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Chemistry A-level > Acid–base and redox reactions > Flashcards

Acid–base and redox reactions Flashcards

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1
Q

Explain water of crystallisation

A

Crystals of compounds contain water molecules bonded into the crystal structure

(These compounds are called hydrated compounds)

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2
Q

How is water of crystallisation shown

A

It is shown by a dot in the formula

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3
Q

What happens when a hydrated compound is heated

A

When a hydrated compound is heated the bonds holding the water molecules into the crystals are broken. This drives the water molecules off as steam. We are now left with an anhydrous compound

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4
Q

What is an anhydrous compound

A

It does not contain water of crystallisation

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5
Q

How can we determine the value of the water of crystallisation by carrying out experiments use an example

A

hydrated compounds (hydrated copper sulfate) is heated to drive off the water of crystallisation.This leaves us with anyhydrous copper sulfate.

We take the empty boiling tube and weigh it with a balance. We place several spatulas of hydrated compounds (blue hydrated copper sulfate) into a boiling tube and weigh it.

We then calculate the mass of hydrated copper sulfate added.

The boiling tube is heated over a roaring Bunsen burner flame. This drives off the water of crystallisation as steam.We weigh the boiling tube every couple of minutes until the mass stops decreasing. All of the water is driven off and we are left with an anhydrous compound (anhydrous copper sulfate which is white)

The boiling tube and contents are weighed and the mass of anhydrous copper sulfate is calculated.

Compare the mass of the hydrated compound and anhydrous compound to get the mass of water. Then divide the mass of each compound/ molecule by their molar mass to get the mole.

Ratio can be found by dividing using the smallest number

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6
Q

Problems with water of crystallisation

A

If the value of the water of crystallisation is less than expected then that suggests that not all of the water molecules were driven off during heating

In other words, once water has been driven off, the anhydrous compound has then decomposed to a different compound (e.g with a metal carbonate which could undergo thermal decomposition to form a metal oxide, carbon dioxide would be released therefore causing the compound to lose more mass than if just water had been removed)

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7
Q

What is titration used for

A

To determine the concentration of a solution for example an acid or alkali.

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8
Q

Explain the general process of titration

A

We take a fixed volume of a solution with a known concentration. We then react it with a solution with an unknown concentration. If we accurately measured the volume needed to react then we can determine the concentration of our solution.

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9
Q

Process of reacting an acid with an unknown concentration with an alkali with a known concentration

A

An accurate fixed volume of our alkali is recorded using a Pipette with a volume of 25cm^3. However, the pipette should be distilled beforehand to remove unwanted chemicals. The water droplets in the pipette is rinsed with alkali and discarded down the sink.

The conical flask is placed right next to the beaker containing the alkali. The conical flask should be rinsed with distilled water to remove traces of unwanted chemicals. Traces of water in the flask would not interfere with the reaction).

The tip of the pipette is placed in the alkali and drawn into the pipette using a pipette filler. ( this is done carefully and slowly to avoid bubbles , make sure the tip of the pipette is below the surface of the alkali)

We should fill up the pipette slightly above the 25cm^3 as the level drops when the pipette is lifted above the alkali.This makes the volume accurate. The bottom of the meniscus should be on the 25cm^3 mark at eye level.

The pipette is moved over the conical flask and released into the alkali. To transfer all of the alkali the tip of the pipette should touch the alkali.

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10
Q

What indicators are used for strong acid and strong base titrations

A

Phenolphthalein or methyl orange

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11
Q

What indicators are used for weak acid and strong base titrations

A

Phenolphthalein

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12
Q

What indicators are used for strong acid and weak base titrations

A

Methyl orange

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13
Q

Why is only a few drops of indicator added

A

A few drops of indicators are added as they are weak acids, therefore a lot could give inaccurate results .

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14
Q

What is a burette used for

A

A burette is used to measure the volume of acid that reacts with our alkali

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15
Q

How is a burette used to measure the volume of acid that reacts with our alkali in titration

A

The burette is rinsed with distilled water to remove traces of unwanted chemicals from the burette. We then rinse the burette with acid that remove any traces of water. The burette is clamped so it is level. A funnel is used to slowly fill the burette with acid.

We want the levels of acid to be above the zero line. The funnel is then removed. This prevents acid from dripping from the funnel into the burette. The tap is opened and acid is slowly released from the burette we want the bottom of the meniscus on the 0cm^3 mark. (Should be read at eye level

Place the conical flask containing an alkali on a white tile. This makes the colour change of the indicator easer to see accurately.

The tap of the burette is opened and acid is slowly released into the conical flask. The conical flask is then swirled. This ensures that acid and alkali mix thoroughly so they can react.

While we swirl we watch the colour of the indicator. we stop adding when the indicator changes colour to show the end point.

The level of acid on the burette is read (uncertainty on burette is 0.05 cm^3). If the meniscus lines between two scale division we record the volume to the nearest 0.05cm^3

Subtracting the start volume from the final volume gives us our titre (volume of acid reacted with alkali)

First titration is considered to be a rough titration. Conical flask is rinsed and titration is repeated. However, as we know the titre at the end point, we add our acid drop by drop. This is done until we have two concordant titres (between 0.1cm^3)

A mean of concordant tires are calculated. (In 2d.p)

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16
Q

How can we determine water of crystallisation by titration

A

Using the mean titre in the calculation:

amount of substance = concentration X volume

(Convert the volume to decimetre cubed)

After find the mole ratio and determine the number of moles for the base. However, the base may only be portion of what it was when it was weighed at the beginning of the experiment.

We need to multiply the base to the amount it was at the start of the experiment.

We then use molar mass = mass / mol

We then use the relative atomic mass to find the molar mass of the anhydrous compound in order to find the molar mass of water

We then divided the molar mass of water by 18 to find the moles of water

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17
Q

What does the atom economy tell us

A

The atom economy tells us that the mass of the reactant atoms ends up in the products we want

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18
Q

Equation for atom economy

A

Atom economy (%)=

molecular mass of desired product
——————————————————— x100
sum of molecular masses of all reactants

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19
Q

Key points of atom economy

A

It is based purely on the chemical equation for the reaction and it assumes that the percentage yield is 100% (all reactant molecules react)

Large numbers in the chemical equation count for atom economy.

Any reaction with one product must have an atom economy of 100%

Reactions with a high atom economy are more sustainable than those with a low atom economy. This is because less of the mass of reactants ends up in waste products.

Amount of energy used is considered in atom economy and the usefulness of waste products (increase overall sustainability)

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20
Q

Why do we rarely achieve the maximum theoretical yield for a chemical reaction

A

Not all reactants may react (reversible reaction)

side reactions take place to form other products

Some product may be lost (filtration)

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21
Q

What is the equation of percentage yield

A

Yield(%)=

Yield
——————————————- X100
Maximum theoretical yield

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22
Q

Key point of percentage yield

A

Either grams or moles should be used for the yield and maximum theoretical yield

Not both

23
Q

What is the excess reagent

A

When there is an unreacted molecule left over at the end of the reaction.

24
Q

What is the limiting reagent

A

It is the chemical which is used up first

25
Q

What are the formulas of hydrochloride acid , sulfuric acid , nitric acid and ethanoic acid

A

Hcl

H2SO4

HNO3

CH3COOH

26
Q

Key idea of acids

A

All acids contain the element hydrogen

When an acids dissolves in water the hydrogen is released as the hydrogen ion (H+ which is a proton. The acid is said to dissociate, which means splits. A negative ion is released when this happens.

When every acid molecule dissociates adn releases the hydrogen ion the acid is said to be a strong acid

When a small percentage of the acid molecule dissociates. This acid is said to be weak as it only partially dissociates.

27
Q

What are the formulas of these bases Copper oxide, Magnesium hydroxide, Sodium carbonate , Ammonia

A

CuO

Mg(OH)2

Na2CO3

NH3

28
Q

What is a base

A

A base can neutralise an acid to produce a salt

29
Q

Explain neutralisation reactions between a metal oxide and acid

A

When a salt is formed the metal ion in the metal oxide has been replaced by the hydrogen ion in the acid

The oxide ion from the metal oxide and the hydrogen ions of the acid has formed water

30
Q

What product are formed from a neutralisation reaction between a carbonate and acid

A

Salt, carbon dioxide, water

31
Q

What are alkalis

A

Alkalis are bases which dissolve in water

32
Q

What group in the periodic table is soluble in water

A

All group 1 metal hydroxides are soluble in water and can form alkalis

33
Q

What are the key features of hydroxides

A

In a solution they releases the hydroxide ion OH-

34
Q

Explain what happens during a neutralisation reaction between an acid and alkali

A

When an alkali react with an acid, we make salt and water .

The alkali releases the hydroxide ion OH- and the acid releases the hydrogen ion H+

35
Q

What is an oxidation reaction

A

When an oxygen atoms transfer from one atom which is less reactive and binds with another atom which is more reactive

Loss of electrons from an atom

36
Q

What is reduction

A

Loss of oxygen from an atom is called reduction

Gain of electrons from an atom

37
Q

What is a redox reaction

A

When reduction and oxidation take place at the same time in a chemical reaction

38
Q

What does oxidation and reduction also apply to

A

Hydrogen which has been removed can also be oxidation.

Hydrogen which has been added is called reduction

39
Q

What are ions called when they remain unchanged

A

Spectator ions

40
Q

Give an example of an half equation for oxidation and reduction

A

Mg —->(oxidation) Mg^2+ + 2e^-

Zn^2+ = 2e^- —->(reduction) Zn

41
Q

What is an oxidising agent

A

The chemical accepting the electrons is called an oxidising agent

The oxidant agent is reduced

42
Q

What is a reducing agent

A

The chemical which is donating electrons is called the reducing agent

The reducing agent is oxidised

43
Q

Explain oxidation numbers

A

If an atom has lost electrons, then it will have a positive oxidation number.

If an atom has gained electrons, then it will have a negative oxidation number.

The sign is always written before the number

44
Q

Rules of assigning oxidation numbers

A

In a pure element, the atoms have an oxidation number of zero.

When elements have chemically reacted with other elements then their oxidation numbers will no longer be zero.

45
Q

Why does fluorine always form on oxidation number of -1

A

Fluorine is the most electronegative element

46
Q

What are the two exception of oxygen forming an oxidation number which isn’t -2

A

Oxygen has an oxidation number of -1 in peroxides such as hydrogen peroxides

Oxygen has an oxidation number of +2 when reacted with Fluorine (as it electronegative enough to remove electrons from oxygen)

47
Q

What oxidation number to halogens form

A

usually form an oxidation number of zero-1

Can be positive oxidation number if they have rarest with fluorine or oxygen

48
Q

Explain the oxidation number of hydrogen

A

Hydrogen has an oxidation number of +1

However, in metal hydrides such as lithium hydride, hydrogen has an oxidation number of -1

49
Q

What is the oxidation number when group 1 elements react , group 2 elements react, aluminum reacting and transition metal reacting

A

Always +1

Always +2

Always +3

Variable- depending on the compound

50
Q

Key idea of oxidation numbers

A

All oxidation numbers of atoms in a compound must add up to the total charge on the compound

51
Q

What does Roman numerals show

A

The oxidation number of the element/molecule/compound.

52
Q

Why can chlorine form both:

Chlorate (I) and Chlorate (III)

A

Chlorate (I) can form an oxidation number of +1

Chlorate (III) can form an oxidation number of +3

In both these example chlorine reacts with oxygen. This is because oxygen is more electronegative than chlorine, it can remove electrons from the chlorine atom.

53
Q

How can you use oxidation in chemical reaction:

Fe(s) + 2HCl(aq) —> Fecl2(aq) = H2(g)

A

Fe and H2 are both unreacted = 0

Iron reacts to form iron chloride. Chlorine has an oxidation number of -1 and there are two chlorine. Therefore has a total oxidation number of -2. The iron chloride has an overall charge of zero. This means that the oxidation number of iron is +2.

On the left side there is hydrochloric acid. Therefore has an oxidation number of hydrogen is +1. There are two molecules of hydrochloric acid although we only consider the oxidation number of a single atom of hydrogen.

In the reaction the oxidation number of iron has increased from 0-+2. This means that iron has been oxidised

The oxidation number of hydrogen has deceased from +1-0. Therefore, hydrogen ash been reduced.

The changes in oxidation numbers are balanced. The oxidation number of chlorine is -1 on both the right and left hand side. The oxidation number has not changed therefore chlorine is a spectator ion.

Chemistry A-level (35 decks)

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