Unit 2: Acids, bases and salt preparations

Describe an experiment for making sodium sulfate crystals.

1. 25cm3 of sodium hydroxide solution is transferred to a conical flask using a pipette and a few drops of methyl orange are added as the indicator 2. Sulfuric acid is run in from the burette until the indicator just turns from yellow to orange (it usually turns red for acids but orange indicates an endpoint) 3. The volume of acid needed is noted and the same volumes of acid and alkali are mixed together in a clean flask without any indicator The equation for the formation of the solution is: 2NaOH + H2SO4 -> Na2SO4 + 2H2O 4. Then the solution is heated to evaporate off some of the water until a saturated solution is formed. It is left to cool and crystals form 5. The crystals are finally separated from any remaining solution by filtration (or the remaining solution is poured away) 6. The crystals are dried by patting them dry with a paper towel or by leaving them in a warm place The equation for the crystallization process: Na2SO4 (aq) + 10H2O (l) -> Na2SO4 . 10H2O. (s)

How can we make soluble salts (except sodium, potassium and ammonium salts)

1. Acid + metal (only for moderately reactive metals from magnesium to iron) 2. Acid + metal oxide or hydroxide 3. Acid + carbonate

What do we mix together to make insoluble salts?

1. All nitrates are soluble 2. All sodium and potassium salts are soluble So all we have to do is mix the nitrate of the metal part of our insoluble salt with the sodium or potassium salt of the non-metal part For example, if we have to make lead (II) iodide: 1. The metal part is lead so we use a solution of lead (II) nitrate 2. The non metal part is iodide so we use a solution of sodium iodide A yellow precipitate will be formed

Describe the proton transfers in acid-base reactions such as: 1. Acids in water 2. Metal oxides in water 3. Sulfuric acid with copper (II) oxide 4. Ammonia in water 5. Hydrochloric acid and sodium hydroxide

1. HA + H2O -> H3O(+) + A(-) The acid donates a proton to H2O which acts as a base 2 (Na+)2(O2-) + H2O -> 2Na(+) + 2OH(-) The O2- ion accepts a proton from the water 3. Cu(2+)O(2-) + H2(+)SO4(2-) -> Cu(2+)SO4(2-) + H2(+)O(2-) H2SO4 is an acid because it donates protons (H+) to CuO (O2-), the base 4. NH3 + H2O -> NH4(+) + OH(-) The ammonia accepts a proton (H+) so it's a base 5. NaOH + HCl -> NaCl + H2O NaOH + H2O -> Na(+) + OH(-) HCl + H2O -> H3O(+) + Cl(-) Na(+) + OH (-) + H3O(-) + Cl (-) -> Na(+)Cl(-) + 2H2O Simplified: OH(-) + H(+) -> H2O

How do you know whether to heat the mixture or not?

Carbonates and magnesium react with cold dilute acids Most other substances need to be heated to react with dilute acids

What happens when you react acids with carbonates? (example: copper (II) carbonate and dilute acids)

Carbonates react with cold dilute acids to produce salt, carbon dioxide and water: Carbonate + acid -> salt + carbon dioxide + water For example: Green copper (II) carbonate reacts with the common dilute acids to five a blue or blue-green solution of copper (II) sulfate, copper (II) nitrate or copper (II) chloride Carbon dioxide is given off. You can recognize it because it turns limewater milky Formulas: CuCO3 (s) + H2SO4 (aq) -> CuSO4 (aq) + CO2 (g) + H2O (l) CuCO3 (s) + 2HNO3 (aq) -> Cu(NO3)2 (aq) + CO2 + H2O CuCO3 (s) + 2HCl (aq) -> CuCl2 (aq) Copper carbonates aren't soluble so they don't dissociate into ions

How do you make insoluble salts?

Method: 1. Mix two solutions of two soluble salts to form an insoluble salt and a solution of a soluble one 2. For example, to make insoluble silver chloride we would mix together solutions of silver nitrate and sodium chloride AgNO3 + NaCl -> AgCl + NaNO3 This is called a precipitation reaction A precipitate is a solid that is formed by a chemical reaction involving liquids or gases A precipitation reaction is simply a reaction that produces precipitate In this reaction, white precipitate is formed because silver chloride won't dissolve in water and so it is seen as a fine, white solid

Why is methyl orange good for titrations?

Methyl orange actually changes colour between pH 3.1 and pH 4.4 so below pH 3.1 methyl orange is red and above pH 4.4 it's yellow Between these pHs methyl orange will be a mixture of red and yellow (orange) so it is yellow in a solution of pH 7 Therefore, it doesn't give us an exact neutralisation point, but because the pH changes very rapidly at the endpoint of titration, if we do the titration carefully we will be able to find the neutralisation point to the nearest drop of acid

What happens when you react magnesium with dilute sulfuric acid?

Observations: 1. Rapid fizzing 2. Colourless gas that pops with a lighted splint 3. The reaction mixture becomes very warm as heat is produced 4. The magnesium gradually disappears to leave a colourless solution of magnesium sulfate Formula: Mg (s) + H2SO4 (aq) -> MgSO4 (aq) + H2 (g) Explanation: Since magnesium is more reactive than hydrogen, it displaces it The magnesium loses electrons and so is oxidised and the and the hydrogen gains electrons and so is reduced

Using information from previous experiments, what will a reaction between zinc and acid be like?

Observations: 1. Same as with the other acids but slower since zinc is lower in the reactivity series than magnesium Formula for sulfuric acid: Zn (s) + H2SO4 (aq) -> ZnSO4 (aq) + H2 (g) Ionic equation: Zn + H2 (+) -> Zn (2+) + H2 Formula for hydrochloric acid: Zn (s) + 2HCl (aq) -> ZnCl2 (aq) + H2 (g) Ionic equation: Zn + H2 (+) -> Zn (2+) + H2

Describe the reaction between sodium carbonate and dilute acids?

Sodium carbonate is soluble in water The equation for the reaction of sodium carbonate with hydrochloric acid is: Na2CO3 (aq) + 2HCl (aq) -> 2NaCl (aq) + CO2 (g) + H20 (l) If we show the ions (remembering that HCl splits apart in solutions): 2Na(+) + CO3(2-) + 2H(+) + 2Cl(-) -> 2Na(+) + 2Cl (-) + CO2 + H2O Ionic equation (without the spectator ions): CO3(2-) + 2H(+) -> CO2 + H2O The CO3 is reduced (loses an oxygen) and the 2H is oxidised (gains oxygen) Since the carbonate ions react by accepting hydrogen ions, we can classify carbonate ions as bases

Write equations that show the process of making sodium/ammonium/potassium chloride/sulfate crystals.

Sodium chloride: NaOH + HCl -> NaCl + H2O (You can actually evaporate this to dryness rather than crystalizing it slowly since sodium chloride crystals don't contain any water of crystallization. You could obtain either a powder or very tiny crystals) Ammonium sulfate: 2NH3 (aq) + H2SO4 (aq) -> (NH4)2SO4 (aq) (this also doesn't have any water of crystallization but you should still crystallise it slowly since heating dry ammonium salts can break them up)

Describe a practical to make copper (II) sulfate crystals

1. Measure 50cm3 of dilute sulfuric acid into a beaker and heat it on a tripod and gauze using a Bunsen burner 2. Add a spatula full of black copper (II) oxide and continue heating. If all the copper (II) disappears, add more copper (II) oxide until there is some left in the beaker. Stir the mixture well to make sure that no more will react. At this stage, we have added excess copper (II) oxide- there is more than enough to react with all the acid present. When there is copper (II) oxide left, we know all the acid has been neutralized 3. Filter off the excess copper (II) oxide and transfer the filtrate (solution) which is blue to an evaporating basin. The solution we have now is copper (II) sulfate The equation for the formation of the solution is: CuO (s) + H2SO4 (aq) -> CuSO4 (aq) + H2O (l) 4. Heat the solution of copper (II) sulfate over a Bunsen burner to boil of some water and concentrate the solution 5. Keep heating until a saturated solution is formed. We can test this by dipping a glass rod into the solution. If crystals for on the glass rod when we remove it we know that the solution is very close to saturated and crystals will also begin to form in the solution 6. Stop heating the reaction mixture and allow it to cool slowly at room temperature so that larger crystals can form 7. Remove the blue crystals from the reaction mixture by filtration, or by just pouring off the remaining solution 8. The crystals can be dried by blotting them with a paper towel or they can be left to dry in a warm place Safety note: 1. Direct heating of the acid is not recommended- it is best heated in a boiling tube placed in a water bath 2. Wear eye protection and don't evaporate the solution to dryness 3. Avoid contact with the crystals are they are harmful and irritating to both the skin and eyes

What are the theories of acids and bases?

1. The Arrhenius theory The theory that an acid is something that produces H+ (aq) ions in solution and an alkali is something that produces OH- (aq) ions in solution For example, when hydrogen chloride gas is dissolved in water to form hydrochloric acid the molecules dissociate to form H+ ions, proving that it's an acid: HCl (aq) -> H(+) (aq) + Cl(-) (aq) A neutralisation reaction can be shown as H+ from an acid reacting with OH- from an alkali to form water: H(+) (aq) + OH(-) (aq) -> H2O (l) The problem with this definition of acids and bases is that it only applies to reactions that occur in aqueous solution and is not more widely applicable 2. The Brønsted-Lowry Theory a) An acid is a proton (hydrogen ion) donor b) A base is a proton (hydrogen ion) acceptor A hydrogen ion is a proton. This is because hydrogen atoms only have one electron in their outer shell so if that electron is removed to make it a positive ion then the hydrogen atom becomes proton A better way of writing the reaction that occur when hydrogen chloride dissolves in water is: H2O (l) + HCl (aq) -> H3O(+) (aq) + Cl (-) (aq) The HCl donates a proton (H+) to an H2O molecule. So therefore, HCl is an acid because it gives a proton and H2O is a base because it accept it H3O+ is a covalent bond. It is unusual because the bond formed between the H2O and the H+ ion is only comprised of H2O electrons. This is called a dative covalent bond or a co-ordinate covalent bond and once it has been formed it is exactly the same as a normal covalent bond The H3O+ ion is called the hydroxonium ion. This is the ion that we usually write simply as H+. Cl- has a negative charge because the H+ left an electron behind when it transferred to the H2O The same occurs when ammonium (NH3) dissolves in water and gains a H+ ion, causing it to become NH4+

Can you write out the ionic equation for the reaction between sodium chloride and silver nitrate?

1. Write out the equation: AgNO3 + NaCl -> AgCl + NaNO3 2. Write out the ions: Ag (+) + NO3 (-) + Na (+) + Cl (-) -> AgCl + Na (+) + NO3 (-) 3. Remove the spectator ions: Ag (+) (aq) + Cl (-) (aq) -> AgCl (s)

Can you write a reaction between a metal and an acid using ions?

Acids in solution form ions Dilute sulfuric acid contains hydrogen ions and sulfate ions Dilute hydrochloric acid contain hydrogen ions and chloride ions So using magnesium as an example: 1. Sulfuric acid Mg + 2H (+) + SO4 (2-) -> Mg (2+) + SO2 (2-) + H2 The SO4 (2-) is a spectator ion The magnesium is oxidised because it loses electrons The hydrogen is reduced because it gains electrons As an ionic equation: Mg + 2H (+) -> Mg (2+) + H2 2. Hydrochloric acid Mg + 2H (+) + 2Cl (-) -> Mg (2+) + 2Cl (-) + H2 As an ionic equation: Mg + 2H(+) -> Mg (2+) + H2

What are salts?

All acids contain hydrogen. When that hydrogen is replaced by a metal, the compound formed is called a salt Examples of acids 1. Hydrochloric acid Formula: HCl Example of a salt: NaCl Name of salts: chlorides 2. Nitric acid Formula: HNO3 Example of salt: KNO3 Name of salts: nitrates 3. Sulfuric acid Formula: H2SO4 Example of salt: CuSO4 Name of salts: sulfates 4. Ethanoic acid Formula: CH3COOH Example of salt: CH3COONa Name of salts: ethanoates 5. Phosphoric acid Formula: H3PO4 Example of salt: K3PO4 Name of salts: phosphates Sulfuric acid can be though of as the parent acid of all the sulfates (the salts formed from sulfuric acid are all called sulfates) It doesn't matter if the replacement of hydrogen can't be done directly. For example, you can't make copper (II) sulfate from copper and dilute sulfuric acid because they don't react However, there are other ways of making it from sulfuric acid so copper (II) sulfate is still a salt Salts are also formed when the hydrogen in an acid is replaced with NH4 (ammonium) These are known as ammonium salts: Ammonium chloride - NH4Cl Ammonium sulfate - (NH4)2SO4 These salts contain the NH4+ ions When ammonia dissolves in water it forms NH4 (+) and OH (-) ions

Describe reactions between acids and metal hydroxides?

All metal hydroxides react with acids in a neutralization reaction: metal hydroxide+ acid -> salt + water For example, reacting dilute hydrochloric acid with sodium hydroxide solution: Observations: 1. Mixing the sodium hydroxide solution and the dilute hydrochloric acid produces a colourless solution so not much seems to have happened 2. However if you put a thermometer in the beaker, the temperature rises several degrees, showing that there has been a chemical change Formula: NaOH (aq) + HCl (aq) -> NaCl (aq) + H2O (l) In terms of ions: H(+) + OH(-) -> H2O All neutralization reactions for an acid reacting with an alkali have the same ionic equation

How does ammonia reacts with water?

Ammonia (NH3) reacts with water to form ammonium ions and hydroxide ions: NH3 (aq) + H2O (l) <-> NH4 (+) (aq) + OH (-) (aq)

Describe the reaction between NH3 and HCl?

Dilute hydrochloric acid reacts with ammonia solution in a neutralisation reaction to form a salt, ammonium chloride: HCl + NH3 -> NH4Cl 1. Brønsted-Lowry Theory: In order to form ammonium chloride, the HCl must donate a proton (H+) to the NH3. The ammonium then becomes a positive ion and joins with the negatively charged chlorine which gained an electron from the hydrogen atom that left it. The HCl is an acid because it donates a proton and the NH3 is a base because it accepts the proton 2. Arrhenius Theory: Ammonia reacts with water to form OH- ions: NH3 + H2O -> NH4(+) + OH(-) The hydrochloric acid dissociates to form H+ ions: HCl -> H(+) + Cl (-) The overall reaction after removing spectator ions: H(+) + OH (-) -> H2O (l) Ammonia and hydrogen chloride gasses react to form ammonium chloride (white smoke) NH3 + HCl -> NH4+Cl-

Why in the reaction above don't we just evaporate off all the water by boiling the solution?

Evaporating to dryness wouldn't give you blue copper (II) sulfate crystals Instead you would produce a white powder of anhydrous copper (II) sulfate (anhydrous means without water) When many salts form their crystals, water from the solution becomes chemically bound up with the salt. This is called water of crystallization A salt which contains water of crystallization is said to be hydrated We can show this part of the reaction from the experiment above as: CuSO4 (aq) + 5H2O (l) -> CuSO4 . 5H2O (s)

Why do we need a different method for making sodium, potassium and ammonium salts?

In the method described previously, you add an excess of a solid to an acid, then filter off the unreacted solid You do this to make sure that all the acid is used up This method doesn't work for making ammonium, sodium and potassium salts because their compounds are soluble in water Therefore, the solid you added would not only react with the liquid, but any excess would dissolve into the water present and you wouldn't have any visible excess to filter off

Can you summarise the methods of mixing salts?

Is the salt soluble? 1. No - use a precipitation method. Mix two solutions, one containing the correct positive ion and one containing the correct negative ion 2. Yes a) Is it a sodium, potassium or ammonium salt? ii) No- react an acid with an excess of solid metal, metal oxide, metal hydroxide or carbonate iii) Yes- use a titration method. React an an acid with a solution of sodium or potassium hydroxide or carbonate (or use ammonia solution)

How can we prepare a pure, dry sample of lead (II) sulfate?

Method: 1. Take 25 cm3 of a solution of lead (II) nitrate in a beaker and add 25 cm3 of a solution of sodium sulfate 2. A white precipitate of lead (II) sulfate will form 3. The equation for the reaction is: Pb(NO3)2 + Na2SO4 -> 2NaNO3 + PbSO4 4. The reaction mixture is filtered 5. The white residue of lead (II) sulfate is left in the filter paper and a colourless solution of sodium nitrate and any excess reactants passes through into the beaker or conical flask 6. The white residue on the filter paper is contaminated with solutions of sodium nitrate and the reactant that was in excess 7. Wash the residue with distilled water by pouring it onto the filter paper and allowing it to filter through. This washes away everything apart from the insoluble lead sulfate and should be repeated 8. We then transfer the filter paper with the lead sulfate on it to a warm oven to evaporate the water

Can you explain why this method works to make insoluble salts?

Silver nitrate solution contains silver ions and nitrate ions in solution The positive and negative ions are attracted to each other, but the attractions aren't strong enough to make them stick together. Similarly, sodium chloride solution contains sodium ions and chloride ions but their attractions aren't strong enough for them to stick together When you mix the two solutions, the various ions interact. The attraction between silver and chloride ions is so strong that the ions clump together and form a solid. The sodium and nitrate ions remain in solution because they aren't sufficiently attracted to each other

Can you describe the reactions involving acids and metals?

Simple dilute acids react with metals depending on their position in the reactivity series: 1. Metals below hydrogen in the series don't react with dilute acids because they can't displace the hydrogen 2. Metals above hydrogen in the series react to produce hydrogen gas 3. The higher the metal is in the reactivity series, the more vigorous the reaction; you would never mix metals such as sodium or potassium with acids because their reactions are too violent In summary: ACID + METAL -> SALT + HYDROGEN 1. Acid + metal oxide -> salt + water (using sulfuric acid and sodium) H2SO4 + NaO -> NaSO4 + H2O 2. Acid + metal hydroxide -> salt + water 3. Acid + ammonia -> ammonium salt (using nitric acid) H(+) + NO3 (-) + NH4 (+) + OH (-) -> NH4NO3 + H2O Hydrochloric acid forms a molecule with two chlorines afterwards except for with metal carbonates and metal hydroxides

How do metal oxides dissolve in water?

Some metal oxides are soluble in water and react with it to form solutions of metal hydroxide: Na2O + H2O -> 2NaOH All the group 1 metals do this reaction so for potassium we would get: K2O + H2O -> 2KOH However, other metals are not soluble in water Calcium oxide dissolves slightly to form calcium hydroxide: CaO + H2O -> Ca(OH)2 All of these solutions contain hydroxide ions since they are alkalis

What happens when you react dilute sulfuric acid with copper (II) oxide?

The copper (II) oxide (black powder) reacts with hot dilute sulfuric acid to produce a blue solution of copper (II) sulfate CuO (s) + H2SO4 (aq) -> CuSO4 (aq) + H2O (l) Most reactions between metal oxides and acids need to be heated for the reaction to start and have the formula: metal oxide + acid -> salt + water Copper (II) is an ionic compound containing the O(2-) ion What happened in this reaction is that the H+ ions in the acid combined with the O(2-) ions to create H2O This is a neutralisation reaction No reduction or oxidation has technically occurred in terms of electron loss and gain which is why copper is able to be part of this reaction: 1. Reactivity is how readily an atom loses or gains electrons to become an ion 2. Since copper is already part of an ionic compound with O (2-) ions and Cu (2+) ions, it doesn't need to give electrons to hydrogen, allowing it to react

What happens when you react magnesium with hydrochloric acid?

The reaction between magnesium and dilute hydrochloric acid looks exactly the same as it does with sulfuric acid except for this time magnesium chloride is produced: Mg (s) + 2HCl (aq) -> MgCl2 (aq) + H2 (g)

How do we solve this problem?

These salts can be made using the metal hydroxide, carbonates or ammonia solution Luckily, all of these are alkaline so you can find out when you have a neutral solution through using an indicator You can do this using titration to tell you when you have added exactly enough of the acid to neutralise the alkali and produce a salt Having found out how much acid and alkali are needed, you can make a pure solution of the salt by mixing those same volumes again but without the indicator

Describe the solubility of ionic compounds in water?

Trends: All the metals included here are: Ammonium A Potassium Particularly Sodium Salty Barium Bitch Calcium Called Magnesium Me Aluminium A Zinc Zebra Iron In Lead Languages Copper Class Siver So 1. All sodium, potassium and ammonium compounds are soluble SAP 2. All nitrates are soluble 3. Most common chlorides are soluble, except lead (II) chloride and silver chloride Chloe Likes Sam 4. Most common sulfate are soluble except lead and barium sulfate. Calcium and silver are slightly soluble Sulfish Little Birds Scare Cows 5. Most common carbonates are insoluble except for sodium potassium and ammonium carbonates 6. Most common hydroxides are insoluble except potassium, barium sodium and ammonium hydroxides. Calcium hydroxide is slightly soluble in water Haha SAPpy baby callum

How do we make magnesium sulfate crystals?

We can add excess magnesium to sulfuric acid. This time the acid doesn't have to be heated When we add the magnesium, the mixture will fix (hydrogen is given off) We keep adding magnesium until the fizzing stops and there is magnesium left in the beaker. This means that all the acid has reacted The equation for the formation of the solution of magnesium sulfate is: Mg (s) + H2SO4 (aq) -> MgSO4 (aq) + H2 (g) The solution is concentrated by heating it and allowed to crystallize The crystallization reaction is: MgSO4 (aq) + 7H2O (l) -> MgSO4 . 7H2O (s)

How can we make barium sulfate?

We can use barium nitrate solution and potassium sulfate solution to form a white precipitate of barium sulfate: Ba(NO3)2 + K2SO4 -> BaSO4 + 2KNO3 The sulfate part could also come from sulfuric acid