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Tag: calorimetry

  • What is Calorimetry?

    What is Calorimetry?

    Calorimetry is a technique which we use to measure the amount of heat released or absorbed during a reaction. To measure the heat of a reaction accurately, it is essential to isolate the system to avoid heat exchange with the environment. We accomplish this using a device known as a calorimeter.

    Calorimeter mainly consists of metallic vessels which are good conductor of heat such as aluminium and copper etc. The vessel includes a built-in stirring mechanism to mix its contents. This vessel is placed inside an insulated jacket to minimize heat loss to the surroundings. A single opening is provided for inserting a thermometer to monitor temperature changes during the reaction.

    Principle of Calorimetry

    The principle of calorimetry is based on the law of conservation of energy, which states that energy cannot be created or destroyed, only transferred from one body to another. When two objects at different temperatures come into contact, the object at the higher temperature transfers heat energy to the one at the lower temperature. This heat transfer continues until both objects reach the same temperature, a state known as thermal equilibrium.

    Example

    Consider a scenario where someone places a hot iron rod into a container of cool water. Since the iron rod is at a higher temperature and the water is at a lower temperature, heat will transfer from the rod to the water.

    As a result:

    • The temperature of the iron rod decreases.
    • The temperature of the water increases.

    This exchange of heat continues until both the rod and the water reach the same final temperature. According to the principle of calorimetry, the total heat lost by the hot object is equal to the total heat gained by the cooler one. Mathematically, we express it as follows:

    qwater + qiron rod = 0

    By rearranging this gives:

    qwater = – qiron rod

    Here, q represent the amount of heat transferred, we calculate it using the formula:

    q = mcΔT

    Where:

    • m is the mass of the substance (in grams),
    • c is the specific heat capacity (a material-specific constant),
    • ΔT is the change in temperature, calculated as:

    ΔT =Tfinal – Tinitial

    Where, Tfinal is final temperature and Tinitial is initial temperature.

    This formula allows us to quantify the heat exchanged between substances during thermal interactions and is fundamental in calorimetry calculations.

    Types of Calorimeter

    1. Bomb Calorimeter
    2. Coffee cup calorimeter

    Let’s discuss about these calorimeter in detail:

    1. Bomb Calorimeter: A bomb calorimeter is a device used to measure the heat of combustion of a substance. It functions based on the principle of calorimetry and operates by burning a sample in a sealed chamber filled with high-pressure oxygen at constant volume. Scientists refer to this sealed chamber as the “bomb” due to its design, which can withstand the force generated during combustion. In most bomb calorimeters, water surrounds the chamber as it absorbs the heat released during combustion process.
    2. Coffee cup Calorimeter: This calorimeter is a simple yet effective tool for measuring heat transfer during chemical reactions in liquid solutions. It operates at constant pressure and typically consists of two nested Styrofoam coffee cups with a lid to provide thermal insulation. Laboratories frequently use this type of calorimeter due to its convenience, affordability, and suitability for basic thermochemical experiments.

    Applications

    1. Food technologists can determine the energy content of food by measuring the heat released during combustion using calorimeter in food laboratories.
    2. Researchers can determine the specific heat capacity of a material by measuring the heat exchanged during temperature changes using calorimeter.
    3. Calorimetry is useful for analyzing medicines and other biological substances by measuring heat changes during molecular interactions, helping to understand their properties and effects.

    Limitations

    1. Assuming the solution is pure water:
    • In reactions involving aqueous solutions (like acids and bases), we often assume the solution has the same density (1 g/mL) and specific heat capacity (4.18 J/g°C) as water.
    • However, if the solution contains dissolved substances (like salts or acids), its actual properties may differ, leading to inaccuracies.

    2. Assuming no heat is lost to the surroundings:

    • In reality, some heat is always lost to the container or environment, especially in simple calorimeters like coffee cup calorimeters.
    • This causes the measured temperature change to be smaller than it would be in a perfectly insulated system, underestimating the true heat change.

  • What is Thermochemistry?

    What is Thermochemistry?

    Thermochemistry is the branch of chemistry in which we study the heat energy involved in chemical reactions and phase changes, such as melting and boiling. For example, adding heat to ice can change its state from solid to liquid.

    Thermochemistry helps us explain how much heat is released or absorbed quantitatively.

    Thermochemical reactions are classified into two categories:

    1. Exothermic Process, and
    2. Endothermic Process

    1. Exothermic Process:

    In an exothermic process, the system releases heat energy into the surroundings, resulting in an increase in the surrounding temperature. This release of energy illustrates an exothermic process.

    Example:

    • Making ice cubes: As the temperature of the water decreases and it transitions from liquid to solid, it releases heat into the surroundings. This release of heat characterizes exothermic reactions, where the system releases energy rather than absorbing it.
    • Mixing water and strong acid: When we mix acid into water, they react vigorously, releasing heat energy into the surroundings. This release of heat conveys an exothermic reaction.

    Note: Always add acid to water, not the other way around, as adding water to acid can cause it to splash or erupt violently.

    2. Endothermic Process

    In an Endothermic process, the system absorbs heat energy from the surroundings, thus decreasing the surrounding temperature.

    Example:

    • Cooking an egg: Egg absorbs heat energy from the pan or water, causing changes to its internal structure. This transformation is what cooks the egg and is characteristic of an endothermic process, where the system absorbs energy.
    • Melting ice cubes: During melting, ice absorbs heat from the surroundings, which is characteristic of an endothermic process. In this process, the temperature of the ice stays constant at the melting point (0° C ) during the phase change from solid (Ice) to liquid(water). The temperature will only increase when the ice completely melts and we continue adding heat.

    Enthalpy of reaction: Enthalpy is used to measure the energy in a system.

    When a chemical reaction is given, we can find out the change in enthalpy by the following formula:

    ΔHrxn = ∑ΔHproducts – ∑ΔHreactants

    Where,

    ΔHproducts = Sum of total enthalpy absorbed/released by the products

    ΔHreactants = Sum of total enthalpy absorbed/released by the reactants

    We can use the above formula to identify whether the reaction is exothermic or endothermic. If ΔH reaction is positive, the reaction will be endothermic, and if ΔH is negative, the reaction will be exothermic.

    Energy

    Energy is the capacity to do work. In thermochemistry, we prioritize heat energy (the heat exchange between a system and its surroundings during phase change and chemical reactions).

    Energy Transfer

    As the term suggests, energy transfer refers to the movement of energy from one system or object to another. In thermochemistry, energy transfer specifically refers to the flow of energy, primarily in the form of heat or work, due to differences in temperature, pressure, or other conditions.

    Latent heat

    Latent heat is the heat energy necessary to change the phase of a substance or object from solid (Ice) to liquid (water), liquid (water) to vapor (gas), and vice versa when its temperature is constant.

    There are mainly two types of latent heat:

    1. Latent heat of fusion: We denote the latent heat of fusion by ‘Hf’. Latent heat of fusion is the heat energy required to melt a solid (Ice) without changing its temperature. When ice melts, only the phase changes. The temperature remains at 0° C, and the liquid water that forms with the phase change will also be at 0° C.
    2. Latent heat of vaporization: We denote latent heat of vaporization by ‘Hv‘. Latent heat of vaporization is the heat energy required to vaporize liquid(water) without changing its temperature.

    Sensible heat

    Sensible heat is the heat energy required to change the temperature of a substance without changing its phase.  This heat is the opposite of latent heat where phase changes without changing temperature.

    The formula to find sensible heat is Q = mcΔT

    Where Q = heat energy

    m= mass of substance or object

    c=specific heat

    ΔT =difference in temperatures (Tf – Ti)

    Specific heat

    It is denoted by ‘c’. Specific heat is the quantity of heat required to raise the temperature of 1 g of substance by 1 degree Celsius or 1 kelvin.

    Calorimetry

    Calorimetry measures the heat energy absorbed or released during physical or chemical changes. It involves an instrument calorimeter for monitoring and quantifying the heat exchange.

    Principle of calorimetry: When two objects or substances with different temperatures come in contact, the heat transfers from hotter objects to colder objects until they reach thermal equilibrium. Here, the Principle of calorimetry indicates the law of conservation of energy. The total heat lost by an object equals the total heat gained by the other object.

    Hess’s law

    Hess’s law states that the total enthalpy change for a reaction is the same whether the reaction takes place in one or more than one step.

    Mathematically, we express it as ΔHtotal =∑ΔHsteps

    For example:  Consider we have a reaction:  X → Y

    If we can break it into two steps:

    X → Z(ΔH1)

                                                                  Z → Y (ΔH2)

    Then, according to Hess’s law

    ΔHtotal = ΔH1 + ΔH2