Thermodynamics

Calorimeter: Measurement of Heat, Delta U, and Delta H

Calorimetry is the process of measuring an amount of heat in physical changes, chemical reactions, or in transitions of phase. A calorimeter is the tool used for calculating Calorimetry. Calorimetry can be used to calculate the amount of heat transfer in substance. It also tests whether a chemical reaction is exothermic or endothermic. Let us see how we can use this technique to measure Delta U and Delta H.

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Calorimetry

Generally, calorimetry refers to an experimental technique that we use for the measurement of enthalpy( delta H) and internal energy( delta U). Calorimetry techniques use the principle of thermometric methods carried out in a vessel. This is the calorimeter that we keep immersed in a specific volume of liquid.

calorimetry

source: opentextbc

We calculate the heat let out in the process with the help of known heat capacities of the liquid and the calorimeter. This, we can do by measuring the temperature differences. There are two different conditions under which we can make these measurements. They are:

  • At constant pressure, qp
  • At constant volume, qv

Browse more Topics under Thermodynamics

Types of Calorimetry Techniques

  1. Sorption Calorimetry
  2. Isothermal Microcalorimetry
  3. Reaction Calorimetry
  4. Isothermal Titration Calorimetry

Measurement of Enthalpy Change( Delta H)

We can define enthalpy as the energy released at constant pressure. Mathematically, it is nothing but the total sum of internal energy with the product of pressure and volume.

H = U + PV

Where,

  • H= enthalpy
  • U= internal energy
  • P= pressure of the system
  • V= volume of the system

We must remember that it is very difficult or near to impossible to calculate the absolute enthalpy of a system directly. Therefore, we calculate the enthalpy about a reference point. Therefore, we generally take into consideration the change in enthalpy.

Enthalpy change, or ΔH, is positive for endothermic reactions. On the other hand, it is negative for exothermic reactions. We measure the enthalpy change in the laboratory through various calorimetry techniques.

We are aware of the fact that enthalpy change is the heat change at constant pressure.  “Coffee-cup calorimeter” is what we commonly use to calculate the enthalpy change.

What is the Coffee-Cup Calorimeter?

In this technique, we fill the cup partially with a known volume of water and insert a thermometer through the lid of the cup. We do it in such a way that its bulb is below the water surface. Whenever a chemical reaction takes place, the water absorbs the heat of the reaction.

The change in the water temperature is what we actually use to calculate the amount of heat that the reaction has absorbed or given up. The cup is usually made of polystyrene foam. This is a very good insulator. Hence, very little heat energy escapes. We can calculate the energy change or enthalpy change in this process:

ΔH  =  qp  =  mcpΔT

Where,

  • m= mass of water
  • c= specific heat capacity of water at constant pressure
  • ΔT = temperature difference

Solved Example for You

Q: How do you measure the change in internal energy (delta U)?

Ans: We generally use a bomb calorimeter for measuring internal energy change(delta U). In this technique, we immerse a steel vessel (commonly called bomb) in a water bath so that no heat is lost to the surrounding.

We burn a combustible substance n oxygen gas supplied in the bomb. The water absorbs the heat evolved and we record the change in temperature. As the volume remains constant, work done is zero for the system. We calculate the energy change or internal energy change in the process as:

ΔU = qv  = mcvΔT

Where,

  • m= mass of water
  • cv= specific heat capacity at constant volume
  • ΔT = temperature differenceBomb calorimeter
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