Chemistry - Unit 4

Front 1

What is Rate of Reaction and how is it calculated?

Back 1

 It is the change in concentration of a reactantor product per unit time.

 Rate = change in concentration / time taken

Front 2

How do you measure rates of reactions?

Back 2

 Measure the gradient at any time along the curve - by drawing a tangent.
 Initial Velocity = gradient taken at time zero.

Front 3

How can data be collected for a concentration-time graph?

Back 3

 Reactions with acids or bases - can measure change in pH, by titrations/pH meter.
 Reactions which produce gases - measure change in volume or pressure/loss in mass of reactants.
 Reactions that produce visual changes - forming of a precipitate/change in colour.

Front 4

What are the 3 common orders and how can they be recorgnised?

Back 4

 Zero order - the rate is uneffected by the concentration of A.
 First order - increase in concentration mes an increase in rate by the same amount.
 Second order - Change in concentrate and a squared affect on the rate ( if concentration is increases by 2 then the rate increasesby 2 squared).

Front 5

What is a rate equation?

Back 5

mA + nB ---->C

rate = K[A]^m x [B]^n

 K = links the rate with the concentration

Front 6

What is Rate of Reaction and how is it calculated?

Back 6

 It is the change in concentration of a reactantor product per unit time.

 Rate = change in concentration / time taken

Front 7

How do you measure rates of reactions?

Back 7

 Measure the gradient at any time along the curve - by drawing a tangent.
 Initial Velocity = gradient taken at time zero.

Front 8

How can data be collected for a concentration-time graph?

Back 8

 Reactions with acids or bases - can measure change in pH, by titrations/pH meter.
 Reactions which produce gases - measure change in volume or pressure/loss in mass of reactants.
 Reactions that produce visual changes - forming of a precipitate/change in colour.

Front 9

What are the 3 common orders and how can they be recorgnised?

Back 9

 Zero order - the rate is uneffected by the concentration of A.
 First order - increase in concentration mes an increase in rate by the same amount.
 Second order - Change in concentrate and a squared affect on the rate ( if concentration is increases by 2 then the rate increasesby 2 squared).

Front 10

What is a rate equation?

Back 10

mA + nB ---->C

rate = K[A]^m x [B]^n

Front 11

What is Half-life?

Back 11

 Is the time taken for the concentration of the reactant to be reduced by half.
 concentration-time graphs the first order has a half life and so can be identified easily. - half life is always the same no matter what the concentration is.

e.g radioactive half-life used to measure stability of radioactive elemens over time - can tell the age of an historical object.

Front 12

How can you tell the order of a reaction from a concentration-time graph?

Back 12

 Zero order - decrease at a constant rate - half life decreases with time.
 First order - Half-life is constant.
 Second order - Coincentration decreases rapidly but the rate slows down - half-life increases with time.

Front 13

How can you tell the order of a reaction from a rate-concentration graph?

Back 13

 Zero order - rate is unaffect by changes to the concentration (---).
 First order - Rate is directly proportional to concentration (/).
 Second order - Rate increases double the amount to the concentration.

Front 14

What is the Rate Constant, k?

Back 14

 K = links the rate with the concentration.
 larger the value of k, the faster the reaction.
 Rate increases with the increase in temperature as there is more energy present in the reaction.

Front 15

What is the Rate-determining Step?

Back 15

 is the slowest step in the reaction mechanism of a multiple step reaction.
 overall reaction cant be faster than its slowest step.
 overall equation does NOT tell you anything about the reaction mechanism - some experiments have to be carried to find this.
 intermediates are generated in the first step then consumed in the second step - doesnt appear in the overall reaction - short-lived.

Front 16

What is the Equilibrium Constant, Kc?

Back 16

 tells you the relative proportions of the reactants adn the products present at equilibrium.

 aA + bB <----> cC + dD
therefore quilibrium law states that :
Kc = [C]^c x [D]^d / [A]^a x [B]^b

Front 17

When is Dynamic Equilibrium reached?

Back 17

 forward reaction takes place at the same time as the reverse reaction.

Front 18

What is the significance of a Kc value?

Back 18

 magnitude of Kc indicates the extent of a chemical reaction.
 Kc value of 1 indicates equilibrium is half way between reactants and products.
 if Kc > 1 - then reaction is in favour of products/ lies to the right of the reaction.
 if Kc < 1 - then reaction is in favour of reactants/lies to the left.

Front 19

How do changes in temperature affect Kc?

Back 19

 increase in temperature shifts equilibrium in the endothermic direction (+ve) - Kc increases the quilibrium yeild of the products.
 decrease in temperature shifts equilibrium in the exothermic direction (-ve) - Kc increases the quilibrium yeild of the reactant.

Front 20

How do changes in Pressure and Concentration affect Kc?

Back 20

 Kc is unaffected by change in pressure or concentration
 If pressure or concentration is changed then the reaction is no longer in equiibrium and so system needs to be altered to bring back to equilibrium.

Front 21

How does the preence of a catalyst affect Kc?

Back 21

 They affect the RATE of chemical reations but NOT the POSITION of the quilibrium.
 Speed up both forward and reverse reactions.
 Reach equilibrium much quicker.

Front 22

Equilibrium Constant, Kc.....

Back 22

 Compares the concentration of reactants and products present at equilibrium
 Larger the value for Kc = equilibrium lies to the right (favours products)
 If forward reaction in endothermic - Kc increase with an increase in temperature
 If forward reaction is exothermic - Kc decreases with increase in temperature

Front 23

Rate Constant, K.....

Back 23

 measures rate of reaction
 large value of K = fast rate of reaction
 K increases with an increase in temperature - so rate increases with increase in temperature
 Only measure K experimentally

Front 24

What is the importance of compromising?

Back 24

 low temperatures - high yeild but slow
 high temperatures - quilibirum reached quickly but low yeild.

Front 25

Describe Bronsted-Lowry's acid base theory.....

Back 25

 acid-bade reaction involves transfer of H+ ions (ie protons)
 A Bronsted-Lowry acid = any substance from which a proton can be removed.
 A Bronsted-Lowry base = any substance that can remove a proton from an acid.

Front 26

What is the definition of Neutralisation?

Back 26

 a chemical reaction in which an acid and base react together to produce salt adn water.

Front 27

Acid + metal ---->

Back 27

--->salt + hydrogen

Front 28

What is a Conjugat Acid?

Back 28

 A species formed when a proton is added to a base.

e.g. H2) accepts a proton to form its conjugate acid H3O+

Front 29

What is a Conjugate Base?

Back 29

 A species formed when a proton is added to an dacid.

e.g. HNO2 releases a proton to form its conjugate base NO2-

Front 30

How do you calculate pH levels and what are they?

Back 30

 -log[H+(aq)]

 lower pH value = larger [H+]
 higher pH value = smaller [H+]
 A pH change of 1 changes [H+] by 10 times!

Front 31

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

Back 31

 Strong acid is an acid which completely dissociates in solution.
 Weak acid is an acid which partially dissociates in solution.

Front 32

What is Acid Dissociation Constant,Ka, defined as?

Back 32

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

 Large Ka value indicates a large extent of dissociation - the acid is strong
 Small Ka value indicates a small extent of dissociation - the acid is weak.

Front 33

What is Ionisation of Water, Kw?

Back 33

 Kw = [H+(aq)] [OH-[(aq)]

 acidic solution = [H+(aq)] > [OH-(aq)]

 Alkaline solution = [H+(aq)] < [OH-(aq)]

Front 34

What is an alkali?

Back 34

 It is a soluable base that releases hydroxide ions, OH-

Front 35

Describe bases and their strengths.....

Back 35

 the strength of a base is a measure of its dissociation in solution to generate OH- ions.
 Strong bases are 100% dissociated in aqueous solution.
 stronger bases tend to be hydroxides of the metals in group 1 and 2.

Front 36

How do you calculate the pH of a strong base?

Back 36

 [H+(aq)] = Kw
----------------
[OH-(aq)]