Change in - PowerPoint Presentation

Slide1

Change in Enthalpy

Unit

11Slide2

Learning ObjectiveYou should be able to define and calculate the enthalpy of a reaction.You should be able to identify whether a reaction is endothermic or exothermic

from a

chemical reaction or grap

h.Slide3

Chemical ChangesChemical reactions involve the breaking of existing bonds (reactants) and the formation of new bonds (products).Breaking of bonds

requires energy

.

Formation of bonds

releases energy

. CH4 + Cl2  CH3Cl + HCl + Cl-Cl H-Cl +Slide4

Enthalpy

Measure

of the

heat

content of the system

The change in enthalpy for a reaction lets us know if energy is being

taken in or if

energy

is

being

released

.Slide5

Endothermic Reactions

Reaction

consumes

energy

Energy

is listed on the reactant (left) side of the

reaction

ΔH

is

a positive value

Products

have more

enthalpy

(

energy

)

than

the

reactantsSlide6

Endothermic ReactionsThe endothermic reaction

absorbs heat; energy is a reactantSlide7

Exothermic Reactions

Reaction

releases

energy

Energy

is listed on the product (right) side of the reaction

ΔH

is

a

negative

value

Reactants

have more

enthalpy

(

heat

)

than

the

productsSlide8

Exothermic ReactionsThe exothermic reaction

releases heat; energy is a productSlide9

Changes in Enthalpy ΔH

To measure and study energy changes in reactions, chemists defined a property called enthalpy (H)

Enthalpy is a measure of heat energy of a system at constant pressure

It can’t be measured directly, we measure the change in enthalpy.

Δ

HrxnSlide10

2 NaHCO3(s)

+ 129 kJ

 Na

2

CO

3(s) + H2O(g) + CO2(g)Find the energy required to react 1mole of NaHCO3

Slide11

2 NaHCO3(s) + 129 kJ  Na

2

CO

3

(s) + H

2O(g) + CO2(g)1.Find the energy required to produce ½ mole of CO2(g) ?2.Find the energy required to produce 1/3 mole of H2O(g) ?Slide12

Enthalpy Question

For the decomposition of hydrogen peroxide, it is known that:

H

2

O

2(l) → H2O(l) + 1/2 O2(g) ΔH = -98.2 kJUsing this information, determine ΔH for the reaction:2 H2O(l) + O2(g) → 2 H2O2(l) Slide13

Find ΔH for each given the pair of equationsCaO(s) + H2O(l) 

Ca

(OH)

2

(s) + 65.2 kJ

½ Ca(OH)2(s)  ½ CaO(s) + ½ H2O(l) Slide14

Calculating Changes in EnthalpyChange in enthalpy is called heat of reaction Δ

H

rxn

Δ

Hrxn = ΣHf(products) - ΣHf(reactants)

“Δ Hf0” is called standard heat of formation - the change in enthalpy from the formation of 1 mole of a compound from its elements

This information will come from a table

“

Δ

” is the letter delta – it means “change”

“

Σ

” is the letter sigma – it means “sum”

“

o

” refers to standard temperature and pressure (

STP

)

Units: kJ/

molSlide15

Change in Enthalpy

All elements in their standard states have an enthalpy of zero

Because there is no change involved in their formation

Endothermic

When N

2 and O2 combine to form NO2, the ΔH = +33.2 kJ/molExothermicWhen S and O2 combine to form SO3, the ΔH = -396 kJ/mol. Slide16

Calculate ΔH for the following reaction:8 Al(s) + 3 Fe3O4(s) --> 4 Al2O3(s) + 9 Fe(s)Slide17

8 Al(s) + 3 Fe3O4(s) --> 4 Al2O3(s) + 9 Fe(s)

ΔH = Σ

ΔH

f

products - Σ

ΔHf reactantsΔH = sum of ΔHf products – sum of ΔHf reactants:ΔH = (0) + 4 ΔHf Al2O3(s) - 3 ΔHf Fe3O4(s) + (0)Slide18

The values for ΔHf may be found in the Heats of Formation of Compounds table. Plugging in these numbers:ΔH = 4(-1669.8 kJ) - 3(-1120.9 kJ

)

Δ

H = -3316.5 kJSlide19

Enthalpy Change ProblemUse standard heat of formation to calculate ΔH

rxn



for the combustion of methane. Then, decide if the reaction is exothermic or endothermic.

Molecule

Standard Heat of Formation

ΔH

f

0

CH

4

(g)

-75 kJ/mole

C

2

H

2

(g)

227 kJ/mole

C

2

H

6

(g)

-85 kJ/mole

C

6

H

12

O

6

(s)

-1268 kJ/mole

O

2

(g)

0 kJ/mole

H

2

(g)

0 kJ/mole

CO

2

(g)

-394 kJ/mole

H2O (l)-286 kJ/mole

CH

4

(g) + 2O

2

(g) → CO

2

(g) + 2H

2

O(l)