Acid–base and redox reactions Flashcards by Russell Miller

Explain water of crystallisation

Crystals of compounds contain water molecules bonded into the crystal structure

(These compounds are called hydrated compounds)

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How is water of crystallisation shown

It is shown by a dot in the formula

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What happens when a hydrated compound is heated

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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What is an anhydrous compound

It does not contain water of crystallisation

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

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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Problems with water of crystallisation

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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What is titration used for

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

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Explain the general process of titration

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

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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What indicators are used for strong acid and strong base titrations

Phenolphthalein or methyl orange

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What indicators are used for weak acid and strong base titrations

Phenolphthalein

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What indicators are used for strong acid and weak base titrations

Methyl orange

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Why is only a few drops of indicator added

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

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What is a burette used for

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

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

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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How can we determine water of crystallisation by titration

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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What does the atom economy tell us

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

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Equation for atom economy

Atom economy (%)=

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

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Key points of atom economy

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

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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What is the equation of percentage yield

Yield(%)=

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

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Key point of percentage yield

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

Not both

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What is the excess reagent

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

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What is the limiting reagent

It is the chemical which is used up first

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

Hcl H2SO4 HNO3 CH3COOH

Key idea of acids

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.

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

CuO Mg(OH)2 Na2CO3 NH3

What is a base

A base can neutralise an acid to produce a salt

Explain neutralisation reactions between a metal oxide and acid

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

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

Salt, carbon dioxide, water

What are alkalis

Alkalis are bases which dissolve in water

What group in the periodic table is soluble in water

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

What are the key features of hydroxides

In a solution they releases the hydroxide ion OH-

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

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+

What is an oxidation reaction

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

What is reduction

Loss of oxygen from an atom is called reduction Gain of electrons from an atom

What is a redox reaction

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

What does oxidation and reduction also apply to

Hydrogen which has been removed can also be oxidation. Hydrogen which has been added is called reduction

What are ions called when they remain unchanged

Spectator ions

Give an example of an half equation for oxidation and reduction

Mg —->(oxidation) Mg^2+ + 2e^- Zn^2+ = 2e^- —->(reduction) Zn

What is an oxidising agent

The chemical accepting the electrons is called an oxidising agent The oxidising agent is reduced

What is a reducing agent

The chemical which is donating electrons is called the reducing agent The reducing agent is oxidised

Explain oxidation numbers

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

Rules of assigning oxidation numbers

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.

Why does fluorine always form on oxidation number of -1

Fluorine is the most electronegative element

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

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)

What oxidation number do halogens form

usually form an oxidation number of -1 Can be positive oxidation number if they are bonded with fluorine or oxygen

Explain the oxidation number of hydrogen

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

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

Always +1 Always +2 Always +3 Variable- depending on the compound

Key idea of oxidation numbers

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

What does Roman numerals show

The oxidation number of the element/molecule/compound.

Why can chlorine form both: Chlorate (I) and Chlorate (III)

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.

How can you use oxidation in chemical reaction: Fe(s) + 2HCl(aq) —> Fecl2(aq) = H2(g)

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.

How can you prepare standard solution

1) Place the weighing boat on the balance and measure its weight. Then place the weighing boat with the solute on the balance. After transfer the solute into a 100cm^3 beaker then weigh the weighing boat again using the balance. 2) Pour 100cm^3 of distilled water with the solute in the beaker and stir with a glass rod 3) We then use a funnel and transfer he solution into a volumetric flask. We rinse off the beaker, glass rod and funnel with distilled water and include the rinsing inside the volumetric flask until the bottom of the meniscus reaches 250cm^3. 4) Place a stopper above the volumetric flask and invert it several times to ensure through mixing.

What’s is a standard solution

A standard solution is a a solution of accurately known concentration prepared from a primary standard that is weighed accurately and made up to a fixed volume.

Explain what happens during a neutralisation reaction

During a Neutralisation Reaction, the hydrogen ions (H+) from the acid react with the hydroxide ions (OH-) from the base to form water. At the same time, the remaining positive and negative ions from the acid and base combine to form a salt.

What is the formula for an acid reacting with a metal

Acid(aq) + metal(s) -> salt(aq) + hydrogen gas(g)

What is the formula for an acid reacting with a metal carbonate

Acid(aq) + metal carbonate(aq) + carbon dioxide(g) + water(l)