“activity”的概念、定义、翻译、参考文献-科学参考

activity

Physics

1. Symbol a. A thermodynamic function used in place of concentration in equilibrium constants for reactions involving nonideal gases and solutions. For example, in a reaction

$A ⇌ B + C$

the true equilibrium constant is given by

$K = a_{B} a_{C} / a_{A}$

where a_A, a_B, and a_C are the activities of the components, which function as concentrations (or pressures) corrected for nonideal behaviour. Activity coefficients (symbol γ‎) are defined for gases by γ‎=a/p (where p is pressure) and for solutions by γ‎=aX (where X is the mole fraction). Thus, the equilibrium constant of a gas reaction has the form

$K_{p} = γ_{B} p_{B} γ_{C} p_{C} {/ γ}_{A} p_{A}$

The equilibrium constant of a reaction in solution is

$K_{c} = γ_{B} X_{B} γ_{C} X_{C} {/ γ}_{A} X_{A}$

The activity coefficients thus act as correction factors for the pressures or concentrations. See fugacity.

2. Symbol A. The number of atoms of a radioactive substance that disintegrate per unit time. The specific activity (a) is the activity per unit mass of a pure radioisotope. See radiation units.

Chemistry

1. Symbol a‎. A thermodynamic function used in place of concentration in equilibrium constants for reactions involving nonideal gases and solutions. For example, in a reaction
$A ⇌ B + C$
the true equilibrium constant is given by
$K = a_{B} a_{C} / a_{A}$
where a_A, a_B, and a_C are the activities of the components, which function as concentrations (or pressures) corrected for nonideal behaviour. Activity coefficients (symbol γ‎) are defined for gases by γ‎ = a/p (where p is pressure) and for solutions by γ‎ = aX (where X is the mole fraction). Thus, the equilibrium constant of a gas reaction has the form
$K p = γ_{B} p_{B} γ_{C} p_{C} {/ γ}_{A} p_{A}$
The equilibrium constant of a reaction in solution is
$K_{c} = γ_{B} X_{B} γ_{C} X_{C} {/ γ}_{A} X_{A}$
The activity coefficients thus act as correction factors for the pressures or concentrations. The activity is given by an equation
${μ = μ}^{⦵} + R T ln a$
where μ‎ is chemical potential See also fugacity.
2. Symbol A. The number of atoms of a radioactive substance that disintegrate per unit time. The specific activity (a) is the activity per unit mass of a pure radioisotope. See radiation units.

Chemical Engineering

1. The change in one condition to another expressed as a ratio. Examples include water activity, and chemical activity. The activity of a chemical reaction is used in place of concentration in equilibrium constants for reactions involving non-ideal gases and solutions.

2. A quantitative term used to characterize the number of atomic nuclei that disintegrate in a radioactive substance per unit time. It is measured in becquerel (Bq) where one Bq is equal to one disintegration per second. The unit of activity replaces the former unit of the curie (Ci) where one Ci is equal to 37 × 10¹⁰ Bq. The specific activity is the activity per unit mass of a pure radioisotope.

3. (Symbol U) The amount of an enzyme present in a biologically catalyzed reaction. It is usually expressed in terms of units of activity based on the rate of the reaction that the enzyme catalyzes. The international unit of activity is the amount of enzyme that will convert one μmol of substrate to a product in one minute under defined conditions. These are usually 25ºC and the optimum pH.

4. A thermodynamic parameter that measures the so-called active concentration of a substance, a, in a chemical system, and is in contrast to the molecular concentration, c. It is related $a = f c$ where f is a dimensionless parameter and approaches unity in dilute solutions.

Electronics and Electrical Engineering

The ratio of the peak value of the oscillations in a piezoelectric crystal to the peak value of the exciting voltage.

Geology and Earth Sciences

A broadly used term which refers to the rate or extent of a change associated with some substance or system. For example, it may be the tendency of a metal high in the electromotive series to replace another metal lower in the series, e.g. magnesium displacing copper from most of its compounds. It may also be used to describe the rate of decay of atoms by radioactivity.

单词	activity
释义	activity Physics 1. Symbol a. A thermodynamic function used in place of concentration in equilibrium constants for reactions involving nonideal gases and solutions. For example, in a reaction $A ⇌ B + C$ the true equilibrium constant is given by $K = a_{B} a_{C} / a_{A}$ where a_A, a_B, and a_C are the activities of the components, which function as concentrations (or pressures) corrected for nonideal behaviour. Activity coefficients (symbol γ‎) are defined for gases by γ‎=a/p (where p is pressure) and for solutions by γ‎=aX (where X is the mole fraction). Thus, the equilibrium constant of a gas reaction has the form $K_{p} = γ_{B} p_{B} γ_{C} p_{C} {/ γ}_{A} p_{A}$ The equilibrium constant of a reaction in solution is $K_{c} = γ_{B} X_{B} γ_{C} X_{C} {/ γ}_{A} X_{A}$ The activity coefficients thus act as correction factors for the pressures or concentrations. See fugacity. 2. Symbol A. The number of atoms of a radioactive substance that disintegrate per unit time. The specific activity (a) is the activity per unit mass of a pure radioisotope. See radiation units. Chemistry 1. Symbol a‎. A thermodynamic function used in place of concentration in equilibrium constants for reactions involving nonideal gases and solutions. For example, in a reaction $A ⇌ B + C$ the true equilibrium constant is given by $K = a_{B} a_{C} / a_{A}$ where a_A, a_B, and a_C are the activities of the components, which function as concentrations (or pressures) corrected for nonideal behaviour. Activity coefficients (symbol γ‎) are defined for gases by γ‎ = a/p (where p is pressure) and for solutions by γ‎ = aX (where X is the mole fraction). Thus, the equilibrium constant of a gas reaction has the form $K p = γ_{B} p_{B} γ_{C} p_{C} {/ γ}_{A} p_{A}$ The equilibrium constant of a reaction in solution is $K_{c} = γ_{B} X_{B} γ_{C} X_{C} {/ γ}_{A} X_{A}$ The activity coefficients thus act as correction factors for the pressures or concentrations. The activity is given by an equation ${μ = μ}^{⦵} + R T ln a$ where μ‎ is chemical potential See also fugacity. 2. Symbol A. The number of atoms of a radioactive substance that disintegrate per unit time. The specific activity (a) is the activity per unit mass of a pure radioisotope. See radiation units. Chemical Engineering 1. The change in one condition to another expressed as a ratio. Examples include water activity, and chemical activity. The activity of a chemical reaction is used in place of concentration in equilibrium constants for reactions involving non-ideal gases and solutions. 2. A quantitative term used to characterize the number of atomic nuclei that disintegrate in a radioactive substance per unit time. It is measured in becquerel (Bq) where one Bq is equal to one disintegration per second. The unit of activity replaces the former unit of the curie (Ci) where one Ci is equal to 37 × 10¹⁰ Bq. The specific activity is the activity per unit mass of a pure radioisotope. 3. (Symbol U) The amount of an enzyme present in a biologically catalyzed reaction. It is usually expressed in terms of units of activity based on the rate of the reaction that the enzyme catalyzes. The international unit of activity is the amount of enzyme that will convert one μmol of substrate to a product in one minute under defined conditions. These are usually 25ºC and the optimum pH. 4. A thermodynamic parameter that measures the so-called active concentration of a substance, a, in a chemical system, and is in contrast to the molecular concentration, c. It is related $a = f c$ where f is a dimensionless parameter and approaches unity in dilute solutions. Electronics and Electrical Engineering The ratio of the peak value of the oscillations in a piezoelectric crystal to the peak value of the exciting voltage. Geology and Earth Sciences A broadly used term which refers to the rate or extent of a change associated with some substance or system. For example, it may be the tendency of a metal high in the electromotive series to replace another metal lower in the series, e.g. magnesium displacing copper from most of its compounds. It may also be used to describe the rate of decay of atoms by radioactivity.
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