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chemistry > C3 - chemical reactions > Flashcards

C3 - chemical reactions Flashcards

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1
Q

what happens to mass in a chemical reaction?

A

in a chemical reaction, mass is always conserved

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2
Q

what might you observe happening to an unsealed reaction vessel during a reaction?

A

a change of mass

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3
Q

what may be the reason for an increase in mass to an unsealed reaction vessel during a reaction?

A

probably because at least one of reactants is gas found in air and products are solids, liquids or aqueous
- before reaction, gas is floating around in air
- it’s not contained in reaction vessel so you can’t account for its mass
- when gas reacts to form part of product, it becomes contained inside reaction vessel
- causing mass of stuff inside vessel to increase

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4
Q

what may be the reason for an decrease in mass to an unsealed reaction vessel during a reaction?

A

probably because some/all of reactants are solids, liquids or aqueous and at least one product is gas
- before reaction, any solid, liquid or aqueous reactants contained in reaction vessel
- if vessel not enclosed, gas can escape from reaction as it’s formed
- when the gas isn’t contained in vessel, you can’t account for its mass
- so total mass of stuff inside vessel decreases

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5
Q

what are ionic compounds made up of?

A

positively charged part and negatively charged part

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6
Q

what is the overall charge of any compound?

A

zero

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7
Q

what are chemical changes shown using?

A

chemical equations

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8
Q

what do state symbols tell you?

A

the state of a substance in an equation

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9
Q

what do half equations show?

A

the movement of electrons

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10
Q

what is a mole?

A

a mole is an amount of substance which contains 6.02x10^23 elementary entities

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11
Q

when do reactions stop?

A

when one reactant is used up

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12
Q

what does the amount of product depend on?

A

the limiting reactant

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13
Q

how is the number of moles calculated?

A

n = mass/Mr

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14
Q

what is an exothermic reaction?

A

a reaction which gives out energy to the surroundings, usually in the form of heat and usually shown by a rise in temperature of the surroundings

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15
Q

what is an endothermic reaction?

A

a reaction which takes in energy from the surroundings, usually in the form of heat and usually shown by a fall in temperature of the surroundings

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16
Q

what can show if a reaction is exo or endo?

A

reaction profiles

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17
Q

what is the activation energy?

A

the minimum amount of energy needed for bonds to break and a reaction to start

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18
Q

what is the activation energy on a reaction profile?

A

the energy difference between the reactants and the highest point on the curve

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19
Q

what will happen if the energy input is less than the activation energy?

A

there won’t be enough energy to start the reaction so nothing will happen

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20
Q

what must be supplied to break bonds?

A

energy

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21
Q

what happens to bonds during a chemical reaction?

A

old bonds are broken and new bonds are formed

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22
Q

is bond breaking endothermic or exothermic?

A

energy must be supplied to break existing bonds so bond breaking is an endothermic process

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23
Q

is bond formation endothermic or exothermic?

A

energy is released when new bonds are formed so bond formation is an exothermic process

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24
Q

what does every chemical bond have associated with it?

A

every chemical bond has a particular bond energy associated with it

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25
Q

what can you use known bond energies to calculate?

A

the overall energy change for a reaction

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26
Q

how is overall energy change calculated?

A

overall energy change = energy required to break bonds - energy formed by releasing bonds

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27
Q

is a positive energy change endothermic or exothermic?

A

endothermic

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28
Q

is a negative energy change endothermic or exothermic?

A

exothermic

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29
Q

what is the pH scale a measure of?

A

how acidic or alkaline a solution is

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30
Q

what ions do acids form in water?

A

acids form H^+ ions in water

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31
Q

what ions do alkalis form in water?

A

alkalis form OH^- ions in water

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32
Q

how is the value of the pH related to the concentration of hydrogen ions?

A

the value of the pH is inversely proportional to the concentration of hydrogen ions in a solution

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33
Q

what is used to measure pH?

A

universal indicator

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34
Q

what is used to measure pH electronically?

A

a pH probe attached to a pH meter can be used to measure pH electronically

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35
Q

why is measuring pH electronically more accurate than an indicator?

A

the probe is placed in the solution you are measuring and the pH is given on a digital display as a numerical value

36
Q

what has to be done to the pH probe in between readings?

A

it’s important to calibrate it correctly (by setting it to read pH 7 in a sample of pure water), and rinse the probe with deionised water in between readings

37
Q

what is the reaction between acids and bases called?

A

neutralisation

38
Q

what does neutralisation produce?

A

a salt and water

39
Q

what can neutralisation reactions in aqueous solutions also be shown as?

A

an ionic equation in terms of H^+ and OH^- ions

40
Q

when an cid neutralises a base (or vice versa) what does this mean the product is?

A

neutral

41
Q

what do titration curves show?

A

pH changes with volume
- titration curves are used to show where neutralisation happens during a titration; there’s a vertical point in the curve which is where the solution is neutral (pH 7)

42
Q

what is the vertical point in the curve where the solution is neutral called?

A

the end point

43
Q

what are titrations used for?

A

titrations are used to work out how much of an acid is used to neutralise a base of unknown concentration (or vice versa)

44
Q

what can acids produce in water?

A

protons
- since an H^+ ion is just a proton and acids ionise in water

45
Q

examples of some strong acids?

A

sulfuric, hydrochloric and nitric acids

46
Q

what are some characteristics of strong acids?

A
  • ionise almost completely in water
  • a large proportion of acid molecules dissociate to release H^+ ions
  • tend to have low pHs (0-2)
47
Q

examples of some weak acids?

A

ethanoic, citric and carbonic acids

48
Q

what are some characteristics of weak acids?

A
  • do not fully ionise in solution
  • only a small proportion of acid molecules dissociate to release H^+ ions
  • pHs tend to be around 2-6
49
Q

is the ionisation of a weak acid reversible?

A

the ionisation of a weak acid is a reversible reaction, which sets up an equilibrium mixture

50
Q

what does acid strength tell you?

A

acid strength tells you what proportion of the acid molecules ionise in water

51
Q

what does concentration measure?

A

how much acid there is in a litre of water

52
Q

when is an acid described as concentrated?

A

an acid with a high proportion of acid molecules compared to the volume of water is said to be concentrated

53
Q

when is an acid described as dilute?

A

an acid with a low proportion of acid molecules compared to the volume of water is said to be dilute

54
Q

what does changing the concentration of an acid affect?

A

it’s pH

55
Q

if the concentration of H^+ ions increases by a factor of 10, what would the pH decrease by?

A

1

56
Q

MASH

A

metal + acid = salt + hydrogen

57
Q

what is a salt?

A

an ionic compound, formed as part of a neutralisation reaction

58
Q

acid + metal carbonate = ?

A

salt + water + carbon dioxide

59
Q

acid + alkali = ?

A

salt + water

60
Q

how can soluble salts be made?

A
  • using acid/alkali reaction
  • using an acid and an insoluble reactant
61
Q

wha type of reactions can be used to make insoluble salts?

A

precipitation reactions

62
Q

what type of reaction is it if electrons are transferred?

A

a redox reaction

63
Q

oxidation is …

A

loss of electrons

64
Q

reduction is …

A

gain or electrons

65
Q

what is an oxidising agent?

A

it accepts electrons and gets reduced

66
Q

what is a reducing agent?

A

donates electrons and gets oxidised

67
Q

what do half equations show?

A

if things have been oxidised or reduced

68
Q

what is electrolysis?

A

the breaking down of a substance using electricity

69
Q

what are cations?

A

positive ions

70
Q

what are anions?

A

negative ions

71
Q

what is the cathode in electrolysis?

A

negative electrode

72
Q

what is the anode in electrolysis?

A

positive electrode

73
Q

how does electrolysis work?

A
  1. an electric current is passed through an electrolyte (a molten or dissolved ionic compound), causing it to decompose
  2. the cations in the electrolyte will move towards the cathode and are reduced; the anions in the electrolyte move towards the anode and are oxidised
  3. as ions gain or lose electrons they form the uncharged substances and are discharged from the solution
74
Q

what is an electrochemical cell?

A

a circuit, made up of the anode, cathode, electrolyte, a power source and the wires that connect the two electrodes

75
Q

how many sources of ions are there in molten ionic compounds?

A

one

76
Q

why can’t an ionic solid be electrolysed?

A

because the ions are in fixed positions and can’t move

77
Q

why can molten ionic compounds be electrolysed?

A

the ions can move freely and conduct electricity

78
Q

what happens to positive metal ions at the cathode?

A

they are reduced (they lose electrons) to atoms

79
Q

what happens to negative ions at the anode?

A

they are oxidised (they lose electrons) to atoms

80
Q

why is electrolysis of aqueous solutions complicated?

A

as well as the ions from the ionic compound, there will be hydrogen and hydroxide ions from the water

81
Q

what happens at the cathode during electrolysis of an aqueous solution?

A
  • if H^+ ions and metal ions are present, hydrogen gas will be produced if the metal is more reactive than hydrogen
  • if the metal is less reactive than hydrogen then a solid layer of the pure metal will be produced instead
82
Q

what happens at the anode during electrolysis of an aqueous solution?

A
  • if halide ions are present, molecules of chlorine, bromine or iodine will be formed
  • if no halide ions are present, then oxygen will be formed from the hydroxide ions
83
Q

how do you set up an electrochemical cell?

A
  1. get two electrodes; clean the surfaces of the electrodes using a piece of emery paper
  2. from this point on, be careful not to touch the surfaces of the metals with your hands (you could transfer grease back onto the strips)
  3. place both electrodes into a beaker filled with your electrolyte
  4. connect the electrodes to a power supply using crocodile clips and wires
84
Q

what can happen to non-inert electrodes?

A

they can decompose into the electrolyte

85
Q

what happens to the mass of the non-inert electrodes during electrolysis?

A

e.g. electrolysis of copper sulphate
- the mass of the anode will decrease and the mass of the cathode will increase
- since copper is transferred from the anode to the cathode

chemistry (9 decks)

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