic positive- endothermic

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Heat Capacity and Specific Heat

A calorie is defined as the quantity of heat needed to raise the temperature of 1 g of pure water 1 oC.

• Used except when referring to food

• a Calorie, written with a capital C, always refers to the energy in food

• 1 Calorie = 1 kilocalorie = 1000 cal.

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Heat Capacity and Specific Heat

The calorie is also related to the joule, the SI unit of heat and energy

• named after James Prescott Joule

• 4.184 J = 1 cal

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Thermochemistry Reference Units

1 J = 0.2390 cal 1 cal = 4.184J 1 kJ = 1000 J 1 Calorie = 1 kcal 1 kcal= 1000 cal K = °C + 273 °C = K - 273

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Heat Capacity and Specific Heat

Heat Capacity - the amount of heat needed to increase the temperature of an object exactly 1 oC

Molar Heat Capacity Depends on the Number of Atoms

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Heat Capacity and Specific Heat Specific Heat Capacity - the

amount of heat it takes to raise the temperature of 1 gram of the substance by 1 oC (abbreviated “C”)

• often called simply “Specific Heat”

• Note Table 17.1, page 508 Water has a HUGE value,

compared to other chemicals

Solving for Specific Heat• measured under constant pressure

ΔTmcq OR ΔTm

qc

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Heat Capacity and Specific Heat

heat abbreviated as “q” T = change in temperature C = Specific Heat m = mass (g) Units are either J/(g oC) or cal/(g oC)

Examples The specific heat of graphite is 0.71 J/gºC.

Calculate the energy needed to raise the temperature of 75 kg of graphite from 294 K to 348 K.

q= (.71 J/gºC) (75000 g) (75 ºC – 21ºC) =2875500 J or 2875.5 kJ or 2.9x103 kJ

ΔTmcq OR ΔTm

qc

Practice Problems1. A 4.0 g sample of glass was heated from 274 K to 314 K and

was found to have absorbed 32 J of energy. Find the specific heat.

2. How much energy will the same glass sample gain when heated from 314 K to 344 K?

3. If 200. g of water at 20°C absorbs 41,840 J of heat, what will its final T be? The specific heat of water is 4.184 J/g°C.

4. Aluminum has a specific heat of 0.900 J/g°C and water has a specific heat of 4.18 J/g°C. How high must the temperature of the aluminum be to cause the water temperature to raise from 22°C to 28°C? You are using 25 g of aluminum and 50. grams of water.