First Law of Thermodynamics, Basic Introduction - Internal Energy, Heat and Work - Chemistry

00:11:26
https://www.youtube.com/watch?v=NyOYW07-L5g

摘要

TLDRThe video explains the first law of thermodynamics and its application in understanding energy transfer and internal energy changes within a system. According to the first law, energy cannot be created or destroyed but can be transferred between a system and its surroundings through heat and work. The video elaborates on how heat, represented by 'q', and work, represented by 'w', contribute to changes in a system's internal energy, symbolized as ΔU. Different equations for internal energy change are used in chemistry and physics, reflecting the perspectives of the system and surroundings, respectively. The video covers the basics of thermodynamic systems—open, closed, and isolated—emphasizing their properties and how they allow for the transfer of energy and matter. Additionally, it explains the concepts of endothermic (absorbing heat) and exothermic (releasing heat) reactions in thermodynamic terms. The concept is likened to financial transactions to illustrate energy conservation and transfer.

心得

  • 🔍 Energy cannot be created or destroyed, only transferred.
  • 🌀 Internal energy is the sum of heat and work transferred.
  • 📈 Increases in internal energy occur via heat absorption or work done on the system.
  • ⚗️ Chemistry and physics differ in their approach: systems vs surroundings.
  • 🌡️ Endothermic: heat absorbed, exothermic: heat released.
  • 🔓 Open systems allow both energy and matter transfer.
  • 🔒 Closed systems allow only energy transfer.
  • 🔗 Isolated systems permit neither energy nor matter exchange.
  • ⚖️ The system's perspective uses q + w, while surroundings use q - w.
  • ✍️ Sign conventions are essential: q positive (heat absorbed), q negative (heat released).

时间轴

  • 00:00:00 - 00:05:00

    The first section discusses the first law of thermodynamics, which states that energy cannot be created or destroyed but can only be transferred. It introduces the concept that energy can enter or leave a system through heat or work. The internal energy of a system increases when heat flows in or when work is done on the system, with an example of energy transfer using a financial transaction analogy.

  • 00:05:00 - 00:11:26

    The second section explains the different types of systems: open, closed, and isolated. An open system allows energy and matter to be transferred in and out, a closed system allows only energy, and an isolated system allows neither. The change in internal energy is expressed as q + w in chemistry and q - w in physics, with differences based on perspectives. In chemistry, work done by the system reduces internal energy (negative w), while in physics, it's based on the surroundings' gain in energy, hence w is positive. The sign conventions for endothermic (positive q) and exothermic (negative q) processes are also discussed.

思维导图

Mind Map

常见问题

  • What is the first law of thermodynamics?

    The first law of thermodynamics states that energy cannot be created or destroyed; it can only be transferred from one place to another.

  • How can the internal energy of a system increase?

    The internal energy of a system can increase by the transfer of heat energy into the system or by work being performed on the system by the surroundings.

  • What are the different types of systems in thermodynamics?

    There are three types of systems: open systems, closed systems, and isolated systems.

  • How does energy transfer in an open system?

    In an open system, both matter and energy can be transferred into and out of the system.

  • What is an isolated system?

    An isolated system is one where neither energy nor matter can enter or leave the system.

  • Why do chemistry and physics use different equations for internal energy change?

    The difference arises from the point of view taken. Chemistry takes the system's point of view, while physics considers the surroundings' perspective.

  • What happens in an endothermic process in terms of heat transfer?

    In an endothermic process, heat energy is absorbed by the system, making the energy (q) positive.

  • What is the significance of 'w' in the internal energy equation?

    'W' represents work; in chemistry, it is negative when the system does work, and positive when work is done on the system.

  • What does it mean when a process is exothermic?

    An exothermic process means heat is released from the system to the surroundings, making the energy (q) negative.

  • How does a closed system differ from an open system?

    In a closed system, matter cannot flow in or out, but energy can, unlike in an open system where both matter and energy can flow.

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  • 00:00:01
    in this video we're going to talk about
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    the first law of thermodynamics
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    and how it relates to internal energy
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    heat and work
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    so what is the basic idea behind the
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    first law of thermodynamics
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    the gist of it is this
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    energy cannot be created or destroyed
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    it can simply be transferred from one
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    place to another
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    so let's say
  • 00:00:28
    if
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    we have a system
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    and everything outside of it
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    is the surroundings
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    energy can flow into or out of the
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    system
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    and it's two ways in which energy can do
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    so
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    and that is through heat and work
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    so if heat flows into the system
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    the system gains energy and that energy
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    is known as the internal energy of the
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    system
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    represented by
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    the symbol capital u
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    now the surroundings can do work on a
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    system
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    so those are the two ways in which a
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    system
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    can increase its internal energy
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    it's by the transfer of heat energy into
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    the system or if the surroundings
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    perform work on a system
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    so let's say
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    if the surroundings perform
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    300 joules
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    on a system
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    that means
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    the system's internal energy
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    goes up by 300 so the change is positive
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    so delta u increases
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    the energy of the surroundings however
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    has to decrease by 300 and so energy is
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    not created or destroyed
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    it simply transferred from one place to
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    another
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    the system
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    didn't just get the 300 joules from
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    nowhere
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    that 300 joules of energy came from
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    somewhere it came from the surroundings
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    a good way to illustrate this
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    is to use money
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    so let's say if
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    you decide to sell a laptop for five
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    hundred dollars
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    someone
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    will buy the laptop for 500 let's say if
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    someone does buy it
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    your bank account will increase by 500
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    but that person's bank account will
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    decrease by 500 and so the money just
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    doesn't come from nowhere just doesn't
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    just
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    magically appears
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    it comes from somewhere
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    so in order for you to gain 500 someone
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    else has to lose 500.
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    now granted
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    the government could print more money if
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    they wanted to
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    but
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    in a practical sense in everyday
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    life
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    when someone gains money someone has to
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    lose money and so
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    in that sense within that transaction
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    money is not created or destroyed it
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    simply transferred from one place to
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    another and energy follows that same
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    principle
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    unless you're god
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    energy cannot be created or destroyed
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    as far as we know
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    but in today's
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    world under practical conditions energy
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    is simply transferred from one place to
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    another
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    so in our example
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    if the system gains 300 joules of energy
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    that means the surroundings loses 300
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    joules of energy
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    if the system loses 500 joules of energy
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    the surroundings have to gain 500 joules
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    of energy and in that sense within that
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    practical area
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    energy is not created or destroyed thus
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    we have the first law of thermodynamics
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    now there's three types of systems that
  • 00:03:49
    you want to be familiar with
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    the first one
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    is an open system
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    the second
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    is a closed system
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    and the third
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    is an isolated system
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    so in the open system
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    matter
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    can enter into an open system so oxygen
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    from the air
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    can go inside of it and also heat energy
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    can flow into an open system
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    so matter and energy can be transferred
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    into and out of an open system
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    now in a closed system
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    matter cannot flow into it so oxygen
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    just
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    it can go inside of a closed system
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    however energy
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    can still flow into a closed system
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    so in a closed system only energy can go
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    into and out of a closed system but
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    matter cannot
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    in an isolated system
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    energy or matter cannot enter or leave
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    an isolated system
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    so the mass within an isolated system
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    is fixed it doesn't change and the total
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    energy of an isolate system
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    also doesn't change
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    because no energy can flow into it or
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    out of it
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    the equation for the change
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    in the internal energy of a system
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    is q plus w
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    and this is the equation that you'll see
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    in a typical chemistry textbook
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    q represents the heat energy that flows
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    into or out of the system and w
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    represents the work
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    now in physics
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    the equation is a bit different
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    in the physics textbook you'll see
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    delta u
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    is equal to q minus w
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    now you might be wondering why is it
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    different
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    why is it not the same
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    and the reason for that
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    is
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    the point of view taken by the
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    scientists
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    in chemistry we take the system's point
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    of view
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    in physics
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    engineers take
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    the viewpoint of the surroundings
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    so in chemistry
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    w is negative when work is done
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    by the system
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    anytime work is done by the system
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    the system has to expand energy to do
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    work and so the internal energy of the
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    system decreases
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    imagine if you're going to the gym to
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    work out
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    and
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    as you lift weights
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    your body is burning energy the internal
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    energy of your body decreases
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    as you burn calories
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    and so work is being done by the system
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    that is you're the system you're doing
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    the work and the surroundings might be
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    the waste that you're lifting so as you
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    burn calories to lift up a weight from
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    let's say the ground to an elevated
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    position the potential energy of that
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    weight increases
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    but the internal energy of your body
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    decreases because you're burning
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    calories you're losing weight
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    in this case
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    whenever w is negative work is done by
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    the system in the case of chemistry
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    and
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    when w is positive work is done on a
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    system
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    now in physics
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    it's different
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    we take the surroundings point of view
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    in physics
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    so w is negative when work
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    is done
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    on the system
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    and w is positive when work is done
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    by the system
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    so let's analyze
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    these two cases when work is done by the
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    system so in chemistry
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    when work is done by the system as we
  • 00:08:01
    said before
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    the internal energy decreases
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    and because the energy of the system is
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    decreasing
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    work is negative remember we're taking
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    the viewpoint of the system
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    in physics we want to take the viewpoint
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    of the surroundings
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    so when work is done by the system the
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    internal energy of the system still
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    decreases
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    however we say w is positive because
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    the surroundings is gaining energy we're
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    taking the viewpoint of the surroundings
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    and so since the surroundings gain
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    energy
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    w is positive
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    in the case of
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    chemistry
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    when work is done by the system the
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    system loses energy so we say w is
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    negative and so it's the viewpoint and
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    that's why the equations are a bit
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    different
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    in chemistry we take the viewpoint of
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    the system but in physics we take the
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    viewpoint
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    on the surroundings so when work is done
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    by the system the system loses energy
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    the surroundings gain energy so if
  • 00:09:01
    you're focused on the surroundings
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    then w is positive because the energy of
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    the surroundings go up
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    if you focus on a system the system
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    loses energy when work is done by the
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    system and so
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    w is negative
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    but in this video
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    i'm going to take
  • 00:09:19
    the system's perspective
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    so going back to this equation
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    delta u is q plus w
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    you need to know that q is positive
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    whenever the system
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    meaning the reactants and the products
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    absorb heat energy
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    so anytime heat flows into the system
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    heat energy is absorbed by the system so
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    q is positive
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    this is an endothermic process
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    whenever heat energy is absorbed
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    now q is negative
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    whenever the system
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    releases
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    heat energy
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    and so heat energy flows out of the
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    system into the surroundings
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    and so this is an exothermic process
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    so anytime heat energy is released
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    it's exothermic
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    and when heat energy is absorbed by the
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    system
  • 00:10:18
    it's endothermic
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    so for an exothermic process q is
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    negative and for an endothermic process
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    q is positive
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    and as was mentioned before four
  • 00:10:31
    w
  • 00:10:33
    is positive
  • 00:10:34
    when work is done on the system
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    this is in chemistry
  • 00:10:40
    and
  • 00:10:41
    w is negative whenever work is
  • 00:10:44
    done by the system
  • 00:10:51
    so you need to be familiar with the sign
  • 00:10:53
    conventions
  • 00:10:54
    if you're gonna use this
  • 00:10:55
    equation so in another video i'm gonna
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    give you some practice problems on
  • 00:11:00
    calculating the change in internal
  • 00:11:02
    energy
  • 00:11:03
    using heat and work
  • 00:11:25
    you
标签
  • thermodynamics
  • internal energy
  • heat transfer
  • work
  • systems
  • first law
  • energy conservation
  • endothermic
  • exothermic
  • chemistry
  • physics