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Chemistry Leaving Cert > Rates of Reactions > Flashcards

Rates of Reactions Flashcards

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1
Q

What are Rates of Reactions?

A

The rate of reaction is the change in concentration per unit time of any one reactant or product

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2
Q

What are factors affecting rate of reaction?

A

Nature of reactants, particle size, concentration, temperature, catalysts

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3
Q

How do you draw rate graphs?

A

Concentration v. (1/Time) or Temp v. (1/Time)
(1/Time) used as Rate and Time inversely related (shorter time means faster rate)
Be careful with units of 1/time

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4
Q

Catalyst

A

is a substance that alters the rate of reaction but is not consumed in the reaction

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5
Q

Homogeneous catalysis

A

Homogeneous catalysis occurs when the reactants and the catalyst are in the same phase. example =?

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6
Q

Heterogeneous catalysis

A

Heterogeneous catalysis occurs when the reactants and the catalyst are in different phases

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7
Q

Autocatalysis

A

Occurs when on of the products of the reaction catalyses the reaction. e.g. Mn2+ ions in KMnO4 titrations (purple changes to colourless more quickly as titration proceeds

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8
Q

Mechanisms of Catalysis

A

Intermediate Formation theory

Surface Adsorption theory

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9
Q

Collision Theory

A

For a reaction to occur the reacting particles must collide with each other. A collision only results in a product being formed if a certain minimum energy is exceeded (called activation energy)

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10
Q

Activation Energy

A

The minimum energy which colliding particles must have for a reaction to occur

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11
Q

How to calculate instantaneous rate?

A
  1. Draw a tangent to the curve
  2. Draw a right angle with the tangent as the hypotenuse.
  3. Slope = tan Theta = DeltaV over Delta T
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12
Q

Draw how to calculate instantaneous rate and explain each part.

A

.

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13
Q

What conditions need to be in place for a reaction to occur?

A

Particles must collide if a reaction is to take place.
They must do so with sufficient energy.
They must collide effectively and frequently.

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14
Q

Factors that effect the rate of a reaction

A

Concentration, Catalyst, Area, Temperature and Nature of reactants.

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15
Q

What are the nature of reactants of covalent compounds?

A

Slow Reactions

Bonds are broken before new ones are formed

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16
Q

What are the nature of reactants of Ionic compounds?

A

Fast Reactions

Oppositely charged ions come together

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17
Q

How does temperature affect rates of reaction?

A

Hotter the temperature - the more energy the particles have - the faster the particles move. Therefore, more effective and frequent collisions and the reaction occurs quicker. Therefore, the higher the temperature, the shorter the reaction time.

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18
Q

At lower temperatures, the reaction time is ______.

A

longer

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19
Q

At higher temperatures, the reaction time is _______.

A

shorter

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20
Q

Draw an example graph of a reaction with time on the y axis and temp. on the x axis.

A

.

21
Q

Does increasing concentration decrease or increase the rate of a reaction?

A

Increasing concentration, increases the rate of a reaction.

22
Q

For H2O2, what does increasing concentration do and why?

A

For H2O2 - increasing concentration, increases the amount of product (gas) produced, as it is the only reactant present

23
Q

What does increasing concentration cause for hydrochloric acid and sodium thiosulphate?

A

For hydrochloric acid and sodium thiosulphate - increasing concentration causes the same amount of gas to produced but in a shorter time.

24
Q

At lower concentration the reaction time is _____.

A

longer

25
Q

At high concentrations the reaction time is _______.

A

shorter

26
Q

Draw an example graph of a reaction with time on the y axis and conc. on the x axis.

A

.

27
Q

What is a catalyst?

A

A catalyst is a substance that alters the rate of a chemical reaction but is not consumed in the reaction.

28
Q

What does hydrogen peroxide decompose into at room temperature and what happens if you add manganese dioxide?

A

Hydrogen peroxide decomposes into water and oxygen at room temperature, the addition of manganese dioxide as a catalyst causes hydrogen peroxide to decompose faster.

29
Q

What are the properties of catalysts?

A

Remain chemically unchanged at end of reaction.
Specific - work for one reaction but not another.
Only needed in small amounts.
In equilibrium reactions, a catalyst helps equilibrium to be achieved quicker.

30
Q

What are the 3 types of catalysis?

A
  1. Homogenous 2. Heterogeneous 3. Autocatalysis
31
Q

What is homogenous catalysis?

A

Both reactants and catalyst are in the same phase (same physical state). There is not boundary between the reactants and the catalyst.

32
Q

Give an example of homogenous catalysis.

A

E.g. Both reactants and catalyst are liquids - aqueous potassium iodide and hydrogen peroxide, potassium iodide breaks H2O2 into water and oxygen

33
Q

What is heterogeneous catalysis?

A

Reactants and catalyst are in different physical states. There is a boundary between the reactants and the catalyst.

34
Q

Give an example of heterogeneous catalysis.

A

E.g. Hydrogen peroxide (liquid) and manganese dioxide (solid) or oxidation of methanol by platinum catalyst.

35
Q

What is autocatalyis?

A

Autocatalysis is where one of the products of a reaction catalyses the reaction.

36
Q

Give an example of autocatalysis.

A

E.g. Reduction of Manganate (VII)
MnO4- + 8H+ + 5Fe2+ -> Mn2+ + 5Fe3+ + 4H2O
-Mn2+ catalyses the reaction
-Purple colour decolorises slowly
-Then decolorising speeds up due to Mn2+ ions produced, catalysing the reaction.

37
Q

What is chemical equilibrium?

A

Chemical Equilibrium is a state of dynamic balance where the rate of the forward reaction equals the rate of the reverse reaction.

38
Q

What is Le Chatelier’s Principle?

A

If a stress is applied to a system at equilibrium the system readjusts to oppose the stress applied

39
Q

Describe Le Chatelier’s Principle and Gases

A

Le Chatelier’s Principle predicts that in an all-gaseous reaction an increase in pressure will favour the reaction which takes place with a reduction in volume

40
Q

What is the equilibrium constant?

A

Kc

41
Q

What do [ ] mean in terms of the equilibrium constant?

A

concentration in moles per litre

42
Q

What is the equation for the equilibrium constant?

A

Kc = [C] to the power of c x [D] to the power of d over [A] to the power of a x [B] to the power of b for aA + bB -> cC + dD
(Products of products conc. over products of reactants conc.)

43
Q

Le Chatelier and Industry

A

Ammonia and Haber Process predict max yield at high press. /low temp reality = 200 atmospheres and 500 degrees celsius.
Sulfuric Acid and Contact Process predict max yield at high press./low temp reality = one atm and 450 degrees celsius

44
Q

What does large Kc mean?

A

equilibrium far to right (lots of product produced)

45
Q

What does small Kc mean?

A

equilibrium far to left (v. little product formed)

46
Q

What does Kc do?

A

tells us how far not how fast a reaction occurs

47
Q

What are the units for Kc?

A

depend on reaction

48
Q

do you need to quote temp. for Kc?

A

Most quote temp e.g. Kc = X @ 450 degrees celsius

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