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the study of transfers of energy as heat that accompany che the study of transfers of energy as heat that accompany che

the study of transfers of energy as heat that accompany che - PowerPoint Presentation

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the study of transfers of energy as heat that accompany che - PPT Presentation

Thermochemistry Energy absorbed or released as heat in a chemical or physical change is measured in a calorimeter In some calorimeters known quantities of reactants are submersed in water and combusted The energy given off is noted by the temperature change in water ID: 159119

energy heat temperature reaction heat energy reaction temperature specific change absorbed enthalpy h2o capacity water sample released thermochemical matter

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Slide1

the study of transfers of energy as heat that accompany chemical reactions and physical changes.

ThermochemistrySlide2

Energy absorbed or released as heat in a chemical or physical change is measured in a

calorimeter

.In some calorimeters, known quantities of reactants are submersed in water and combusted. The energy given off is noted by the temperature change in water.

Heat and TemperatureSlide3

Heat and TemperatureSlide4

Temperature – a measure of the average kinetic energy of the particles in a sample of matter.

The greater the kinetic energy, the greater the temperature.

Joules – the SI unit of heat as well as all other forms of energy

Heat and TemperatureSlide5

Heat – the energy transferred between samples of matter because of differences in their temperature

Energy transferred as heat always moves spontaneously from matter at a higher temperature to matter at a lower temperature.

Heat and TemperatureSlide6

Specific heat – is the amount of energy required to raise the temperature of one gram of substance by one Celsius degree or one Kelvin.

Joules per gram per Kelvin or joules per gram per Celsius

degreeQ = (m)(C

p

)(ΔT)

C

p

= specific heat at a given pressure ; q = energy lost or gained; m = mass of sample; ΔT = the change in temperature

Heat Capacity and Specific HeatSlide7

EXAMPLE: A 4.0 g sample of glass was heated from 274 K to 314 K, a temperature increase of 40 K, and was found to have absorbed 32J of energy as heat.

What is the specific heat of this type of glass?

Heat Capacity and Specific HeatSlide8

Given: m = 4.0 g q = 32 J ΔT = 40 K C

p

= ??? Q = (m)(Cp)(ΔT)

32 J = (4.0 g)(C

p

)(40 K)

32 J = 160 (C

p

)

0.20 J/

gK

= C

p

Heat Capacity and Specific HeatSlide9

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

Heat Capacity and Specific HeatSlide10

(

0.20 J/

gk)(4.0g)(71K – 41K)(0.8)(30) = 24 J

Heat Capacity and Specific HeatSlide11

Determine the specific heat of a material if a 35 g sample absorbed 48 J as it was heated from 293K to 313K

.

If 980 kJ of energy are added to 6.2 L of water at 291 K, what will the final temperature of water be?

Now You Try….Slide12

Heat of ReactionHeat of reaction – the quantity of energy released or absorbed as heat during a chemical reaction.

Heat of ReactionSlide13

2 H2

(g) + O

2 (g)  2 H2

O (g)

In the above equation 2 mol of hydrogen gas is ignited to consume 1 mol of oxygen gas and form 2 mol of water.

This is an explosive reaction and 483.6 kJ are produced.

We would re-write this as a

thermochemical

equation.

2 H

2

(g) + O

2

(g)

2 H

2

O (g) + 483.6 kJ

Heat of ReactionSlide14

Doubling the reaction would likewise produce double the heat released.

4 H

2 (g) + 2 O

2

(g)

4 H

2

O (g) + 967.2 kJ

Fractional co-

efficients

are sometimes used in

thermochemical

equations

.

H

2

(g) + ½ O

2

(g)

H

2

O (g) + 241.8 kJ

Heat of ReactionSlide15

The physical states of reactants and products must always be included in thermochemical

equations because they influence the overall amount of energy exchanged.

(s) = Solid (aq

) =

aqueous

(g

) = gas

(

l) = liquid

Heat of ReactionSlide16

The energy absorbed or released as heat during a chemical reaction at constant pressure is represented by ΔH.

“H” is the symbol for a quantity called enthalpy.

Only changes in enthalpy can be measuredEnthalpy change – the amount of energy absorbed or lost by a system as heat during a process at constant pressure.

 

ΔH =

H

products

-

H

reactants

Heat of ReactionSlide17

Thermochemical equations are usually written by designating the enthalpy change, rather than the energy as a reactant or product

.

2 H2

(g) + O

2

(g)

2 H

2

O (g) ΔH = -483.6

kJ/mol

Note how enthalpy change is a negative number. This means energy is evolved, or given off, during the reaction.

Heat of ReactionSlide18

The opposite would look like the following:

2 H

2O (g) 

2 H

2

(g) + O

2

(g) ΔH = + 483.6 kJ/mol

Heat of ReactionSlide19

An

exothermic

reaction gives off energy, and therefore has a negative enthalpy change.

Heat of ReactionSlide20

An

endothermic

reaction receives energy, and therefore has a positive enthalpy change.Heat of ReactionSlide21