Rules of Chemistry Flashcards
(16 cards)
The First Law (equation & explanation)
∆U = q + w
The First Law: the energy change is due to the transfer of heat and work between the system and the surroundings
Heat Capacity 1 (equation & explanation)
q = mCs∆T
Heat capacity 1: the energy in the form of heat transferred to a specific mass of a substance is related to its temperature rise by its specific heat capacity, Cs
Heat Capacity 2 (equation & explanation)
∆H = -mCs∆T
Heat capacity 2: in calorimeter experiment, the minus sign ensures that the correct value of ∆H (exothermic or endothermic) is obtained
In an exothermic reaction…
In an exothermic reaction, heat is transferred from the system to the surroundings at constant pressure
∆H<0
In an endothermic reaction…
In an endothermic reaction, heat is transferred from the surroundings to the system at constant pressure
∆H>0
The change in physical quantity, ∆X…
∆X = Xfinal - Xinitial
The change in physical quantity, ∆X, is always the final value - the initial value, regardless of whether Xfinal is numerically bigger or smaller than Xinitial
The heat formation…
The heat of formation of any element in its standard state (p = 1 atm and specified T) is 0
∆fH^0 = 0
The heat (enthalpy) of reaction =…
The heat (enthalpy) of reaction = the sum of the heats of formation of the products - the sum of the heats of formation of the reactants. This is the most general statement of Hess’ law.
∆fH = Σv∆fH(products) - Σv∆fH(reactants)
Rate…
∆[X]/∆t
Rate is the variation in the concentration of X (e.g. in mol l^-1) as a function of time (in convenient units) and can be related to the slope of the graph of [X] versus t.
Rate =
Rate = k[X]^a[Y]^b
The general rate equation: the exponents (powers) a,b… are established by experiment as in the rate constant, K.
k =
k = Ae^-Ea/RT
The Arrhenius equation relates the rate constant (K), the activation energy (Ea) in JOULES and the temperature (T) in KELVIN. In this equation, the 8.314 J mol^-1 K^-1 value of R must always be used
Rate = K
Rate = K
In a zero-order reaction, the rate does not depend on [X]. Thus, the rate is constant until X is used up. The units of K are concentration per time.
Example: metabolism of ethanol.
Rate = K{X}
In a first-order reaction, the rate is proportional to the concentration of X. Thus, as [X] gets smaller, the rate slows down. The units of K are inverse time.
Example: metabolism of warfarin
[A]t =
[A]t = [A]0 x e^-kt
The very important ‘integrated first-order rate equation’ allows us to work out any one of the four components of the equation, if we know the other three. Most commonly, [A]t (the concentration of A after time t) is the unknown.
t1/2 =
t1/2 = 0.693/k
The half-life and the rate constant of a first-order reaction are related by this very simple (and important) equation. It can equally be stated as k = 0.693/t1/2 or even k x t1/2 = 0.693.
Km occurs at 1/2vmax
The Michaelis constant for an enzyme-catalysed reaction is read from a graph of v versus [S] {v = velocity(rate)}. A small value of Km (in concentration units) indicates a strong interaction between enzyme and substrate and therefore the reaction is likely to proceed to form products. Other graphical methods to determine Km, such as a ‘double inverse’.
