Acids, Bases and Salts

An Overview

- Some substances, when dissolved in water will give acidic, alkaline or neutral substances.- The pH scale (0-14) tells you how acidic or alkaline a solution is. - Acids have a pH under 7, while Alkalines have a pH over 7.- pH can be measured by using indicators. - Universal indicator will go more and more red the lower the pH and more and more blue/purple the higher the pH.- Litmus paper will go red in an acid and blue in an alkaline.- Methyl Orange will go red in an acid and yellow in an alkaline.- Phenolphthalein will go colourless in an acid and will go pink in an alkaline.- Adding an acid and an alkaline will always produce water and a salt. - Certain bases will not react with acids if they are lower than hydrogen on the reactivity series.

- The Reactivity Series is as follows in descending order of reactivity:PotassiumSodiumCalciumMagnesiumAluminiumCarbonZincIronTinLeadHydrogenCopperSilverGoldPlatinum

The Reactivity Series

Strong and Weak Acids + Alkalis/Bases

- And acid/alkaline can either be strong or weak.- A strong acid/alkaline will dissociate 100% in water.- A weak acid/alkaline will dissociate <100% in water.- Strong Acids include: Hydrochloric Acid (H-Cl), Sulphuric Acid (H2-S-O4), Perchloric Acid (H-Cl-O4) and Nitric Acid (H-N-03).- Weak Acids include: Phosphoric Acid (H2-P-O4), Carbonic Acid (H2-C-03), Citric Acid (C6-H8-O7), Hydroflouric Acid (H-F) and Ethanoic/Acetic Acid (C-H3-C-O-O-H).- Strong Alkalis/Bases include: Sodium Hydroxide (Na-O-H), Potassium Hydroxide (K-O-H) and Lithium Hydroxide (Li-O-H)- Weak Alkalis/Bases include: Ammonia (N-H3)

Definition of Acids/Bases

-According to the Bronsted-Lowry definition of acids and bases, and acid is a proton donor and a base is a proton acceptor.- The strength of an acid is determined by how willing it is to donate a proton to a base; the strength of a base is likewise determined by how willing it is to steal a proton from an acid.- Some substances can act like acids and bases; water is a good example. In this example, it acts as an acid. N-H3 + H2-O → N-H4 (1+) + O-H (1-)In this example, its acts as a base. H-Cl + H2-O → H3-O (1+) + Cl (1-)- For something to be an alkaline it needs to contain an OH (1-) ion. In the first example ammonia steals the proton to create an OH (1-) ion with its N-H4 (1+). Therefore while N-H3 is a base N-H4-O-H is an alkali. This is why water is required to form alkalies. Water acts as an acid to form OH (1-) ions. For soluble hydroxides it simply serves to dissociate the ions like as in acids, with the dissociation of Hydrogen, or a Proton.- Soluble bases = Alkalies as a result. Metallic Oxides, Metallic Hydroxides, Metal Carbonates and Ammonia are all basic, but only those soluble in water can form alkaline solutions.

Solubility

- The solubility of a substance is the maximum amount that can be dissolved in 100g of water.- If a substance has a solubility <1g/100g then it is described as insoluble.- A saturated solution can dissolve no more solute at the same temperature.- For liquids as the temperature increases the solubility increases.- For gases as the temperature decreases the solubility increases.- It is incredibly important to note that acids and alkalies can only behave so because they are dissolved in water; if you dissolve them in an organic solvent such as Toluene they will not exhibit their acidic/alkaline, as they do not dissociate in organic solvents or when not dissolved in water. Eg. H-Cl will not be electrically conductive, react with bases or corrosive in toluene while it will be all three in a solution in water; likewise Na-O-H will act the same except it will not be able to react with any acids in toluene but will in water instead of reacting with bases as it is a base!

Solubility of Salts

There are  some general rules when it comes to solubilities: All common sodium, potassium and ammonium compounds are soluble.   All common chlorides are soluble, except for silver and lead chlorides  All common sulphates are soluble except for lead barium and calcium sulphates   All common hydroxides are insoluble , except for sodium, potassium and ammonium hydroxides   All common nitrates are soluble

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Preparation of Salts

- To prepare a soluble salt one of two methods depending on the constituent parts must be followed: Reacting a metal or an insoluble base with an acid or reacting a soluble base (an alkali) with an acid. These methods are known as the "excess reagent" method and the "titration" method respectively. - To prepare an insoluble salt only one method is required, as the precipitate is the only thing to filter out.- Salts have many commercial uses and come in two forms - hydrous and anhydrous. Hydrated salts have water incorporated into their crystalline structures, which can often change the colour such as with Copper Sulphate, which is blue when hydrous and white when anhydrous. - To notate hydrated salts simply write down the formula of the salt, then write a central point, then write the number of H2-0 molecules for each molecule of the salt. - To dehydrate a salt simply heat it so that the water leaves through evapouration. Be careful not to heat it too much as it may start to decompose.

The "Excess Reagent" Method

- The "Excess Reagent" Method is fairly simple. You add in small amounts the insoluble base to the acidic solution and you let it react, and you keep adding the substance until it has stopped appearing to disappear and begins collecting , with no visible signs of reaction. In other words, you add and excess of the substance to the acidic solution. Then, you filter out the excess of the substance, and heat the solution until around half of the water is gone to get a saturated solution. You then carry out the class rod test. If crystals form on a cool glass rod dipped in the solution then it is saturated. You then leave the solution to cool. As the solution cools the salt begins to crystallize out of the solution until you are left will pure crystals. You then filter the crystals out, rinse them with water, and dry them by using a paper towel and/or leaving the crystals on the windowsill in the sunlight so that they are dried through evapouration. - For instance, you would use this method for creating Zinc Sulphate out of Zinc and Sulphuric Acid.

The "Precipitation" Method

- A precipitate is an insoluble chemical produced during a chemical reaction.- With a soluble base and an acidic solution, add a solution of the base with the acidic solution and then filter out the precipitate. Rinse the precipitate, then dry it by leaving it on the windowsill to dry via evapouration and/or using paper towel.- In this method you are effectively combining two sets of ions together to form the salt, by mixing them together in the water. Only the insoluble salt will appear as it is not dissociated and dissolved by the water hence it precipitates out.- For instance, one and make white Silver Chloride by combining a solution of HCl with a solution of AgCl in excess.

The "Titration Method"

- The "Titration" Method or simply titration is rather long winded compared to the other two. - First of all, you add some Phenolphthalein to a certain known amount of the acid in a conical flask. It will stay colourless. Then, you (using a burette and a stopcock to control the rate of addition) add the alkaline until the solution turns pink, and the moment it turns pink you note down the amount of acid added. In other words, you add the alkaline until the solution is neutral, not acidic or alkaline. You then repeat the reaction with the same volume of acid and the measured volume of acid without the indicator (because of the impurities the indicator would impart to the salt) and you then heat the solution, partially evapourating it until the solution is saturated. You then carry out the glass rod test, and when you are sure it is saturated you leave it to cool. The salt will eventually crystallize, and then you filter out the crystals, and rinse them. You would then dry them by leaving them on the windowsill to dry via evapouration and/or using a paper towel.- It is important that the acid and alkaline neutralize exactly leaving a neutral solution because if you do not then there is no way to get rid of the solution in excess as it is soluble; and thus the salt's crystals will be contaminated by the solution in excess.- For example, titration would be used if a crystal of NaCl was to be obtained from a solution of H-Cl and a solution of Na-O-H.

Concentrations

- Titration can also be used to work out the concentration of a acid/ alkaline if the conentration of the opposing solution is known. - There are two ways to define concentration: the mass concentration and the molar concentration.- To work out Molar Concentration simply divide the number of moles by the volume of the solution in dm^3.- To work out the Mass Concentration simply divide the mass by the volume of the solution in dm^3- Mass Concentration can also be defined as the number of moles multiplied by the Mr. You then divide the answer by the volume in dm^3.- It can also be defined as the Molar Concentration multiplied by the Mr. -Mass Concentration is measured in g/dm^3 while Molar Concentration is measured in mol/dm^3.- Molar Concentration can also be defined as the mass of the substance divided by the volume of it. You then divide the answer of that by the Mr.