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Acids and Alkalis

Acids and Alkalis. Forming Acids Acids are formed from soluble non metal oxides. They have a ph range between 1 and 7 They turn pH paper of universal indicator red/ orange / yellow. Forming Alkalis. Alkalis are formed from soluble metal oxides. They have a pH range between 8 and 14

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Acids and Alkalis

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  1. Acids and Alkalis • Forming Acids • Acids are formed from soluble non metal oxides. • They have a ph range between 1 and 7 • They turn pH paper of universal indicator red/orange/yellow

  2. Forming Alkalis • Alkalis are formed from soluble metal oxides. • They have a pH range between 8 and 14 • They turn universal indicator and pH paper Dark green/blue/purple

  3. Examples of Acids and Alkalis

  4. Neutral Substances • A neutral substance is neither acid nor alkali. • They have a pH =7 • They turn universal indicator and pH paper green. Example – Water, Tea, Alcohol!

  5. Acid Rain • Acid rain is formed when acidic gases dissolve in rain water. • The main gas responsible is Sulphur dioxide! • Oxides of Nitrogen can also contribute to the problem. • Acid rain damages the environment: • Eats away buildings, kills plants and fish, corrodes iron structures.

  6. Preventing Acid Rain! • Remove Sulphur from fuels before you burn them. • Reduce the amount of fossil Fuels you burn.

  7. Dissociation of Water • Water exists mainly as molecules – however it sometimes splits into 2 ions – Hydrogen and Hydroxide. • H2O(l) <—–>H+ (aq) + OH- (aq) • Water will conduct electricity – slightly because it contains these ions. • Water is neutral because it contains equal numbers of each ion.

  8. Conductivity • Acids and Alkalis can conduct electricity because they contain ions. • Acids = H+ ion • Alkalis = OH- ion • The stronger the acid or alkali is – the more ions it contains – the greater the conductivity!

  9. Diluting! • When we add water to an acid or alkali– we dilute it – reduce the concentration of the ions! • Acids – the concentration oh H+ ions gets less – the pH will increase – moving towards pH7 • Alkalis – the concentration of OH- will get less – the pH will move down -towards pH 7

  10. Electrolysis of Acids • Acids will split up when you pass an electric current through them. • This is because they contain ions – Hydrogen ions. • When we electrolyse Sulphuric Acid the following happens: • Bubbles of Hydrogen gas appear at the negative electrode. • The H+ ions move to the – electrode – they gain electrons to form H2 gas • 2H+ (aq) + 2 e —> H2 (g)

  11. Metal Hydroxides • We can find out if a compound is soluble using p5 in the data book. • Trends • Metal hydroxides in Group 1 and 2 are soluble. E.g NaOH, KOH, Ca( OH )2 • All other metal hydroxides are insoluble. e.g. Cu(OH)2, Zn(OH)2

  12. Formula Mass • This is the total mass of a compound. • It takes into consideration the number and mass of each element in a compound. • Steps • Correct Formula – Use valency rules! • Write down numbers of each elements in compound. • Add in mass of element – use data book. • Add up total mass.

  13. Formula Mass – example! • Work out the formula mass of Calcium hydroxide. 1. Formula – Ca 2+ OH- Cross over – Ca (OH)2 2. Numbers of each element (1 x Ca) + (2 x O) + ( 2 x H) 3. Add in mass ( 1 x 40) + ( 2 x 16) + ( 2 x1) 4. Add total mass 40 + 32 + 2 = 74 amu.

  14. The Mole! – Gram Formula Mass • The mole is basically the formula mass in grams. • The steps are required. • Example – Calculate the mass of 1 Mole of Water • Symbol – H2O • Numbers of elements – ( 2 x H) + ( 1 x O) • Add in Mass - (2x1) + (1x 16) • Total Mass – (2x1) + 16 = 18 g • 2 moles of water would be- 2 x18g = 32g

  15. More Calculations! n C V n = Number of Moles C = Concentration (mol/l) V = Volume (l) n = C x V C = n/V V = n/C

  16. Examples • 1) Calculate – The number of moles if you have a solution of with 2 mol/l HCl in 100cm3 of water. • n = CxV 2 x 100/1000 2 x 0.1 = 0.2 moles

  17. More! • 2) Calculate the volume of water required to make a 2 mol/l solution of NaOH using 3 moles of the alkali. • V = n/C =3/2 = 1.5 litres

  18. Even More! • Calculate the concentration of solution made when 2 mol/l of acid dissolves in 50 cm3 of water. • C = n/V = 2/(50/1000) = 2/ 0.05 = 40 mol/l

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