Rates, Equilibrium and pH

Question 1

Define rate constant

Answer 1

The constant that links the rate of reaction with the concentrations of the reactants, raised to the powers of their orders in the rate equation

Question 2

Define order

Answer 2

The power to which the concentration of the reactant is raised in the rate equation

Question 3

Define Half-life

Answer 3

The time taken for the concentration of a reactant to reduce by half

Question 4

Define rate determining step

Answer 4

The slowest step in the reaction mechanism of a multi-step reaction

Question 5

What can be said about the half-life of a first order reaction?

Answer 5

It is independent of the concentration

Question 6

What would a rate-concentration graph look like for reactants of order 0,1 and 2

Answer 6

0: straight horizontal line
1: straight diagonal line
2: curved line (think y=x^2)

Question 7

How would you determine the initial rate of a reaction?

Answer 7

Method 1: draw a tangent at t=0 on a concentration-time graph; rate is gradient
Method 2: measure time for a certain amount of product to form

Question 8

How would you determine the rate equation?

Answer 8

rate=k(conc. A)^order(conc. B)^order. To find k, rearrange to k=rate/(conc. A)^order(conc. B)^order. Put some numbers in (hopefully given in question)

Question 9

How does the rate constant, k, change due to temperature?

Answer 9

As temperature increases, k increases, resulting in a faster reaction. This is because more collisions occur, and more collisions have the required activation energy to react: more successful collisions.

Question 10

How might you determine what the rate determining step of a multi-step reaction is?

Answer 10

The reactants that appear in the rate equation must be in the rate determining step. Their order gives you how many of them must be in that step.

Question 11

How might you determine the mechanism for a reaction from the rate equation?

Answer 11

You know the reactants in the rate determining step; work out what else must be produced (if 2 molecules of A in rate determining step, but only 1 in the overall equation, A must be produced at some point). Fill in the gaps like an addition sum!

Question 12

For reation aA + bB -> cC + dD, what is Kc?

Answer 12

Kc = (conc.C^cconc.D^d)/(conc.A^aconc.B^) Then suss out units!!

Question 13

What is Kc affected/unaffected by?

Answer 13

Effected by temperature

Unaffected by changes in concentration, pressure or a catalyst

Question 14

How is Kc effected by temperature?

Answer 14

When the forward reaction is endothermic, a temperature increase will result in an equilibrium shift to the right: more products. Think of Kc equation: therefore, Kc must increase.
When forward reaction is exothermic, a temperature increase will result in an equilibrium shift to the left: more reactants. Think of the Kc equation: therefore, Kc must decrease

Question 15

What is the chemical significance of Kc?

Answer 15

Dictates where the equilibrium position lies: if much bigger than 1, to the right, if much smaller than 1, close to the left.

Question 16

Define acid

Answer 16

A proton donor

Question 17

Define base

Answer 17

A proton acceptor

Question 18

Define conjugate acid-base pairs

Answer 18

A pair of 2 species that transform into each other by the gain or loss of a proton

Question 19

What is the difference between a strong acid and a weak acid?

Answer 19

Strong acids dissociate fully in aqueous solution:
HA -> H+ + A-
Weak acids only partially dissociate in solution:
HA H+ + A-

Question 20

What is the acid dissociation constant, Ka?

Answer 20

Ka = (conc.H+*conc.A-)/(conc.HA) = 10^(-pKa) It shows the extent of acid dissociation

Question 21

What is pKa?

Answer 21

pKa = -logKa. High pKa means low Ka (weak acid); low pKa means high Ka strong acid)

Question 22

Define pH

Answer 22

pH = -log(conc.H+)

Question 23

Define conc. H+

Answer 23

conc. H+ = 10^-pH

Question 24

Define ionic product of water, Kw

Answer 24

Kw = (conc. H+)(conc. OH-)

Question 25

How would you calculate the pH of a strong acid?

Answer 25

HA -> H+ + A-, so conc. H+ = conc. HA. pH = -log(conc. H+)

Question 26

How would you calculate the pH of a weak acid?

Answer 26

HA H+ + A-; Ka=(conc.H+conc.A-)/conc.HA
See that conc.H+ = conc.A-
Make assumption that conc.HA(start)=conc.HA(equilibrium)
Get equation conc.H+ = sqrt(Kaconc.HA)
Work out pH

Question 27

How would you calculate the pH of a strong base (2 methods)?

Answer 27

Method 1: XOH -> X+ + OH-
See conc. OH- = conc. XOH
Use Kw to find H+: Kw=conc.H+*conc.OH-
so conc.H+ = Kw/conc.OH-
Work out pH
Method 2: work out OH- conc. as above
pOH = -log(conc.OH-)
pH=14 - pOH

Question 28

Define buffer solution

Answer 28

A system that minimises changes in pH on addition of small amounts of an acid or a base

Question 29

How can a buffer solution be made?

Answer 29

With a weak acid and the salt of the weak acid

Question 30

What is the role of conjugate acid-base pairing in acidic buffer solutions

Answer 30

The salt of the acid (the conjugate base) means that very little of the weak acid dissociates, as the equilibrium is pushed to the left. Have large reservoirs of weak acid and conjugate base. If H+ ions are added, react with conjugate base; weak acid dissociation equilibrium shifts to the left. If OH- ions are added, H+ ions react with them, forming water, so weak acid dissociation equilibrium shifts to right.

Question 31

How would you calculate the pH of a buffer solution?

Answer 31

Method 1: Ka=(conc.H+conc.A-)/(conc.HA)
Rearranging gives conc.H+ = Kaconc.HA/conc.A-
Work out pH
Method 2: pH = pKa + log (conc.A-/conc.HA)