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Chemistry > 6. Chemical reactions > Flashcards

6. Chemical reactions Flashcards

1
Q

Redox reactions

A

Reactions involving simultaneous, oxidation and reduction reactions. (These reactions involve the loss or gain of oxygen or electrons)

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2
Q

Oxidation

A

Loss of electrons or gain of oxygen.

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3
Q

Reduction

A

Gain of electrons or loss of oxygen.

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4
Q

Mnemonic for loss or gain of electrons in redox

A

O - Oxidation
I - Is
L - Loss
R - Reduction
I - Is
G - Gain

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5
Q

Oxidation state

A

A number assigned to an element showing the number of electrons lost or gained (or even shared) in a compound or ion.

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6
Q

Oxidation states related to redox reactions

A

Oxidation states show the relative state of how much an element is oxidised or reduced.

  • The more positive an oxidation state becomes –> The more oxidised the element is
  • The more negative an oxidation state becomes –> The more reduced the element is

The oxidation state can appear in the name of ions (shown as roman numerals)

Eg. Fe (II)

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7
Q

Oxidation state of an element

A

An element always has an oxidation state of zero.

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8
Q

Oxidation state of a compound

A

The sum of the oxidation states in a compound is zero.

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9
Q

Oxidation state of an ion

A

The sum of the oxidation state of an ion equals the charge

(sum bc ions can be polyatomic

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10
Q

Oxidation states of a monoatomic ion

A

The oxidation number of a monatomic ion is the same as the charge on the ion.

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11
Q

Usual oxidation states

A

Group 1 metals = +1
Group 2 metals = +2
Hydrogen = +1
Oxygen = -2
Flourine = -1
Chlorine = -1

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12
Q

Half equations

A

A symbol equation that only shows the atoms and ions that react together.

An ionic half equation also includes the charges on ions.

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13
Q

How to form redox half-equations
(magnesium + copper sulfate –> magnesium sulfate + copper)

A

Mg(s) + CuSO4(aq) → Cu(s) + MgSO4(aq)

  1. Using oxidation states, work out which substance is being oxidised and which is reduced and form a simple equation showing this.

eg. Mg –> Mg²⁺, Cu²⁺ –> Cu

  1. Balance both equations in terms of atoms.

eg. (both sides already balanced)

  1. Add electrons to the equation to show how the substance is being reduced / oxidised.

Mg –> Mg²⁺ + 2e⁻ (oxidation half-reaction)

Cu²⁺ + 2e⁻–> Cu
(reduction half-reaction)

(sulfate ion = spectator ion)

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14
Q

Spectator ion

A

Ions that do not participate in a chemical reaction so are not included in an ionic equation.

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15
Q

Oxidising agent

A

A substance which oxidises another substance and itself is reduced.

Substance that gives away oxygen from another substance.

Substance that takes away/gains electrons from another substance.

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16
Q

Reducing agent

A

A substance which reduces another substance and itself is oxidised.

Substance that takes away/gains oxygen from another substance.

Substance that gives away electrons from another substance.

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17
Q

What product is formed in a redox reaction which turns the solution from colourless to red-brown?

A

An iodide ion is oxidised into iodine

Turns the solution red-brown

(Reducing agent)

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18
Q

What product is formed in a redox reaction which turns the solution from purple to colourless?

A

Manganate (VII) ion is reduced into Manganese (II) ion

(Manganate (VII) is a good oxidising agent as it has a 7+ charge meaning it wants to gain 7 electrons (very powerful))

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19
Q

Physical change

A

A change that does not alter what the substance is, but the physical state changes, for example melting.

(Can usually be reversed)

  • Allow mixtures to be made that can be seperated using physical techniques, eg. distillation, filtration, sieving etc.
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20
Q

Chemical change

A

A change that alters what the substance is during the process of a chemical reaction, so that the reactants are different from the products.

(Cannot be easily reversed)

  • Allow new compounds to be made which can be separated by chemical separating techniques such as electrolysis
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21
Q

Rate of reaction

A

The speed at which a reaction takes place.

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22
Q

Two ways of following a chemical reaction to investigate rate of reaction:

A
  • Measure the rate of appearance of products
  • Measure the rate of disappearance of the reactants
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23
Q

How to measure rate of reaction in a reaction

A

If gas is formed…

Use a gas syringe/ displacement of water from an upturned measuring cylinder.

If no gas is formed…

Measure mass loss in reactants / mass gain in products.

24
Q

How does the rate of a reaciton change during a chemical reaction?

A

1) Fast rate of reaction at start (gradient is steepest)

2) Rate of reaction slows down (gradient less steep)

3) Reaction is finished (gradient is flat)

25
Q

Explaining the change of gradient during a reaction –> linked to reaction rate.

A

1) At the start of the reaction, many reactant particle are present so the rate is fast.

2) Over time, one of the reactant particles starts to get used up (the limiting reagent), so the rate begins to slow down.

3) At the end, there are no more reactant particles, so the reaction stops.

26
Q

Measuring rate of reaction quanitively

A

Rate = (amount of products formed) / time

or

Rate = (amount of reactants disappeared) / Time

Eg.

Rate = Volume of gas in cm³ / time in seconds

or

Rate = mass loss in grams / time in seconds

27
Q

What needs to happen to the particles for a reaction to occur

A
  • Particles need to collide to react.
  • More frequent collisions betewen particles mean a faster rate of reaction.
28
Q

Factors that affect the rate of reaction

A

Concentration of solution - Increased concentration, increases the rate of reaction

Pressure of gases - Increase in pressure –> increases the rate of reaction

Surface area - Increase in surface area of solid reactant –> increases rate of reaction

Temperature - Increase in temperature –> increases the rate of reaction

Catalyst - Increase the rate of reaction

29
Q

How does a higher concentration of a solution affect the rate of reaction and why?

A

Rate of reaction increases bc there is an increased chance of successful collisions due to an increase in particles able to collide –> higher frequency of successful collisions.

(Directly proportional) –> concentration doubles = rate of reaction doubles.

30
Q

How does a higher pressure of a gas affect the rate of reaction and why?

A

If pressure increases (often by decreasing volume), the no of particles in a given volume also increases –> rate of reaction increases.

31
Q

How does the surface area of a solid affect the rate of reaction and why?

A

More surface area exposed = more particles available to react –> increases the rate of reaction bc higher chance of successful collisions.

(why powdered substances sometimes cause an explosion)

32
Q

Why do powdered substances explode when ignited in air.

A

Explosions are caused by the large surface area of the particles –> causing a very fast rate of combustion.

eg.

  • Flour dust in flour mills
  • Metal dust /wood dust in factories
  • Coal dust in coal mines
33
Q

How does a catalyst affect the rate of reaction and why?

A

A catalyst is a substance that speeds up the rate of a reaction but it does not get used up in the reaction –> can thus be reused or recycled.

Enzymes = biological catalysts –> increase rate of biochemical reactions. eg. respiration.

Need optimum pH + temperaute. If temp or pH = too extreme –> enzymes lose structure + denature.

34
Q

How does temperaure affect the rate of reaction and why?

A

Temperature increases –> particles gain more energy –> move faster –> collide more often –> more frequent successful collisions.

(More particles have activation energy necessary to react)

35
Q

Collision theory

A

For a successful reaction to occur…

  • Particlse must collide
  • The particles must collide with the correct orientation (head-on).
  • The particles must collide with sufficient kinetic energy. (Min. amount of energy needed for reaction to occur = activation energy, Ea .

Greater no. of successful collisions = greater the rate of the reaction

36
Q

Catalysts explained further

A

Catalyst works by decreasing activation energy of reaction –> decreased bc catalyst provides an alternativ pathway with a lower activation energy

–> allows more particles to successfully collide bc they have activation energy.

(The mass of the catalyst doesn’t change during the reaction)

37
Q

Dynamic equilibrium

A

The condition when the rate of the forward reaction is equal to the rate of the reverse reaction and there is no change in the proportion of reactants or products.

38
Q

Reversible reactions

A

Reactions that can go in both directions i.e. reactants to products (forward reaction) then back from products to reactants (reverse reaction).

These reactions are represented by the symbol ⇌.

39
Q

How does chemical equilibrium occur?

A

If you start a reaction with only reactants and no products, the forward reaction will occur.

The forward reaction will simultaneously occur as the backwards reaction.

However, in the beginning the forward reaction has more particles to react, meaning a greater proportion of it will react.

The backward reaction however, has a smaller proportion and thus less particles, meaning a smaller proportion will react.

Eventually a point will be reached where the rate of the forward reaction is equalt to the rate of the reverse reaction.

40
Q

Characteristics needed for chemical equilibrium to occur.

A

1) The rate of the forward reaction equals the rate of the backward reaction

2) The concentrations of reactants and products remain constant

3) The reaction occurs in a closed system. A closed system is when there is no exchange of substances with the surroundings. Eg. a beaker with a lid on it.

41
Q

Position of equilibrium

A

Where the equilibrium reaction lies. For instance more to the left-hand side or the right-hand side of a reversible reaction.

If there are more products than reactants at equilibrium, the position of equilibrium is to the right.

42
Q

Le chateliers principle

A

When conditions change in a reversible reaction at chemical equilibrium, then the equilibrium moves in the direction that will oppose that change and bring the system back to equilibrium.

eg. if temperature is increased (position of equilibrium is affected), the endothermic reaction will occur to reduce the temperature.

43
Q

Effect of temperature on the position of equilibrium.

A

A reversible reaction always has an endothermic and an exothermic reaction. They can be either one.

Lets assume the forward reaction is exothermic and the backward reaction is endothermic.

If the temperature of a system increases, le chateliers principle states that the change will be counteracted to bring conditions (temperature) back to the same as it was before.

Therefore to oppose the increase of temperature, the endothermic reaction will occur (backward reaction).

This means more backwards reactions occur, resulting in an increased concentration of reactants than products.

Once the temperature has been restored, equilibrium will restore and the position of equilibrium will have changed.

44
Q

Effect of concentration on the position of equilibrium

A

If the concentration of the reactants is artificially increased by someone adding more reactants, le chateliers principle states that this will be counteracted until conditions go back to normal.

Lets assume the position of equilibrium is in the middle to begin with.

Then a large amount of reactants is added, pushing the position of equilibrium to the left.

The forward reaction will begin occuring at a faster rate until the condition of equilibrium is restored. (the equilibrium will shift right until it is in the middle again)

By taking out the products, the concentration of the reactants is increased again, causing the forward reaction to happen again. By doing this repeatedly the yield of the products is greatly increased. (used in the Haber process)

45
Q

Effect of pressure on the position of equilibrium

A

If pressure increases, according to le chateliers principle, the reaction which reduces pressure will occur until conditions are restored.

Therefore if the reactants have more moles than the products, and the pressure is increased, the forward reaction will occur. This is because more moles will react into less moles, decreasing the no. of particles and thus decreasing pressure.

Therefore the position of equilibrium will shift to the right as there will be more products than reactants.

46
Q

Effect of catalysts on the position of equilibrium

A

In a reversible reaction, a catalyst speeds up the rate of the forward reaction and reverse reaction equally as it reduces the activation energy.

This has no net effect on the actual position, it just increases the rate at which equilibrium is reached.

Therefore catalysts are vital in industrial processes for economic reasons.

47
Q

Increase in concentration of reactant - Effect on equilibrium

A

Moves to the right-hand side

48
Q

Increase in concentration of product - Effect on equilibrium

A

Moves to the left-hand side

49
Q

Increase in pressure - Effect on equilibrium

A

Moves to the side with fewer gas moles

50
Q

Decrease in pressure - Effect on equilibrium

A

Moves to the side with more gas moles

51
Q

Increase in temperature - Effect on equilibrium

A

Moves to the endothermic reaction side

52
Q

Decrease in temperature - Effect on equilibrium

A

Moves to the exothermic reaction side

53
Q

Haber process - The reaction

A
  • Reversible reaction producing ammonia from hydrogen and nitrogen (ammonia is a necessary fertiliser)
  • Nitrogen is easily acquired from air, whereas hydrogen has to be made from hydrocarbons (difficult)

N2 + 3H2 ⇌ 2NH3

54
Q

Haber process - How it works

A

1) Hydrogen and Nitrogen enters the reaction vessel. (Large metal container with conditions of 450 C and 200 atm and an iron catalyst in the middle)

2) Forward reaction occurs to produce ammonia. (But bc its reversible, a large amount of the reactants have still not reacted)

3) Ammonia has a fairly low boiling point so the gaseous ammonia is piped into a condesner where it condenses to form a liquid. Hydrogen and nitrogen stay gaseous though.

4) The hydrogen and nitrogen is recyled back into the reaction vessel so as to be able to carry out the reaction again. (Ammonia product is taken out so as to increase concentration of reactants and shift equilibrium to the right).

55
Q

Haber process - Conditions of the closed system, Explained.

A

450C

  • Lower temperature increases % yield
  • But higher temperature increases rate of reaction
    –> 450C = compromise (+ heat is expensive to generate so lower heat is used)

200 atm

  • Higher pressure –> shifts equilibrium to the right bc less moles of gaseous ammonia than of nitrogen and hydrogen
  • Higher pressure also increases rate of reaction
  • 200 atm is higher possible pressure that is still profitable + able to safely achieve.

Iron used as catalyst.

56
Q

What is the source of hydrogen used in the Haber process?

A

Methane

Chemistry (13 decks)

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