Chemistry self made flashcards

what is a rxn order and what effect do different rxn orders have on ROR?

rxn orders (i.e. m & n) demonstrate the rate of rxn’s dependance on the concentration of the respective reactant.

If conc. of given reactant is doubled:
Zero^th (n = 0) : Rate is unaffected (2[A]⁰ = 1 ∴ 𝒙 ⨉ 1 = 𝒙)
First (n = 1) : Rate is doubled
Second (n = 2) : Rate quadruples
Third (n = 3) : 8 ⨉ Rate
(2^ⁿ)

How do you find the overall rxn order?

Sum the rate law powers

What is the rate law?

rate of rxn = k[A]^m[B]ⁿ
k = rate constant
[A] & [B] are reactant concentrations
m & n are rxn orders

Rate law k units?

rearrange for k and solve units, mol L^-1 = M
0^th : mol L^-1 s^-1
1^st : s^-1
2^nd : L mol^-1 s^-1

3^rd : L² mol^-2 s^-1

What graphs respond to what orders of rxn?

0^th : Linear, -kt
1^st : Natural log, ln([A]) - kt
2^nd : Inverse, 1/[A] + kt

How to do half life questions?

Solve decay question for k (k = (ln(2)/t_half)
Amount left = initial amount ⨉ e^-k(t)
A_t = A_ie^-kt

What is the Arrhenius equation and what does it mean?

how does the rate constant change w.r.t to activation energy, E_A, and Temperature, T.
(R is gas constant)

What is a complex rxn? (Hint: “and ∴ a elementary rxn”)

A complex rxn is a rxn that takes a path of elementary rxn’s to reach its final products

n & m if a rxn is xA + yB → zC

n, m = x, y
e.g. 3A + 2B gives the rate law k[A]³[B]²

What’s the difference between a catalyst and a intermediate?

A catalyst (X) is used in one part of the reaction then reproduced later
An intermediate (Y) is produced in one part of a rxn then is later used up

What is a rate determining step?

The slowest of the elementary reactions which therefore determines the rate at which the reaction will occur.
(Think of a pipe with sections with different sized holes)

In the Beer-Lambert Law (A = ε c l), what are the name and generally used units for the symbol ε?

Molar absorptivity with units L mol^-1 cm^-1

What is the Beer-Lambert law?

The Beer–Lambert Law states that light absorbance is directly proportional to both the concentration of a solution and the path length the light travels through it. Expressed as A = ε ⋅ c ⋅ l

What is an equilibrium constant?

It tells us which side equilibrium is favoured (products over reactants)

K >> 1	Products favoured
K << 1	Reactants favoured
K ≈ 1	Neither strongly favoured

What is the reaction Quotient and how is it different from the equilibrium ȼ?

Reaction quotient gives a value for any concentration of reactants/products in a rxn which when compared with the known eq. ȼ will tell us if the rxn is forward or backwards.

What are the activities used in equalibrium equations?

pure liquid = 1
pure solid = 1
gas = P_(paskal) / 1 bar (100 000Pa)

What is the condition for K_c = K_p

Equilibrium constant for concentrations and partial pressures are equal when there is no change in the number of moles of gas in the overall rxn

What is K_p?

K_p is the equalibrium constant calculated with partial pressures (same eq as K_c but w/ pressure of gasses instead of conc.)

What happens if the pressure a rxn is happening at changes? (Le Châtelier’s)

The direction with fewer gaseous particles is favoured

What happens if the temperature of the rxn environment is decreased? (Le Châtelier’s)

The more exothermic direction is favoured

What happens if the Volume of the rxn environment is decreased?

the direction with the fewer gaseous particles is favoured

Rxn changes effects on K?

Rvs rxn: 1/K (Inverse)

Multiply coefficents by n: K^ⁿ
Adding rxn’s: multiply K’s together

What is the 3rd law of thermodynamics? (For CHEM111)

The entropy of a perfect crystal at 0K is 0.

∆S_surroundings = ^-∆H/_T = S_f - S_i

∆_rS = ∑v_iS_(products) - ∑v_iS_(reactants), where v_i is the stoichiometric coefficient
e.g. A_(g) + 3B_(g) → 2C_(g), gives ∆_rS = 2S_c - (S_A + 3S_B)

What is G?

Gibbs free energy, which is a concept that determines the thermodynamic favourability of a rxn and which direction it will evolve, i.e. whether a rxn is favourable or not w/ ȼ: T, P

What are the formulae for G?

∆G = ^-T/∆S_universe
∆G = ∆H - T∆S

What are the directionalities related to G?

At constant Temp. and Pressure

How do you solve dilution questions?

Given C₁, V₁, and V₂ you can use C₁V₁ = C₂V₂ to find the new concentration. Do for each step of the dilution process.
OR
Shortcut by multiplying by dilution factors i.e. 10ml of a / 100ml flask: 10/100
C_f = C_i (D_n)

Light spectrum values:

<400nm Ultraviolet UV
400 - 700nm Visible spectrum
700nm + Infrared (IR)
1mm - 1m Microwave
Big = Radiowave

What is a catalyst?

A catalyst provides an alternative rxn pathway with a lower E_a, increasing the rxn rate of both fwd AND rvs rxn’s.
K DOES NOT shift

What are A, B, C, D and F?

A: Starting energy (reactants)
B&C: Transition states (uncatalysed and catalysed)
D: Intermediate
F: Final energy (products)

What is generally the rate determining step?

The step with the highest activation energy (increase in energy with previous step) generally takes the longest to fully react.

What is a psudo-n^th-order rate law?

When one of the reactants are in comparatively significant excess then the value can be ignored in the rate law

What do you need to find the Rate of rxn w/ Temperature change.

molar ∆_rH of the rxn and Q = mc_sol∆T

0^th order rxn’s final conc. questions approch?

recognise conc. change pattern and extrapolate

Why does temperature increase reaction rate?

Think of the Arrhenius equation, larger T results in less negative exponent and thus higher k in the rate law.

What is heterogeneous catalysis?

The catalyst is a different state (aq, s, l, g) from reactants
Homogeneous is where they are the same state

What is an enzyme?

A biological catlyst

Weak-acid/base equilibrium constants

[H⁺] = [A^-] or [OH^-] = [A⁺]

U and S values for a gas expansion into a insulated vacuum volume?

U = 0 (no ∆T)
S_sys > 0

How can you determine the sign for entropy in a rxn?

Which side has more disordered state moles.

Why can’t you determine the spontaneity of a rxn off solely entropy?

Because spontaneity is determined by Gibbs energy, ∆G = ∆H - T∆S and the rxn will only be spontaneous if G is negative

What are the laws of thermodynamics?

1^st : Energy is conserved
2^nd: The entropy of an isolated system can’t decrease
3^rd : A perfect crystal has zero entropy at 0 K

How is molar entropy related to molecule size?

Smaller molecules have smaller entropy’s because they have less ways to store energy

What is the sign of ∆G?

Positive, G_f - G_i > 0
(Right is higher than left)

Why does water form a meniscus?

Because the H₂O forms Hydrogen bonds with the glass

Adding a salt to water does what to its boiling point?

Increases it

Bonds breaking is… endo/exothermic?

Endothermic

Forming bonds is endo/exothermic?

Exothermic

What happens when water evaporates from a saturated salt solution containing excess solid, at constant temperature?

More solid precipitates, but the equilibrium ion concentrations stay the same because (K_sp) is constant.

How do you approach K_sp questions for ZY given solubility of XY questions

X = Z = ^K_sp/[Y]

What is a conjogate acid/base?

NH₄⁺ is the conjugate acid of NH₃

How do you get pH from pKa and conc. of acid/base-conjugatebase/acid pair?

What is K_w?

1 × 10^-14 = [OH^-][H₃O⁺]

How do you prepare a buffer with pH = pKa using a weak acid and NaOH?

Add enough NaOH to neutralise half the weak acid, producing equal amounts of HA and A^-

How do you find pK_a from K_a?

pK_a = -log(K_a)
∴ K_a = 10^-pKa

What does pKa actually represent?

intra vs intermolecular bonds: