The symbol **H **in thermodynamics represents enthalpy. **Enthalpy** is defined as total heat content in a system. It is the sum of internal energy and product of pressure-volume work.

H=U+PV

The sign delta is similar to the sign of a triangle .** Delta** means change. The term is called the** enthalpy change.**

# Sign Convention

If H is positive it means energy is transferred from the surroundings to the system or the system requires energy for the reaction.

If H is negative it means energy is transferred from the system to the surroundings or the system releases energy to do a reaction.

Summary,

**H=+ve (Endothermic reaction)**

**H=-ve (Exothermic reaction)**

Here are some methods to find enthalpy change.

## From the formula of Enthalpy

From the above formula, enthalpy change can be calculated as

H=U+PV

H=U+(PV)

## From the formula of heat capacity

Heat capacity at constant pressure is defined as the ratio of change in enthalpy to change in temperature at constant pressure.

C_{p}=()_{p}

Rearrange the equation to find the enthalpy change

H=C_{p}T

This formula is derived from heat capacity.

**Heat capacity** is defined as the amount of heat required to raise the temperature of a system through 1^{o}C.

C=q/T

q=CT

**Specific heat capacity** is defined as the amount of heat required to raise the temperature of 1g of substance through 1^{o}C.

q=mcT

At constant pressure

(dq)_{p}=(dH)_{p}

So we can write enthalpy in the place of heat.

**Example **

**How many joules are required to heat 100g of copper (c=0.389J/g.K) from 10 ^{o}C to 100^{o}C?**

**Solution**

We are given the values of the mass of copper, change in temperature, and specific heat capacity. Use the above formula to find enthalpy change.

H=mC_{p}T

=(100g)x(0.389J/g.K)x(100-10)K

=3500J

## From change of phase of a substance

H can be calculated from phase changes of substance. There are some processes where the phase of the substance changes.

**Enthalpy Change of Atomisation**

It is the enthalpy change when 1 mole of gaseous atoms are formed from its element. It is denoted by .

The enthalpy change of atomization of lithium is

=+161kJ/mol

The value of enthalpy change is positive because the reaction is endothermic.

**Enthalpy Change of Solution**

It is the amount of heat absorbed or liberated when a substance is dissolved in a solvent to form an infinitely dilute solution.

Enthalpy change of solution may be positive or negative.

(s) +aq (aq) = -55kJ/mol

NaCl(s)+aq NaCl(aq) =+3.9kJ/mol

**Enthalpy Change of Hydration**

It is the change in enthalpy when 1 mol of gaseous ion is dissolved in water to form an infinitely dilute solution.

It is always negative.

_{(g)}+O_{(aq)} _{(aq) } = -364kJ/mol

**Enthalpy Change of Neutralization**

It is the change in enthalpy when 1 mole of acid reacts with a base to form salt and water.

HCl+NaOH NaCl+O = -57.9kJ/mol

**Enthalpy change of Vaporization**

It is also called heat of evaporation. Amount of heat required to convert 1 mole of a liquid into gas.

The heat of vaporization of water is 40.8kJ/mol.

O_{(liquid) } O_{(gas)} = -40.8kJ/mol

**Enthalpy of hydrogenation**

It is the enthalpy change when 1 mole of unsaturated compound reacts with hydrogen to form a saturated compound.

1-butene + H_{2} 1-butane =-30.3kcalmol^{-1}

**Enthalpy of Denaturation**

It is defined as the enthalpy change required to denature 1 mol of a compound.

**Enthalpy of sublimation**

It is defined as the enthalpy change required to convert 1 mol of solid into a gaseous state.

I_{2(s)}I_{2(g) }=106kJmol^{-1}

**Lattice Enthalpy**

It is defined as enthalpy change required when 1 mol of an ionic compound is converted into gaseous ions to an infinite distance apart.

NaCl_{(s)}Na^{+}_{(g)}+Cl^{–}_{9g) } =769kJmol^{-1}

**Enthalpy of formation**

There is a change in enthalpy that accompanies a chemical reaction. It is defined as an enthalpy change that forms when one mole of a substance is formed from its constituent elements.

It is represented asor .

For example

C_{(graphite)}+O_{2}CO_{2(g)} H_{f}=-393.51kJ

H_{2(g)}+I_{2(s)}2HI H^{o}_{f}=+52.72kJ

### Calculate **H** by using enthalpy of formation

The standard enthalpy change of reaction can be calculated by the sum of enthalpy change of formation of products minus the sum of enthalpy change of formation of reactants.

H^{o}_{reaction}=H_{f(products)}-H_{f(reactants)}

**Standard Enthalpies of formation at 25 ^{o}C**

Substance | H^{o}_{f}kJ/mol |
Substance | H^{o}_{f}kJ/mol |
---|---|---|---|

Al_{2}O_{3}(s) |
-1675.7 | HNO_{3}(l) |
-174.10 |

B_{2}O_{3}(s) |
-1272.8 | H_{2}O(g) |
-241.81 |

Br(g) | +111.84 | H_{2}O(l) |
-285.83 |

C(diamond) | +1.88 | H_{2}O_{2}(l) |
-187.8 |

CF_{4}(g) |
-924.7 | H_{2}S(aq) |
-20.6 |

CH_{3}OH(g) |
-200.7 | I_{2}(g) |
+62.4 |

CO(g) | -110.53 | KCl(s) | -436.9 |

CO_{2}(g) |
-393.51 | KClO_{3}(s) |
-397.7 |

CaC_{2}(s) |
-59.4 | LiAlH_{4}(s) |
-101.3 |

Cao(s) | -634.3 | NO(g) | +89.75 |

HI(g) | +26.36 | NO_{2}(g) |
+35.98 |

HF(g) | -271.1 | N_{2}O_{4}(l) |
-19.50 |

HCl(g) | -92.31 | N_{2}O_{4}(g) |
+9.16 |

HBr(g) | -36.38 | O_{3}(g) |
+142.7 |

FeS(s) | -100 | SO_{2}(g) |
-296.81 |

NH_{3} |
+46.19 | SO_{3}(g) |
-395.72 |

CS_{2} |
-87.9 | POCl_{3}(g) |
-558.48 |

All these values are the standard heat of the formation of compounds, not the elements. Because standard heat of formation for elements in their ground state at 25^{o}C and 1 atm is taken to be zero.

For example, the standard state of H_{2}, O_{2}, Cl_{2,} or N_{2} is gaseous; Fe, Na, I_{2}, or Cr is solid; and Br_{2}, Hg, or Cs is liquid. So the standard heat of formation of these substances is zero(0). The standard state of carbon is graphite, not diamond so the heat of formation is zero.

**Example **

**Calculate the enthalpy change for the reaction**

**4NH _{3}+5O_{2}→4NO_{2}+6H_{2}O**

**Solution**

The formula for the enthalpy change of a reaction is

H^{o}_{reaction}=nH_{f(products)}-mH_{f(reactants)}

Where n and m are the numbers of moles of products and reactants from a balanced equation respectively.

The heat of formation for NH_{3}=+49.19

The heat of formation for O_{2}=0

The heat of formation for NO_{2}=35.98

The heat of formation for H_{2}O=-285.83

H_{f(products)}=(4mol of NO_{2})(35.98kJ/mol)+(6mol of water)(-285.83kJ/mol)

H_{f(products)}=143.92-1714.98

H_{f(products)}=-1571.06kJ

Similarly heat of formation for reactants

H_{f(reactants)}=(4mol of ammonia)(49.19kJ/mol)+5(0)

H_{f(reactants)}=196.76kJ

Put these values in the above equation to find enthalpy change for a reaction

H^{o}_{reaction}=H_{f(products)}-H_{f(reactants)}

=-1571.06-196.76

=-1767.82kJ

The negative sign shows that the reaction is exothermic.

## 4. From Hess’s law

Hess’s law is another method to find delta H. this law states that

*‘’The amount of heat evolved or absorbed during chemical reaction remains the same, whether that change takes place in one or several steps.’’*

**Example **

**CS _{2}+3O_{2}→CO_{2}(g)+2SO_{2}(g)**

This reaction is carried out in three steps.

**Step-1 **

C(s)+O_{2}(g)**→**CO_{2} H_{f}=-393.5kJ/mol

**Step-2**

S(s)+O_{2}(g)**→**SO_{2} H_{f}=-60.3kJ/mol

**Step-3**

C(s)+2S(s)**→**CS_{2} H_{f}=87.9kJ/mol

Rewrite the above reactions to get the equation given in the question.

Reverse step-3 because CS_{2} is present on the reactant side.

CS_{2 }**→**C(s)+2S(s) H_{f}=-87.9kJ/mol

Step-1 remain the same.

C(s)+O_{2}(g)**→**CO_{2} H_{f}=-393.5kJ/mol

Multiply step-2 with 2 to get 2 moles of SO_{2}.

2S(s)+2O_{2}(g)**→**2SO_{2} H_{f}=2(-60.3)kJ/mol=-120.6kJ/mol

Now,

H_{f(products)}=-393.5+(-120.6)

H_{f(products)}=-514.1kJ/mol

Similarly

H_{f(reactants)}=-87.9+0

H_{f(reactants)}=-87.9kJ/mol

Put these values in the above equation to find enthalpy change for a reaction

H^{o}_{reaction}=H_{f(products)}-H_{f(reactants)}

=-514.1-(-87.9)

=-426.2kJ/mol

## 5. From Bond energies

Calculate H for the following reaction, using average bond enthalpies, assuming all compounds are there in a gaseous state.

**CH _{4(g)}+2O_{2(g)}→CO_{2(g)}+2H_{2}O_{(g)}**

**Solution**

*The amount of energy required to break a bond between two atoms in their gaseous state is called **bond energy.*

*Breaking of bond always requires energy.*

The enthalpy change for the reaction is equal to the sum of bond energies of the bonds broken minus the sum of bond energies of the bonds formed.

H_{reaction}=n(enthalpies of bonds broken)-m(enthalpies of bonds formed)

Where n and m are the numbers of moles of a bond broken and bonds formed from a balanced equation respectively.

Let’s write the structure of a given equation.

**Avearage Bond Energies(kJ/mol)**

CH | 413 |
---|---|

CC | 348 |

CO | 358 |

OH | 463 |

O_{2} |
495 |

CO | 799 |

Now,

**Enthalpies of Bonds broken**

There are 4 CH bonds that are break and the enthalpy of the CH bond is 413.

(4mol of methane)(413kJ/mol)=1652kJ

There are two O_{2} bonds that are broken so enthalpy is

(2mol of oxygen)(495kJ/mol)=990kJ

**Enthalpies of Bonds formed**

There are two C=O are formed so enthalpy is

2(799)=1598kJ

The number of OH bonds formed are 4 so enthalpy is

4(463)=1852kJ

By putting these values in the formula, we get

H_{reaction}=(enthalpies of bonds broken)-(enthalpies of bonds formed)

H_{reaction}=(1652+990)-(1598+1852)

H_{reaction}=2642-3450

H_{reaction}=-808kJ

## Concepts Berg

- Why cannot carbon tetrachloride be directly prepared by combining carbon and chlorine?

Carbon tetrachloride cannot be prepared directly because it is a free radical reaction. It is not a single-step reaction rather it involves initiation, propagation, and termination steps.

Carbon tetrachloride is formed in the propagation step along with CH_{3}Cl, CH_{2}Cl_{2}, CHCl_{3,} and HCl.

- How to calculate the enthalpy of a reaction?

Enthalpy of reaction can be calculated by a formula which is given below.

H_{reaction}=nH_{f(products)}-mH_{f(reactants)}

This equation states that enthalpy of reaction is equal to the sum of heat of formation of products minus the sum of heat of formation of reactants.

n and m are the numbers of moles of products and reactants from the balanced chemical equation.

- What is the molar enthalpy of formation for ammonia?

The molar enthalpy for the formation of ammonia is -46.19kJ/mol.

- How to calculate ΔH Numerically?

ΔH can be calculated numerically by using the heat of formation of reactants and products according to the formula.

H_{reaction}=nH_{f(products)}-mH_{f(reactants)}

This equation states that enthalpy of reaction is equal to the sum of heat of formation of products minus the sum of heat of formation of reactants.

n and m are the numbers of moles of products and reactants from the balanced chemical equation.

- How do you find delta H?

There is a list of some methods to find delta H.

(i). From the formula of enthalpy

ΔH= ΔU+P ΔV

(ii). From the formula of the heat of formation

H_{reaction}=nH_{f(products)}-mH_{f(reactants)}

(iii). From Hess’s law

(iv). From bond energies

(v). From the formula of heat capacity

H=mC_{p}T

- How to calculate delta H with bond energies?

The enthalpy change for the reaction(delta H) is equal to the sum of bond energies of the bonds broken minus the sum of bond energies of the bonds formed.

H_{reaction}=n(enthalpies of bonds broken)-m(enthalpies of bonds formed)

Where n and m are the numbers of moles of a bond broken and bonds formed from a balanced equation respectively.

- How do I calculate the ΔH of Mg(s) + HCl(l)→ MgCl
_{2}(aq) + H_{2}(g)?

The formula used to find ΔH is

H_{reaction}=nH_{f(products)}-mH_{f(reactants)}

Heat of formation of Mg(s)=0kJ/mol

Heat of formation of HCl(l)=-166.94kJ/mol

Heat of formation of MgCl_{2}(aq)=-801.2kJ/mol

Heat of formation of H_{2}(g)=0kJ/mol

Put these values in above equation

H_{reaction}=nH_{f(products)}-mH_{f(reactants)}

H_{reaction}=(-801.2kJ/mol+0)-(0-166.94kJ/mol)

H_{reaction}=-634.26kJ/mol

- What is delta S in chemistry and what is the difference between delta H and delta S?

The symbol ‘**S’** in chemistry represents the term entropy. Delta S means a change in entropy. Entropy is a measure of disorderliness in the system. It can also be defined as the amount of heat transferred reversibly in and out of the system per unit temperature.

When a chemical reaction takes place, we measure the change in entropy of the system whether it increases or decreases. If reactants are in solid-state and products are in a gaseous state then entropy becomes positive and vice versa.

**Delta H **means the enthalpy change. It is a measure of the amount of heat absorbed or released by a system. In a chemical reaction, positive delta H means the process is endothermic, and negative delta H means the process is exothermic.

Delta H for elements in their standard state is zero whereas delta S for elements is never zero. We cannot find the absolute value of H but we can find the absolute value of S.

- When does q = delta H?

In the isobaric process i.e the process which is carried out at constant pressure the amount of heat added or removed from the system is equal to the change in enthalpy. In this case, there is no work energy but the pressure-volume work.

- What is the difference between delta H and delta U. Can anyone explain to me in detail and intuitively the concepts of delta H delta U and PV work?

Delta H is the enthalpy change and delta U is the change in internal energy. The main difference between them is that when heat is exchanged between the system and surrounding at the constant volume it is equal to delta U.

When the heat is exchanged at a constant pressure it is equal to enthalpy change.

q(at constant volume)=delta U

q(at constant pressure)=delta H

Both these terms are related as

H=U+PV

If V=0 then

H=U

- Given that the reaction of CO + 2H
_{2}yields CH_{3}OH gas and the H= -91 kJ/mol. What is the delta H if CH_{3}OH is a liquid Heat of the evaporation is -37 kJ/mol?

As formation of methanol is an exothermic reaction and H=-91kJ/mol. The heat of evaporation of liquid is H=-37kJ/mol.

We have to find the value of H if methanol(gas) is converted into methanol(liquid) i.e condensation process. So it will be the sum of these two enthalpies change.

H=-91kJ/mol-37kJ/mol

H=-128kJ/mol

- A certain reaction has an enthalpy of ΔH=-59 kJ and activation energy of Ea=23 kJ. What is the activation energy of the reverse reaction?

The activation energy of the reverse reaction is the sum of energy of reactants and existing activation energy. It can be easily described with the help of a graph.

From the graph it is clear that it is an exothermic reaction because reactants have negative enthalpy. The activation energy for the reverse reaction will be

59+23=82kJ

- How to determine whether the reaction is endothermic or exothermic if the value delta energy is not given?

Endothermic is a reaction in which heat is absorbed by the reactants and exothermic is a reaction in which heat is released by the reactants. As the breaking of bonds requires energy and formation of bonds always releases energy.

So, if the energy of reactants is greater than the energy of products then the reaction will be exothermic. And if the energy of products is greater than the energy of reactants then the reaction will be endothermic.

- What is the standard enthalpy of formation of H
_{2 }and why?

The standard enthalpy of formation is defined as the amount of heat change when 1 mol of a compound is formed from its constituent elements.

The standard enthalpy of formation for H_{2} is zero as the enthalpy of formation for the elements in their natural state is zero because there is no heat change when elements are formed themselves.

The second reason is that in thermochemistry, we measure the enthalpy change for the process which takes place in a system. And the change in a system can be measured by using variables(P, T, U, H, V). But in the case of the formation of free elements, such a system does not exist.

- What is the specific enthalpy?

Specific enthalpy is defined as enthalpy per unit mass of the system. Specific enthalpy is represented by **h**. It is also defined as the sum of specific internal energy and product of pressure and specific volume.

**h=H/m**

Enthalpy is an extensive property and to make it intensive we divide it with mass. So specific enthalpy is an intensive property.

- What’s the formula for the pressure difference?

Bernoulli’s equation tells the formula of pressure difference.

P=gh

Where,

P=pressure difference

=density of a fluid

g=acceleration due to gravity

h=height of fluid

- How to find a derivative by the limit process?

The formula of derivative from 1st principle is

f^{′}(x)=Lim_{h→0}

From this equation, we can find derivatives by the limit process.

- C
_{2}H_{6}+O_{2}→CO_{2}+H_{2}O. What is the value of delta H?

First of all we have to balance the chemical equation.

2C_{2}H_{6}+7O_{2}→4CO_{2}+6H_{2}O

The formula used to find ΔH is

H_{reaction}=nH_{f(products)}-mH_{f(reactants)}

Heat of formation of ethane=-84kJ/mol

Heat of formation of CO_{2}=-394kJ/mol

Heat of formation ofH_{2}O=-286kJ/mol

Put these values in above formula

H_{reaction}=-

H_{reaction}=(-1576-1716)-(-168)

H_{reaction}=-3292+168

Reaction=-3124kJ

- What is an equation that relates temperature to enthalpy?

The equation of specific heat capacity at constant pressure relates to temperature and enthalpy.

C_{p}=(dH/dT)_{p}

To find the enthalpy change, rewrite this formula

dH=C_{p}dT

The change in enthalpy is directly linked to the change in temperature.

- What is the formula for power when mass height and time are given?

Power is defined as work done per unit time.

P=W/t

Work is a product of force and distance.

P=F.d/t

P=m.ad/t

Replace a with g and d with h.

P=mgh/t

m is the mass of a body, g is the acceleration due to gravity, h is the height of the body fallen and t is the time of the fallen body.

- What is the difference between activation enthalpy and the enthalpy of reaction?

The amount of energy required to start a reaction is called activation energy of activation **enthalpy.** It is the difference between the energy of the transition state and the energy of reactants.

**Enthalpy of reaction** is the amount of energy released or absorbed during a chemical reaction. There is a difference between the enthalpy of a product and the enthalpy of reactants.

- In chemistry, how do you use the equation Q=mc∆t?

This equation is used to find heat when values of the mass of substance, and change in temperature are given.

Let suppose we have 150g of substance and specific heat capacity is always 4.18J/g and change in temperature is -9^{o}C.

Q=(150)(4.18)(-9)

Q=-5643J

-ve sign indicates that it is an exothermic reaction.

- What does the mathematical symbol Δ mean? Can you give some very simple examples of how it is used?

Δ means **the change** in mathematics. It is the difference between the final value and the initial value. For example, if y represent height then change in height will be equal to

Δ y=y_{2}-y_{1}

- How to calculate enthalpy for superheated steam?

Enthalpy is the amount of energy at some specific temperature and pressure. If these two properties are known, the enthalpy for superheated steam can be calculated from a table. The table is provided in thermodynamics books.

- What is the enthalpy change for a reversible reaction?

The enthalpy change for reversible reaction can be determined by using Gibbs free energy formula.

dG=dH-TdS

At equilibrium dG=0

So,

dH=TdS

For reversible reaction, the enthalpy change is equal to the product of temperature and entropy change.

It can be described in such a way that enthalpy change for forward reaction is equal to enthalpy change for a reverse reaction but of the opposite sign.

- What does the symbol Δ (delta) mean in chemistry?

The symbol delta in chemistry represents heat. This symbol is used in a reaction where heat is absorbed or released to indicates heat change. It is written on the right side of the equation for the endothermic reaction and the left side for an exothermic reaction.

For example,

CH_{4}(g)+2O_{2}(g)→CO_{2}(g)+2H_{2}O(l)+Δ

It is also used to show the change. For example, the change in temperature is

ΔT=T_{2}-T_{1}

27. What is the change in enthalpy in a constant volume process?

Enthalpy is defined as the sum of internal energy and product of pressure-volume work.

H=U+PV

The change in enthalpy is equal to

ΔH=ΔU+ΔP.V+PΔV

At constant volume ΔV=0, so the change in enthalpy becomes equal to

ΔH=ΔU+ΔP.V

28. What is the effect of pressure and temperature on enthalpy?

Enthalpy has a direct relation to temperature. When temperature increases the K.E and P.E of substance increase which increases the internal energy of the system. So the enthalpy also increase because from the definition of enthalpy

H=U+PV

Enthalpy is defined as the amount of heat transferred at constant pressure. It is independent of pressure. It is a function of temperature only.

29. What is the activation energy for a reverse reaction?

The amount of energy required by the reactants to form a product is called activation energy. It is a difference of energy between the energy of reactants and of the transition state.

For a reverse reaction, the reactants become products and the product of the forward reaction becomes reactants for the reverse reaction.

Now activation energy is a difference of energy between the energy of products and of the transition state.

Now there are two cases.

For an endothermic reaction, the activation energy for the reverse step is lower than the forwarding step. So reverse step is more feasible.

For exothermic reactions, the activation energy for the forward step is lower than the reverse step. So the forward step is more feasible.

30. What is the difference between heat and enthalpy?

**Heat** is defined as the amount of energy transfer from one place to another. While **enthalpy** is the total heat content in a system. When **heat **is added to a system it increases its internal energy while **enthalpy** is equal to the sum of internal energy and product of pressure-volume work. **Heat **is a path function while **enthalpy** is a state function.

31. How to determine the formula to calculate depth?

If you find the depth of square with area A then its formula will be

Depth = Volume/Area

32. What is the difference between hfg and latent heat?

HFG is the amount of heat required to convert liquid into vapors and it is called the latent heat of vaporization. It is the difference in enthalpies of gas and liquid.

HFG=h_{g}-h_{f}

Where subscript ‘f’ stands for fluid.

The term** latent heat** is general. It may be a latent heat of fusion, latent heat of vaporization or latent heat of sublimation. It is the ratio of heat required to phase change and mass.

L=Q/m

33. How do I calculate a theoretical value for the total change in temperature or energy in exothermic chemical reactions?

34. Does an isothermal process have an enthalpy change?

Enthalpy is the sum of internal energy and product of pressure-volume work.

H=U+PV

The change in enthalpy is

H=U+(PV)

For the isothermal process, T=0 so the change in internal energy is zero because internal energy is a function of temperature U=0

H=0+(PV)

For ideal gas PV=nRT

H=(nRT)

H=nRT

So, H=0

It means for isothermal processes there is no enthalpy change.

35. How do intermolecular forces affect a liquid’s heat of vaporization?

The stronger intermolecular forces mean more energy is required to convert liquid into a vapor state. It will cause more heat of vaporization of the liquid and lower its vapor pressure.

36. How do we find the enthalpy change of a reversible reaction? Is it equal to the sum of the enthalpies of the front and reverse reactions?

For a reversible reaction, the enthalpy change for the forwarding reaction is equal to the enthalpy change for a reverse reaction but with opposite signs.

37. If NH_{4}Cl dissolves in water what is the final equation?

NH_{4}Cl is a strong electrolyte. It is formed by the reaction of strong acid HCl and weak base NH_{3}. when it is dissolved in water it completely dissociates into its ions.

NH_{4}Cl+H_{2}O→NH_{4}^{+}+Cl^{–}

It forms an acidic solution in water.

38. Is change in enthalpy dependent on temperature?

Enthalpy is the sum of internal energy and product of pressure-volume work.

H=U+PV

The change in enthalpy is

H=U+(PV)

When the temperature of a system increases its internal energy also increases which increases the enthalpy change.

Also from the formula of specific heat capacity at constant pressure

dH=C_{p}dT

So, from both equations enthalpy change is dependent on temperature.

39. How was the entropy equation ∆S= ∆Q/T formulated and how can it be conceptualized?

From the 2nd law of thermodynamics

dS=dQ/T

This equation states that the change in entropy is equal to the amount of heat transferred reversibly in and out of the system at a given temperature.

∆S=dQ/T

To calculate the difference in entropy between any two states of a system, we find a reversible path between them and integrate the energy supplied as heat at each stage of the path divided by the temperature at which heating occurs.

40. The reaction NaOH(s) NaOH aq has an enthalpy of reaction of 41 kJ If there are 2 g of NaOH what would be the enthalpy of reaction if there were 8 g of NaOH What are the steps?

41. How can we classify Na+Cl= NaCl on the basis of oxidation and reduction? Since the oxidation, no of Na is getting increased but on the other hand, the oxidation no of Cl is getting reduced?

First of all, we balance the equation

2Na+Cl_{2}→2NaCl

Split the equation

2Na→2Na^{+}+2e

Cl_{2}+2e→2Cl^{–}

By combining

2Na^{+}+2Cl^{–}→2NaCl

Oxidation is the loss of electrons and reduction is the gain of electrons. In this reaction sodium loses its electron so it gets oxidized. While chlorine is the reduction of electrons so it gets reduced.

Moreover, the oxidation number increases in oxidation and decreases in reduction. That’s why the oxidation number of sodium increases and the oxidation number of chlorine decreases.

42. In thermodynamics, what are hf, hg, and hfg in steam tables?

In thermodynamics, the symbol ‘h’ represents specific enthalpy.

So the symbol h_{f} represents the specific enthalpy of fluid/liquid.

The symbol h_{g} represents the specific enthalpy of gas/vapor state.

Hfg is the difference between h_{g} and h_{f}.

43. What is the correct formula for ΔH?

Hess’s law states that

If a chemical change takes place by several different routes the overall energy change is the same, regardless of the route by which chemical change occurs, provided the initial and final conditions are the same.

Mathematically,

H=0

H=H_{1}+H_{2}+H_{3}+…

44. What is H in delta H?

The symbol ‘H’ in thermochemistry represents **Enthalpy.** Enthalpy is the sum of internal energy and product of pressure-volume work. Enthalpy measures the amount of heat absorbed or released during a chemical reaction. Delta H is the change in enthalpy.

45. Is Q and Delta H the same?

Q(heat) is the transfer of energy between the system and surroundings. Enthalpy(H) is the total heat content of the system. It is the sum of internal energy and product of pressure-volume work.

They are not the same in normal cases. But if the process is carried out at constant pressure the amount of heat absorbed is equal to enthalpy change.

46. What is Delta H in Gibbs free energy?

Gibbs free energy is the amount of energy that is available to do work. It is equal to

G=H-TS

Where

G=change in Gibbs free energy

H=change in enthalpy

T=temperature

S= change in entropy

47. How to find delta S?

For any chemical reaction delta S is the change in entropy from reactants to products. It is a difference between the entropy of the product and the entropy of the reactant.

S=S(product)-S(reactant)

48 .How to find delta H on a graph?

To find delta H from a graph, plot a graph between reaction progress on the x-axis and energy on the y-axis.

Delta H is equal to the difference between activation energy of forward reaction and activation energy of reverse reaction.

H= E_{a}(forward)-E_{a}(reverse)

49. How to find delta H_{f}?

Delta H_{f }is the heat of formation. It is defined as the amount of heat absorbed or released when 1 mol of the compound is formed from its constituent elements.

The heat of formation for a compound at standard temperature and pressure is given in a table. The heat of formation for elements is regarded as zero. It can be calculated from the formula

H=H_{f}(product)-H_{f}(reactant)

If the heat of reaction and heat of formation of reactants and products are given we can calculate the heat of formation of others.

## Reference Book

Schaums-college chemistry 9th edition by Jerome L. Rosenberg, Ph.D. Lawrence M.Epstein, Ph.D. Peter J. Krieger, Ed.D.