Reaction Enthalpy

Have you heard the term reaction enthalpy? Do you know what it is? What is your opinion about any reaction that happens around us? We mix two components and then we get a product/products. Is it this simple? No, there are many intermediate reactions happening. There is a give and take of heat. In simple terms, this is reaction enthalpy. So without further delay, let’s see what exactly reaction enthalpy is and the ways to calculate it.

What is Reaction Enthalpy?

During any chemical reaction, heat is either absorbed or given out. The heat exchange between the chemical reaction and its environment is reaction enthalpy (H). This cannot be measured directly. For this, there is a measurement of change in the temperature of a reaction over time to the final change in enthalpy denoted by ΔH.

From the value of ΔH, whether in a reaction heat is absorbed from the environment [endothermic] or is given off to the environment [exothermic] is determined. In general, ΔH is given as, ΔH = m×s×ΔT, where m is the mass of reactants, s is the specific heat of products, and ΔT is a change in temperature during the completion of the reaction.

Source: Wikipedia

How to Calculate the Reaction Enthalpy?

To find enthalpy of a chemical reaction we need to follow the below mentioned steps.

• Determine products and reactants of the reaction
• Find total mass of reactants
• Determine the specific heat values of the products
• Find the difference in temperature between the start and end of a reaction
• Finally, multiply the mass of reactants by the heat values and that number, in turn, by the change in temperature

Finally, we will get the enthalpy of the chemical reaction.

Example of Enthalpy Calculation

Hydrogen reacts with oxygen to give water. The chemical reaction is,

2H2 + O2   →   2H2O

Reactants       Products

• Step 1: Determining reactants and products – In this reaction, H2 and O2 are the reactants and H2O is the products.
• Step 2: Determination of the total mass of reactants – We can find total mass of reactants from the molar masses which can be formed in the periodic table

2H2 + O2 = 2H2O

2×[2g] 1×[32g] = 4g + 32g =36g

• Step 3: Determining specific heat of products – Every product has a specific heat value associated with it, which are constant values. Specific heat values carry the unit joule/gram°C.

2H2 + O2 = H2O

The specific heat of water is 4.2 joule/gram°C

• Step 4: Finding the difference in temperature after completion of reaction – For this, we have to subtract initial temperature (T1) of the reaction from the final temperature (T2) of the reaction, expressed in Kelvin.

ΔT = T1  – T2      (T1 = 185K and T2 = 95 K) = 185K – 95K

ΔT = -90K

In the above reaction, the reactants had more temperature, and after the completion of the reaction, the product got cooled off.

• Step 5: Use of formula ΔH = m×s×ΔT

ΔH = 36g×42 jg-1k-1 ×(-90)K = -13608 joule

ΔH < 0

• Step 6: Determining whether this reaction gained or lost energy – ΔH determines whether the reaction gains or loses heat. The positive sign of ΔH means, the reaction gains heat and hence is endothermic. If the sign of ΔH is negative, the reaction is exothermic.

This makes logical sense since hot gases hydrogen (H2) and oxygen (O2) react together and let off heat to the environment. Then water [H2O] forms, indicating that it is an exothermic reaction.

Which other methods are used to calculate enthalpy?

Here is the alternate method

• Use of bond energies: Chemical reactions involve making and breaking of chemical bonds. If we know the energy required to make or break the bonds in the reaction, then we can calculate the enthalpy change for the entire reaction. For example, hydrogen and fluorine react together to give hydrogen fluoride.

H2 + F2    →    2HF

From the chemistry databook, we need bond energy to break H2 and F2 molecules of 436kJ/mol and 158kL/mol, respectively. The bond energy required to form HF molecule is -568kJ/mol.

H2                +                F2          →         2HF

436kJ/mol           158kJ/mol             2*(-568)

= -1136 kJ/mol

Adding these values together we get,

436 + 158 + (-1136) = -542kJ/mol

Use of enthalpies of formation: Enthalpies of formation are a set of values representing enthalpy the changes in enthalpy taking place during the formation of that particular chemical. Let’s understand this through one example.

When ethyl alcohol is heated, it gives carbon dioxide and water.

C2H5OH + 3O2   →    2CO2 + 3H2O

Let’s find enthalpies of formation for C2H5OH, CO2, and H2O

C2H5OH  →   2C + 3H2 + 0.5O2 = 228 kJ/mol

2C + 2O2      →      2CO2 = -394×2 = -788 kJ/mol

3H2 + 1.5O2   →   3H2O = -286×3 = -858 kJ/mol

We can add these values together and find the enthalpy of reaction.

288 + (-788) + (-858) = -1418

The formation of C2H5OH →  2C + 3H2 + 0.5 O2 is a breaking reaction. Its formation value is negative, i.e. (-228) kJ/mol. Here, we have to switch the sign to (+ve) as we are using the product of the reverse reaction

2C + 3H2 + 0.5O2   →  C2H5OH

Let’s see the third method

Observing enthalpy changes experimentally

• Take a clean container and fill it half with clean water
• Measure its temperature by a thermometer, say its 10°C (T1)
• Add one Alka-Selzer tablet to the water. It starts bubbling and fizzing. As it dissolves in water, it breaks down into bicarbonate (HCO3) and citric acid, which reacts in the form of hydrogen ion H+

The reaction further continues as follows.

3HCO3 + 3H+ → 3H2O + 3CO2

• Measure the temperature after the reaction ends. You can feel that the water is slightly cold than before. Measure its temperature. Say it is 8°C (T2)
• Alka-Selzer tablet in water forms water and carbon dioxide. It absorbs energy from water to come out as fizzing bubbles and causes the water temperature to drop

In this experiment, we observed a drop in the water temperature 2°C which indicates that the reaction is mildly endothermic.

ΔT = T2 – T1 = 8 – 10 = -2°C

Solved Example for You

Question: What happens to the heat when a bond is formed or broken?

Solution: When the bond breaks, heat is given out, i.e. the reaction is exothermic, while when the bond is formed, heat is absorbed, i.e. the reaction is endothermic.

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