Ch5.1 Rates, equilibrium and pH

Question 1

What does a rate equation allow you to do?

Answer 1

• to calculate exactly how fast a chemical reaction will proceed

Question 2

What is the general form of the rate equation?

Answer 2

rate = k [A]ᵐ [B]ⁿ

k = rate constant
A and B = concs of reactants a and b
m and n = the orders of reaction with respect to each reactant

the orders can be 0, 1 or 2

Question 3

What must be considered before constructing a rate equation?

Answer 3

• think carefully about how to design our experiment
• decide what we’re going to record as a measure of reaction rate eg. vol of gas
• a trial experiment will establish a suitable range of concs to use
• we decide on actual values for variables such as concs

Question 4

Describe the method for determining the orders of reaction

Answer 4

1. keep conc of A the same and systematically change B
2. draw a reaction curve for each experiment
3. draw a tangent on each curve at the initial rate
4. plot the gradients against the concs on a new graph
5. flat line = 0, straight diagonal = 1, curve upwards = 2
6. repeat for any other reactants

Question 5

When would the half-lives method be used to work out the orders of reaction in a rate equation?

Answer 5

if nothing in a reaction is visibly measurable eg. esterification, 2 clear colourless liquids react to produce 2 clear colourless liquids

Question 6

Describe the half-lives method

Answer 6

1. mix a large volume of the reaction mixture in a big beaker
2. at fixed intervals (usually 1 min) withdraw small samples of the mixture with a volumetric pipette and quench each to stop the reaction
3. when sufficient samples have been collected/quenched, carry out titrations on each
4. plot graph of conc falling against time
5. plot 3 successive half-lives

Question 7

What will the 3 successive hallf-lives tell you at the end of the half-lives method?

Answer 7

• if half-lives decrease, the reaction is 0 order
• if they remain constant, the reaction is 1 order
• if they increase, the reaction is 2 order

Question 8

What 2 things can a rate equation tell you?

Answer 8

• how quickly the reaction will proceed for any given concs
• how many steps there are likely to be in the reaction which will help us to work out the mechanism

Question 9

What is the rate-determining step?

Answer 9

• the slowest step in a reaction
• it determines the overall speed

Question 10

How can the rate equation tell you the rate-determining step?

Answer 10

• if the conc of a reactant is in the rate equation, then that reactant or something derived from it takes part in the slow step of the reaction
• if the conc of a reactant doesn’t appear in the rate equation then neither the reactant nor anything derived from it participates in the slow step

Question 11

If the equation for the reaction of CO and NO2 is this:
CO+NO2 -> NO+CO2
and the rate equation for this is:
rate = k[NO2]^2,
what does this tell us?

Answer 11

• the rate determining step (usually the first step) involves two molecules of NO2 and nothing else
• there must be a separate and faster step involving the products of the first step and CO
• if 2 molecules of NO2 are used in the first steps, we must produce one in the second step to be consistent with the overall equation

Question 12

What should be the only thing changing when we work out the rate equation for a particular chemical reaction?

Answer 12

the only variable that changes should be conc

Question 13

What is the Arrhenius equation?

Answer 13

k=Ae^(-Ea / RT)

k= rate constant
A = pre-exponential factor
T = temp in K
R = gas constant
Ea = activation energy

Question 14

If concentrations are kept the same, what will happen to the value of k as the temp increases?

Answer 14

it will increase

Question 15

What is the shape and bond angles of an oxonium ion?

Answer 15

pyramidal 107’

Question 16

Where do acidic properties come from?

Answer 16

the reaction of H+ ions with water to make oxonium ions

Question 17

word equation for metal and acid

Answer 17

metal + acid -> salt + hydrogen

Question 18

word equation for metal oxide and acid

Answer 18

metal oxide + acid -> salt + water

Question 19

word equation for metal hydroxide and acid

Answer 19

metal hydroxide + acid -> salt + water

Question 20

word equation for metal carbonate and acid

Answer 20

metal carbonate + acid -> salt + water

Question 21

What happens when an acid is mixed with water?

Answer 21

when an acid donates a proton in the presence of water, the water behaves as a base however the water has become H3O+ which is now an acid

acid in reactants becomes conjugate base and base in reactants becomes conjugate acid in products

Question 22

What happens to bases and acids during neutralisation?

Answer 22

• bases become acids
• acids become bases

Question 23

What are all acid-base reactions?

Answer 23

REVERSIBLE
they are EQUILIBRIUM reactions

Question 24

What is Ka?

Answer 24

• acid dissociation constant

if equation is
HA ⇌ H+ + A-
where A is an acid

Ka = ([H+][A-]) / [HA]

Question 25

How are the dissociations of strong and weak acids different?

Answer 25

strong= complete dissociation
weak= partial dissociation

Question 26

What is the general rule for which acids are strong and weak?

Answer 26

strong acids are inorganic and weak acids are organic (contain carbon)

Question 27

Describe the Ka values of strong and weak acids

Answer 27

strong = high Ka value (always >1)
weak= small (between 0 and 1)

Question 28

What is the formula for calculating pH?

Answer 28

pH= -log10[H+]

Question 29

Describe how monobasic, dibasic and tribasic acids behave in dissociation

Answer 29

• monobasic= split apart completely to produce 1 H+ ion
• dibasic = split apart completely to produce 2 H+ ions
• tribasic = split apart completely to produce 3 H+ ions

Question 30

What is Kw?

Answer 30

• the ionic product of water
  Kw = [H+][OH-] = 1x10^-14

Question 31

What assumptions are made for weak acid calculations?

Answer 31

2 CONTRADICTORY ASSUMPTIONS
- none of the weak acid dissociates (this isn’t as strange an assumption as it seems, for ethanoic acid, 1/1000 molecules dissociate)
- obviously some molecules have to dissociate and when they do, they give equal numbers of [H+] and [A-]

Question 32

After assumptions have been made about weak acid calculations, what conclusion can be made?

Answer 32

Ka= ([H+][A-])/[HA]

= [H+]^2/[HA]

Question 33

What is a buffer?

Answer 33

• a solution which is capable of neutralising small additions of H+ and OH-
• this maintains the pH of the solution

Question 34

Which 2 chemical processes are buffers important for and why?

Answer 34

• most dyes are indicators, if pH isn’t kept constant while they’re being used then they change colour
• many drugs exist as optical isomers (they have right and left handed forms). right-handed forms can be harmful so they need to be screened out during manufacture. using an enzyme in one of the reaction steps ensures that only left-handed isomers are produced. buffer needed so enzyme doesn’t denature