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    EBwithReaction I

    Uploaded by

    Dennis Ling
    1. Energy is required to break reactant bonds and released when product bonds form during chemical reactions, necessitating heat transfer to/from the reactor to maintain temperature. 2. The heat of reaction, ΔH_r, is the enthalpy change for a reaction at constant temperature and pressure and determines if heat must be added or removed from the reactor. 3. Hess's law allows calculation of heat of reaction from individual reaction heats if the net reaction can be written as a combination of the individual reactions.

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    EBwithReaction I

    Uploaded by

    Dennis Ling
    43 views19 pages
    1. Energy is required to break reactant bonds and released when product bonds form during chemical reactions, necessitating heat transfer to/from the reactor to maintain temperature. 2. The heat of reaction, ΔH_r, is the enthalpy change for a reaction at constant temperature and pressure and determines if heat must be added or removed from the reactor. 3. Hess's law allows calculation of heat of reaction from individual reaction heats if the net reaction can be written as a combination of the individual reactions.

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    1. Energy is required to break reactant bonds and released when product bonds form during chemical reactions, necessitating heat transfer to/from the reactor to maintain temperature. 2. The heat of reaction, ΔH_r, is the enthalpy change for a reaction at constant temperature and pressure and determines if heat must be added or removed from the reactor. 3. Hess's law allows calculation of heat of reaction from individual reaction heats if the net reaction can be written as a combination of the individual reactions.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    43 views19 pages

    EBwithReaction I

    Uploaded by

    Dennis Ling
    1. Energy is required to break reactant bonds and released when product bonds form during chemical reactions, necessitating heat transfer to/from the reactor to maintain temperature. 2. The heat of reaction, ΔH_r, is the enthalpy change for a reaction at constant temperature and pressure and determines if heat must be added or removed from the reactor. 3. Hess's law allows calculation of heat of reaction from individual reaction heats if the net reaction can be written as a combination of the individual reactions.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    of 19

    Energy Balances with Reaction

    In any chemical reaction, energy is required to break the reactant


    bonds and energy is released when the product bonds are formed
    The large changes in enthalpy and internal energy during a chemical
    reaction necessitates substantial heat transfer (heating or cooling)
    from the reactor in order to maintain the reactor at its desired
    operating temperature
    The net change of enthalpy is called the heat of reaction, and is the
    energy that must either be transferred to or from the reactor to
    maintain the desired reactor temperature

    Heats of Reaction
    By definition, the heat of reaction, r (T,P), is the enthalpy change for a
    process in which stoichiometric quantities of reactants at temperature T
    and pressure P react completely to form products at the same
    temperature and pressure.
    For the following reaction,

    aA + bB cC + dD
    r is calculated as the difference between the product and reactant
    enthalpies (at constant T,P) which are weighted by their stoichiometric
    coefficients. Therefore,
    H (T , P) [kJ/mol] = H
    H
    r

    products

    reactants

    = cH C (T , P ) + dH D (T , P ) aH A (T , P ) bH B (T , P )
    = H (T , P )

    Heats of Reaction
    The units of r are kJ/mol but, per mole of what?? Recall that the
    reported r applies to stoichiometric quantities of each species. For
    example,
    2A + B 3C
    r (100C, 1 atm) = -50 kJ/mol
    the enthalpy change for the given reaction is
    50 kJ
    50 kJ
    50 kJ
    =
    =
    2 mol A consumed 1 mol B consumed 3 mol C generated

    If 150 mol of C/s was generated at 100C and 1 atm,

    150 mol C generated


    50 kJ

    H& =
    = 2500 kJ/s
    s

    3 mol C generated

    Heats of Reaction Extent of Reaction


    In general, if nA,r moles of A are generated or consumed by reaction at a
    temperature T and pressure P and A is the stoichiometric coefficient of
    the reactant or product, the associated enthalpy change is:

    H r (T , P )
    &
    H =
    n A, r = &H r (T , P )
    |A |

    Recall that the extent of reaction, , is a measure of how far a reaction


    has proceeded.
    | (n& A )out (n& A )in | | n A, r |
    &
    =
    =
    | A |
    | A |

    Notes on Heats of Reaction


    1. If r (T,P) is negative, the reaction is exothermic energy must be
    removed from the reactor to keep the temperature from increasing
    2. If r (T,P) is positive, the reaction is endothermic energy must be
    added to the reactor to keep the temperature from decreasing
    3. At low and moderate pressure, r (T,P) is nearly independent of
    pressure. Therefore, r (T,P) r (T)
    4. The value of the heat of reaction depends on how the stoichiometric
    equation is written. For example:
    CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l): r1 (25C) = -890.3 kJ/mol
    2 CH4(g) + 4 O2(g) 2 CO2(g) + 4 H2O(l): r2 (25C) = -1780.6
    kJ/mol

    Notes on Heats of Reaction


    5. The value of the heat of reaction depends on the phase of the
    reactants and products. For example:
    CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l) : r1 (25C) = -890.3 kJ/mol
    CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) : r2 (25C) = -802.3 kJ/mol
    6. The standard heat of reaction, r, is the heat of reaction when
    both reactants and products are at standard conditions, 25C and
    1 atm.
    The symbol denotes standard conditions (i.e., 25C and 1 atm).

    Example
    1. The standard heat of the combustion on n-butane vapour is

    C 4H10 (g) +

    13
    O 2 (g) 4 CO 2 (g) + 5 H2O(l)
    2

    H ro = 2878 kJ/mol

    Calculate the rate of enthalpy change, H& (kJ/s) if 2400 mol/s of CO2 is
    produced in this reaction and the reactants and products are all at 25C.
    2. The heats of vapourization of n-butane and water at 25C are 19.2 kJ/mol
    and 44.0 kJ/mol, respectively. What is the standard heat of the reaction

    C 4H10 (l) +

    13
    O 2 (g) 4 CO 2 (g) + 5 H2O(v)
    2

    Calculate H& if 2400 mol/s of CO2 is produced in this reaction and the
    reactants and products are all at 25C.

    Closed System Reactions


    What if the reaction takes place in a closed system of constant volume?
    Energy balance:

    U + Ek + Ep = Q W

    The internal energy of reaction, r (T), is calculated as the difference


    between the product and reactant internal energies if stoichiometric
    quantities of reactants react completely at temperature T.
    U (T ) = U
    U
    r

    products

    reactants

    Assuming ideal gas behaviour, the internal energy is related to the heat
    of reaction by

    | vi |
    | vi |
    U r (T ) = H r (T ) RT

    gaseous
    gaseous

    products
    reac tan ts

    where I is the stoichiometric coefficient of the ith gaseous reactant or


    product. (See F&R Ex. 9.1-2)

    Measurement of Heats of Reaction


    Heats of reaction are measured in a calorimeter. A calorimeter is a
    closed reactor that is submersed in a fluid and enclosed in an insulated
    vessel. The increase or decrease in fluid temperature determines the
    amount of energy released or absorbed and using the heat capacities of
    the reactants and products, H r can be determined.
    However, this measurement technique will not work for every reaction.
    For example, consider the following reaction:
    1
    H r (25C, 1 atm) = ?
    C(s) + O 2 (g) CO(g)
    2
    - only minimal amounts of CO would form since the rate of reaction
    at 25C is too low
    - higher reaction temperatures (higher reaction rates) would not
    lead to the formation of pure CO but rather a mixture of CO and
    CO2

    Measurement of Heats of Reaction


    What if we cant measure experimentally H r for our desired reaction?
    1
    H r1 = ?
    C + O 2 CO
    2
    However, we can measure these heats of reaction,
    H = 393.51 kJ/mol
    C + O CO
    2

    CO +

    C+

    r2

    1
    O 2 CO 2
    2

    1
    1
    O2 (+ O2 )
    2
    2

    H r3 = 282.99 kJ/mol
    H r1

    H r = H r2

    1
    CO( + O 2 )
    2
    H r = H r3

    CO 2
    H r1 = H r2 + ( H r3 ) = -393.51 + (-282.99) = -110.52 kJ/mol

    Hesss Law
    The previous result could be more readily obtained if we treated the
    stoichiometric equations as algebraic equations. That is,

    1
    C + O 2 CO O 2 CO 2 CO 2 (reaction 2 - reaction 3)
    2

    1
    C + O 2 CO (desired reaction 1)
    2
    H r1 = H r2 H r3 = 393.51 ( 282.99) = -110.52 kJ/mol

    Hesss Law if a set of reactions can be manipulated through a series


    algebraic operations to yield the desired reaction, then the desired heat
    of reaction can be obtained by performing the same algebraic
    operations on the heats of reaction of the manipulated set of reactions

    Example
    The standard heats of the following combustion reactions have been
    determined experimentally:

    7
    O 2 2 CO 2 + 3 H2O
    2
    C + O 2 CO 2
    1
    H2 + O 2 H2 O
    2

    C 2H6 +

    H r1 = 1559.8 kJ/mol
    H r2 = 393.5 kJ/mol
    H r3 = 285.8 kJ/mol

    Use Hesss Law and the given heats of reaction to determine the
    standard heat of the reaction
    2 C + 3 H2 C 2H6

    H r4 = ?

    Calculating r from Heats of Formation


    The H r can be calculated using standard heats of formation. A
    formation reaction of a compound is the reaction in which the compound
    is formed from its elemental constituents as they would occur in nature
    (e.g., O2 rather than O).
    The enthalpy change associated with the formation of 1 mole of the
    compound at 25C and 1 atm is the standard heat of formation H f of
    the compound. H f for many compounds are found in Table B.1.
    For example, from Table B.1 it can be seen that the H f of ammonium
    nitrate (NH4NO3(s)) is -365.14 kJ/mol. This signifies that,
    3
    N2 (g) + 2 H2 (g) + O 2 (g) NH4NO 3 (s)
    2

    H r = 365.14 kJ/mol

    Calculating r from Heats of Formation


    A consequence of Hesss Law is that the H r of any reaction can be
    calculated as:

    H r =

    vi H f i =

    | vi | (H f ) i

    products

    | vi | ( H f ) i

    reactants

    where,

    i is the stoichiometric coefficient of reactant or product species i


    (H f ) i is the standard heat of formation of species i
    The standard heats of formation of all elemental species
    (e.g., O2, N2, Zn, etc.) are zero.

    Example
    Determine the standard heat of reaction for the combustion of liquid npentane.
    C5H12(l) + 8 O2(g) 5 CO2(g) + 6 H2O(l)

    Calculating r from Heats of Combustion


    The standard heat of combustion of a species, H c, is the enthalpy
    change associated with the complete combustion of mole of a species
    with oxygen at 25C and 1 atm such that:
    all the carbon forms CO2(g)
    all the hydrogen forms H2O(l)
    all the sulphur forms SO2(g)
    all the nitrogen forms N2(g)
    H c values for combustible species (if available) are found in Table B.1.

    For example, from Table B.1 it can be seen that the H c of ethanol is
    -1366.9 kJ/mol. This signifies that,

    C 2H5 OH(l) + 3 O 2 (g) 2 CO 2 (g) + 3 H2O(l)

    H r = 1366.9 kJ/mol

    Calculating r from Heats of Combustion


    A consequence of Hesss Law is that the H r of any reaction involving
    only oxygen and a combustible species can be calculated as:
    H r =

    v (H
    i

    where,

    c )i

    | v | (H
    i

    reactants

    c )i

    | v | (H
    i

    c )i

    products

    Note: this is reverse to determining the


    heat of reaction from heats of formation

    i is the stoichiometric coefficient of reactant or product species i

    (H c ) i is the standard heat of formation of species i

    If any reactants or products are combustion products (i.e, CO2,


    H2O(l), SO2,), their heats of combustion are zero.

    Example
    Calculate the standard heat of reaction form the dehydrogenation of
    ethane:
    C2H6 C2H4 + H2

    Determining f from c
    For many substances, it is much easier to measure H c than H f
    For example, the formation of pentane:

    5 C(s) + 6 H2 (g) C 5H12 (l)

    H f = ?

    Carbon, hydrogen and pentane can all be burned and their standard
    heats of combustion determined experimentally. Therefore,

    ( H f )C5H12 (l) = 5( H c )C(s) + 6( H c )H2 (g) - ( H c )C5H12 (l)

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