how far 5.1.1 and 5.1.2 Flashcards
What is rate of reaction?
The change in concentration of a substance per unit time.
This is used for consistency as it’s better than say change in mass per unit time.
Describe the following general rate equation: r = k[A]m[B]n
● Based on the reaction… aA + bB → products.
● Unit of r (rate) is moldm-3s-1, unit of [A] and [B] (conc. of A and
B) are moldm-3, and unit of k (rate constant) varies.
● The rate constant is ONLY dependent on temperature (i.e., it
increases with temperatures).
● The values m and n are called reaction orders and have
nothing to do with the stoichiometric coefficients in the balanced equations. They are worked out experimentally and can be 0, 1, or 2.
● The overall order is m + n.
What are the 3 orders of reaction with their associated rate- concentration and concentration-time graphs of the reactant(s)?
Zero-order reaction - rate is independent of concentration. ● First-order reaction - rate ∝ conc.
● Second-order rate - rate ∝ concX (by some factor).
The concentration-time graph for a second-order reaction isn’t required.
What is the half-life of a reaction? And the half-life
The time taken for the conc. of a reaction to half (which is constant for a first-order reaction).
Give 5 ways of determining the rate of a reaction
- Use a gas syringe to measure the change in volume of gas. 2. Use a top pan balance to measure the change in mass as
gases escapes (works best with a heavy gas like CO2). - Measure the time taken for a precipitate to form.
- Measure a change in electricity conductivity (works well if
ions are released). - Use colorimetry to measure absorbance of a wavelength by a chemical
What should be ensured when continually monitoring [A] and why? (in rates of reactions)
Having a large excess of everything else so their conc. is effectively constant ∴ its conc. won’t change much during the reaction ∴ won’t affect the reaction.
How can you use TLC to monitor the course of a reaction
By taking samples at regular intervals.
Give 2 ways of determining orders of reaction and state the benefit of one over the other
- Continually monitor [A] and form a concentration-time graph for many different conc.’s then use this to plot a rate- concentration graph whose shape you can use.
- Use a clock reaction to get a good approximation of the initial rate.
● Then use the initial rates method.
● The latter method is far less time consuming as a graph doesn’t have to be plotted each time.
What 3 assumptions are made during a clock reaction
- The concentration of reactant doesn’t change significantly over the timescale.
- Constant temperature.
- When the endpoint is observed, the reaction has not
proceeded too far.
This allows us to assume a constant rate.
What is the rate determining step
The slowest step in a reaction mechanism which determines the overall rate of reaction.
What things should you look out for when proposing mechanisms from overall reactions and rate equations
- A catalyst appearing in the rate equation as it will reform later.
- When the slowest step contains an intermediate (not a reactant). It should be replaced by substances that make up the previous step.
What is the Arrhenius Plot, what can it be used to find, and what does it use?
ln(k) = (-Ea/RT) + ln(a)
y = ln(k)
m= -Ea/R
x=1/T
c= ln(a)
Used to determine the pre-exponential factor and activation energy using a set of temperatures and rate constants.
What method should you use for KC questions?
● Initial moles.
● Change in moles.
● Equilibrium moles.
What are the 2 types of equilibria
● Homogeneous - reactants and products are in the same physical states.
● Heterogeneous - reactants and products are in different physical states
How do the KC and KP expressions change for heterogeneous equilibria?
Only gases and aqueous reactants/products are used.
E.g., Cu (s) + 2Ag+ (aq) ⇌ Cu2+ (aq) + 2Ag (s) so KC = [Cu2+] / [Ag+]2.
What are mole fractions and partial pressures? How do they relate
● Mole fraction of A - fraction of the total number of moles. ● Partial pressure of A - the pressure of A if it alone occupied
the same volume vessel alone ● p(A) = xA · Ptot
This means the total pressure is the sum of the partial pressures.
What is KP and why do we use it?
● An equilibrium constant calculated from partial pressures. ● As it’s difficult to get concentrations for gases.
E.g., H2 (g) + I2 (g) ⇌ 2HI (g) so KP = p(HI)2 / p(H2)p(I2).
What method should you use for KP questions?
ICE followed by XA PA.
What is the only thing that can affect KC and KP and why not others
● Temperature.
● Changing the concentration/pressure of a substance won’t
have an effect as as the system restores equilibrium so it’s the SAME.
