A chemical reaction is generally accompanied by an exchange of heat energy with the environment. This exchange may be either absorption of thermal energy from the atmosphere or emission of thermal energy into the atmosphere. This change of thermal energy in the thermodynamic system is known as change of enthalpy or delta h written as ∆H in chemistry and calculated using the formula ∆H = cm∆T.
Note that the delta h in a chemical reaction is entirely independent of how a reaction took place; no matter the route, the enthalpy change would be constant for a given chemical reaction. Learn what delta H is, along with a step-by-guide to how to find delta H in chemistry; all the tips and major, profound concepts regarding enthalpy change (∆H).
Before starting over the main discussion, let’s have a brief overview of key terminologies related to this article.
Endothermic Reaction
A chemical reaction is said to be endothermic if there is absorption of thermal energy or heat from the environment. In all such reactions, there is an apparent or negligible amount of increase in the enthalpy.
For example, photosynthesis and hydrolysis.
Exothermic Reaction
A chemical reaction is said to be exothermic if there is an emission of thermal energy or heat into the environment. In all such reactions, there is an apparent or negligible amount of decrease in the enthalpy of the thermodynamic system. In simple words, we may say that in exothermic reactions, heat energy is transferred from the system into the environment.
For example, the combustion of coal.
Enthalpy
In a thermodynamic system, we may typically define enthalpy as the summation of E (internal energy) and the product of the V (volume) and P (pressure).
To be more precise, we may represent the above-quoted definition mathematically as:
H = E + PV
Where
• H refers to enthalpy
• E refers to internal energy
• P refers to pressure.
• V refers to Volume.
If we assume that the pressure is constant, which is generally kept constant for most of the reactions, we can also state that the enthalpy is equal to the heat transferred during the reaction.
What is Delta H in Chemistry
Delta h in chemistry, also written as ∆H, refers to the change of enthalpy in a thermodynamic system caused by either absorption or emission of thermal energy.
In simpler words, we may elaborate delta h as the total change in the system’s heat before and after a chemical reaction. Delta is symbolically represented by ∆and refers to “change”. So literally, we derive the meaning of ∆H as a change in enthalpy.
Delta h or change in enthalpy is measured in kilojoules per mole according to the system of international units. So that also means that while dealing with calculations of change in enthalpy, make sure that you’re using Kilojoules per mole, also written as KJ/mole.
To observe the enthalpy change of a specific reaction, the pressure is generally kept constant to understand the results better. We may also say that to get a better understanding of change of enthalpy. We may refer to it as the overall flow of heat during a chemical reaction.
The first law of thermodynamics suggests that energy can neither be created nor be destroyed, though it can be transferred. The transfer may be from the environment to the system, or from the system to the environment.
Change of enthalpy in a chemical reaction is generally observed by observing the difference of enthalpy between products and reactants. This change of enthalpy on both sides of the arrow of the chemical reaction is the enthalpy change.
You can deduce whether a reaction is endothermic or exothermic by observation of the enthalpy change. A positive enthalpy change means that the reaction is endothermic, as heat is absorbed by the system from the environment. The other way round, the enthalpy change is negative, which means that the system has lost some part of thermal energy, and by this very fact, you may deduce that the reaction is exothermic.
Delta H Formula:
The change in enthalpy can be quickly calculated in a given thermodynamic system that has undergone a chemical reaction by the following formula:
∆H = cm∆T
Where ∆H refers to change in enthalpy of a reaction,
C refers to the specific heat. It is constant for all substances,
m represents the mass and,
∆T refers to the change in temperature of reactants and products or, say, before and after the chemical reaction.
If you know the enthalpies of the products and reactants, respectively, you may calculate the delta h by simply subtracting the enthalpy of reactants from that of products using the following formula:
∆H = H (products) – H (reactants)
How to calculate Delta H
Now that you know the delta h formula and what is delta h, you might be wondering how to calculate it practically. It totally comes down to the specific situation. Calculation of delta h is simplified below:
Step-by-Guide to Calculate Delta H:
- Identification of Reactants and Products:
You need to identify the reactants and products of the reaction first of all. This would be your first step. Knowing this, you would be able to deduce the other essential factors of the calculation.
- Calculating the total mass of your reactants.
Now, determine the mass m of the reactants that are combining to react; in case you fail to determine their mass, you can quickly figure it out using the molar masses of the elements.
- Determining the Specific heat of the Product:
Every element has a specific heat that you may use to figure out what the specific heat of your product is.
- Calculating ∆T:k
∆T refers to the change in temperature of the thermodynamic system before and after the chemical reaction. For calculation of ∆T, you may simply subtract the temperature of products from the temperature of reactants.
- Calculating delta h through the formula
Now simply deduce the value of ∆H by putting all the above-calculated values in the formula:
∆H = cm∆T
Tips To Calculate Delta H in Chemistry
Make sure to remember the following tips at all times. They will help you avoid the most common mistakes:
- Write down the equation of the reaction you’re studying in a horizontal form. You’ll be writing delta h or ∆H precisely on the top of the arrow of your chemical equation.
- Then, carefully write down the rest of the information of the reaction in the respective positions to design an accurate Hess’s diagram.
- Finally, ensure that the direction of the arrowheads in your diagram is correct. The opposite direction of your arrowhead will make the whole diagram wrong.
Examples
Example 1
Find the Enthalpy (∆H) of Formation of the following reaction.
C4H10 (l) +132O2 (g) 4 CO2 (g) + 5 H2O (l)
147.6 kJ 0kJ -393.5 kJ -285.8KJ
Solution
the negative Sign shows that Reaction is Exothermic
Example 2
Neutralization of 100 cm3 of 0.5 M NaOH at 25⁰C with 100 cm3 of 0.5 M HCl at 25⁰C raised the
temperature of the reaction mixture to 28.5⁰C. Find the enthalpy of neutralization. Specific heat of
water = 4.2 J K-1g-1
Solution
Specific Heat of Water, s = 4.2 J K-1g-1
Density of H2O is around 1gcm-3, so 200 cm3 of total solution is approximately = 200g
Rise in Temperature, ∆T = 28.5⁰-25.0⁰ = 3.5⁰C = 3.5 K
100cm3 of 0.5 M NaOH = 100cm3 of 0.5 MHCl
0.5 M solution means that 1000 cm3 of solution has 0.5 moles of solute
So 100 cm3 of 0.5 M solutions =0.05 moles of HCl and NaOH, respectively
Amount of total heat evolved, (q) = m x s x ∆T
= 200g x 4.2 J K-1g-1 x 3.5K
= 2940J = 2.94 KJ
Since, the reaction is exothermic
so, q = -2.94 KJ
When this heat is divided by number of moles, then ∆ Hn0 is for one mole
Enthalpy of neutralization. (∆ Hn0) = -2.94KJ/0.05mol
= -58.8 KJ/mol
Calculation in Excel
E2 = ∆T = 28.5⁰-25.0⁰ = 3.5⁰C = 3.5 K
Find ∆H
Find Heat of Enthalpy for One mole, ∆H0
Example 3
Calculate the Enthalpy Change for the Combustion of one mol of glucose (C6H12O6) to carbon dioxide and water.
(C6H12O6) (s) + O2 (g) 6CO2 (g) + 6H2O (l)
| ∆Hf (KJ/mol) | |
| (C6H12O6) (s) | -1260 |
| O2 (g) | 0 |
| 6CO2 (g) | -393.5 |
| 6H2O (l) | -285.5 |
∆Hf (KJ/mol) = -2814 KJ/mol
Example 4
Calculate the enthalpy Change for the Following reaction.
NH3 (g) + HCl (g) NH4Cl (s)
| ∆Hf (KJ/mol) | |
| NH3 (g) | -46.2 |
| HCl (g) | -92.3 |
| NH4Cl (s) | -314.4 |
Solution
∆Hf (KJ/mol) = -175.9 KJ/mol
FAQs
| Sr# | Questions | Answers |
| Q.1 | What is negative enthalpy change? | Enthalpy is said to be negative if its value is less than zero. Such an enthalpy shows that the respective chemical reaction is exothermic, i.e. a reaction that emits thermal energy; due to loss of thermal energy, it results in negative enthalpy change. On the other hand, in the case of a positive enthalpy, the chemical reaction is endothermic, i.e. reaction in which heat energy is absorbed from the atmosphere. |
| Q.2 | What is the enthalpy change unit? | SI unit of the change in enthalpy is kilojoule per mole, though, in older times, it was conventionally measured in calories |
| Q.3 | How to calculate ∆h? | ∆h or change in enthalpy is calculated by either observing the difference in enthalpy of products and reactant or by using the formula ∆H = cm∆T |
Conclusion:
Delta H is the change in enthalpy during a chemical reaction, which may possibly be positive or negative. Enthalpy change says a lot about whether a chemical reaction is positive or negative. You use several formulas to calculate the enthalpy change, almost universally used delta H equation is ∆H = cm∆T.
