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It is tempting to use the heat of to calculate the energy of an O-H bond. Since breaking the two O-H bonds in water requires 926.9 kJ mol , then we might infer that breaking a single O-H bond requires 926.9 kJ mol 2 463.5 kJ mol . However, the reaction

H 2 O ( g ) O H ( g ) + H ( g )

has Δ H ° 492 kJ mol . Therefore, the energy required to break an O-H bond in H 2 O is not the same as the energy required to break the O-H bond in the O H diatomic molecule. Stated differently, it requires more energy to break the first O-H bond in water than is required to break thesecond O-H bond.

In general, we find that the energy required to break a bond between any two particular atoms depends upon themolecule those two atoms are in. Considering yet again oxygen and hydrogen, we find that the energy required to break the O-H bond inmethanol ( C H 3 O H ) is 437 kJ mol , which differs substantially from the energy of . Similarly, the energy required to break a single C-H bond in methane( C H 4 ) is 435 kJ mol , but the energy required to break all four C-H bonds in methane is 1663 kJ mol , which is not equal to four times the energy of one bond. As anothersuch comparison, the energy required to break a C-H bond is 400 kJ mol in trichloromethane ( H C Cl 3 ), 414 kJ mol in dichloromethane ( H 2 C Cl 2 ), and 422 kJ mol in chloromethane ( H 3 C Cl ).

These observations are somewhat discouraging, since they reveal that, to use bond energies to calculate the heatof a reaction, we must first measure the bond energies for all bonds for all molecules involved in that reaction. This is almostcertainly more difficult than it is desirable. On the other hand, we can note that the bond energies for similar bonds in similarmolecules are close to one another. The C-H bond energies in the three chloromethanes above illustrate this quite well. We canestimate the C-H bond energy in any one of these chloromethanes by the average C-H bond energy in the three chloromethanes molecule,which is 412 kJ mol . Likewise, the average of the C-H bond energies in methane is 1663 kJ mol 4 416 kJ mol and is thus a reasonable approximation to the energy required to break a single C-H bond in methane.

By analyzing many bond energies in many molecules, we find that, in general, we can approximate the bondenergy in any particular molecule by the average of the energies of similar bonds. These average bond energies can then be used toestimate the heat of a reaction without measuring all of the required bond energies.

Consider for example the combustion of methane to form water and carbon dioxide:

C H 4 ( g ) + 2 O 2 ( g ) C O 2 ( g ) + 2 H 2 O ( g )

We can estimate the heat of this reaction by using average bond energies. We must break four C-H bonds at anenergy cost of approximately × 4 412 kJ mol and two O 2 bonds at an energy cost of approximately × 2 496 kJ mol . Forming the bonds in the products releases approximately × 2 743 kJ mol for the two C=O double bonds and × 4 463 kJ mol for the O-H bonds. Net, the heat of reaction is thus approximately Δ H ° 1648 992 1486 1852 -698 kJ mol . This is a rather rough approximation to the actual heat ofcombustion of methane, -890 kJ mol . Therefore, we cannot use average bond energies to predictaccurately the heat of a reaction. We can get an estimate, which may be sufficiently useful. Moreover, we can use these calculationsto gain insight into the energetics of the reaction. For example, is strongly exothermic, which is why methane gas (the primary component in natural gas) isan excellent fuel. From our calculation, we can see that the reaction involved breaking six bonds and forming six new bonds. Thebonds formed are substantially stronger than those broken, thus accounting for the net release of energy during thereaction.

Review and discussion questions

Assume you have two samples of two different metals, X and Z. The samples are exactly the same mass.

Both samples are heated to the same temperature. Then each sample is placed into separate glassescontaining identical quantities of cold water, initially at identical temperatures below that of the metals. The finaltemperature of the water containing metal X is greater than the final temperature of the water containing metal Z. Which of the twometals has the larger heat capacity? Explain your conclusion.

If each sample, initially at the same temperature, is heated with exactly 100J of energy, which samplehas the higher final temperature?

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Explain how Hess' Law is a consequence of conservation of energy.

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Consider the reaction N 2 O 4 ( g ) 2 N O 2 ( g ) Draw Lewis structures for each of N 2 O 4 and N O 2 . On the basis of these structures, predict whether the reaction isendothermic or exothermic, and explain your reasoning.

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Why is the bond energy of H 2 not equal to Δ H f ° of H 2 ? For what species is the enthalpy of formation related to the bondenergy of H 2 ?

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Suggest a reason why Δ H ° for the reaction C O 2 ( g ) C O ( g ) + O ( g ) is not equal to Δ H ° for the reaction C O ( g ) C ( g ) + O ( g )

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Determine whether the reaction is exothermic or endothermic for each of the following circumstances:

The heat of combustion of the products is greater than the heat of combustion of the reactants.

The enthalpy of formation of the products is greater than the enthalpy of formation of the reactants.

The total of the bond energies of the products is greater than the total of the bond energies for thereactants.

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Questions & Answers

how does Neisseria cause meningitis
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Muhammad
is the branch of biology that deals with the study of microorganisms.
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studies of microbes
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How bacteria create energy to survive?
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Bacteria doesn't produce energy they are dependent upon their substrate in case of lack of nutrients they are able to make spores which helps them to sustain in harsh environments
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they make spores
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the significance of food webs for disease transmission
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food webs brings about an infection as an individual depends on number of diseased foods or carriers dully.
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Assimilatory nitrate reduction is a process that occurs in some microorganisms, such as bacteria and archaea, in which nitrate (NO3-) is reduced to nitrite (NO2-), and then further reduced to ammonia (NH3).
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This process is called assimilatory nitrate reduction because the nitrogen that is produced is incorporated in the cells of microorganisms where it can be used in the synthesis of amino acids and other nitrogen products
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Examples of thermophilic organisms
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Give Examples of thermophilic organisms
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Prevent foreign microbes to the host
Abubakar
they provide healthier benefits to their hosts
ayesha
They are friends to host only when Host immune system is strong and become enemies when the host immune system is weakened . very bad relationship!
Mark
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faisal Reply
cell is the smallest unit of life
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cell is the smallest unit of life
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ok
Innocent
cell is the structural and functional unit of life
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is the fundamental units of Life
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There are nothing like emergency disease but there are some common medical emergency which can occur simultaneously like Bleeding,heart attack,Breathing difficulties,severe pain heart stock.Hope you will get my point .Have a nice day ❣️
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I think infection prevention and control is the avoidance of all things we do that gives out break of infections and promotion of health practices that promote life
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en français
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Many sites of the body have it Skin Nasal cavity Oral cavity Gastro intestinal tract
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skin
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skin,Oral,Nasal,GIt
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part of a tissue or an organ being wounded or bruised.
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Binomial nomenclature
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Source:  OpenStax, General chemistry i. OpenStax CNX. Jul 18, 2007 Download for free at http://cnx.org/content/col10263/1.3
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