Topic 5 National Chemistry Summary Notes. Acids and Alkalis

Transcription

1 Topic 5 National Chemistry Summary Notes Acids and Alkalis Experiment Collect some samples of rain water LI 1 The ph Scale The ph scale is a continuous range of numbers from below 0 to above 14. Acids have a ph number of less than 7 Neutral substances have a ph number equal to 7 pure water is a good example Alkalis have a ph number of more than 7 Note a substance becomes more acidic or alkaline the further it is from a ph number of 7 LI 2 Testing the ph of Solutions Experiment ph of Common Solutions Universal indicator, ph paper or a ph meter can be used to find the ph of solutions. When universal indicator is used the following colours are observed: Acidic solutions (ph below 7) red, orange, yellow Neutral solutions (ph 7) green Alkaline solutions (ph above 7) blue, purple Notes: to test the ph of a solid substance it must be soluble in water the ph of water can only be affected by the addition of soluble substances 1

2 LI 3 Common Acids and Alkalis Acids and alkalis are commonly used in the home and in the laboratory. The following tables give some examples of common acids and alkalis. Laboratory Acids hydrochloric acid sulphuric acid nitric acid Alkalis sodium hydroxide lime water ammonia solution Home Acids Vinegar lemon juice Coca-Cola Alkalis bleach washing powder baking soda 2

3 LI 4 Food and Drink and Acids Many of the foods we eat are naturally acidic, for example, oranges and lemons. Many of the manufactured food and drink you like have acids added to them. They are added for lots of reasons. The three main reasons are: 1. To act as a preservative to stop the food going bad quickly 2. To change the flavour 3. To make baked products rise in the oven The most common cause of tooth decay is by acidic foods and drinks. In general, foods and drinks with a ph below have been known to trigger tooth decay. Numerous clinical and laboratory reports link tooth decay to excessive consumption of drinks. Soft fizzy drinks and wine have been shown to cause tooth decay. Experiment Chicken Bones and Acid Pupil Activity Go home tonight and look at some food labels and read the ingredients list to find out if any acids are present. Ethanoic acid, citric acid and benzoic acid are acids you may have come across when looking at food labels. Try to find out why these are added to certain foods and complete the table below with your findings. Food Acid Present Use ethanoic citric benzoic Experiment Lemon Juice & Apples 3

4 E LI 5 The Effect of Dilution on The ph of Acids and Alkalis Experiment 1. Collect a bottle of acid or a bottle of alkali, a bottle of pure water, a plastic dropper, universal indicator and a colour chart 2. Copy and complete the table below remember to wash out the dropper with water between each instruction Acid or Alkali Test Tube Instructions ph Number 1 10ml of acid or alkali 2 1ml from test tube 1 + 9ml water 3 1ml from test tube 2 + 9ml water 4 1ml from test tube 3 + 9ml water 5 1ml from test tube 4 + 9ml water Now add 5 drops of universal indicator to each of the test tubes The ph of a substance is a measure of the hydrogen ion concentration. Acids contain a greater concentration of hydrogen ions than hydroxide ions. Alkalis contain a greater concentration of hydroxide ions than hydrogen ions. Neutral solutions contain an equal concentration of hydrogen and hydroxide ions. 4

5 Pure water is an example of a neutral solution. Pure water contains mainly molecules with a very small but equal number of hydrogen (H + ) and hydroxide (OH - ) ions, i.e. a very small proportion of water molecules dissociate into an equal number of hydrogen and hydroxide ions. H 2 O (l) H + (aq) + OH - (aq) The higher the concentration of hydrogen ions in an acid, the lower the ph number The higher the concentration of hydroxide ions in an alkali, the higher the ph number The ph number of an acidic solution rises towards 7 as it is diluted The ph number of an alkaline solution falls towards 7 as it is diluted Note: Diluted just means water has been added The reason this happens is that when we add water we are mainly adding molecules, not ions, and are therefore decreasing the concentration of the hydrogen or hydroxide ions in the acid or alkali respectively. Experiment Comparing Conductivities 5

6 LI 6 Solubility of Oxides Experiment Name of Oxide Solubility in Water Acid/Alkali/Neutral Using pg 5 of the data booklet we can see that: Group 1 metal oxides are soluble in water and produce alkaline solutions Some Group 2 metal oxides are soluble in water and also produce alkaline solutions Non-metal oxides that are soluble in water produce acidic solutions Note: Insoluble oxides will not affect the ph of water E When added to water, soluble metal oxides produce metal hydroxide solutions, increasing the hydroxide ion concentration. Soluble non-metal oxides increase the hydrogen ion concentration. 6

7 Problems with Non-Metal Oxides Carbon, Sulphur and Nitrogen Oxides Three important non-metal oxides that are soluble in water are sulphur dioxide, carbon dioxide and oxides of nitrogen. These gases are produced naturally as outlined below: carbon dioxide when living things breathe out sulphur dioxide volcanic eruptions oxides of nitrogen lightning storms However, the increased production of these oxides, for example when fossil fuels are burned, causes environmental problems. When produced these oxides react with water in the atmosphere to produce acid rain. Experiments 1. Using a digital ph meter, test the ph of the rain water you collected at the start of this Topic. 2. Cress Seed Growth & Acid Rain. 7

8 The following table shows some examples of the damaging effects of acid rain. Problem buildings steel structures animal life soils and plant life Damaging Effect erosion of buildings, e.g. nose missing from a statue corrodes steel bridges and car bodies increasing acidity of loch water can cause fish to die makes soil too acidic for plants to grow Carbon Dioxide Carbon dioxide gas is the primary gas responsible for Global warming (the rise in the average temperature of the Earth s atmosphere and oceans) and the resulting alarming changes in our climate. In addition to burning of fossil fuels, another large contribution of carbon dioxide in the atmosphere is made by cement manufacturing. When cement is made the process starts with heating calcium carbonate. This releases carbon dioxide into the atmosphere. In addition, the energy required to produce cement can also release carbon dioxide into the atmosphere. Ocean acidification is the name given to the on-going increase in the acidity of the Earth s oceans caused by the uptake of carbon dioxide from the atmosphere. Note: The term carbon footprint refers to the amount of carbon dioxide we emit individually in any one year period. 8

9 LI 7 Neutralisation Reactions A neutralisation reaction is one in which an acid reacts with a base to form water and a salt. The course of a neutralisation reaction can be followed by using a ph indicator such as universal indicator. However, if the base is insoluble then an indicator is not required as the following will occur when the reaction has finished: some of the insoluble base will be left at the bottom of the beaker no more fizzing will be seen In a neutralisation reaction the ph of the substance moves towards 7, i.e. a neutral ph. Bases Bases are substances which neutralise acids giving a salt and water. Bases are: Metal hydroxides Metal oxides Metal carbonates Note: bases which dissolve in water form alkalis Experiment 1. Collect a bottle of acid and a bottle of alkali, universal indicator, a colour chart and two syringes, one labelled acid the other alkali 2. Measure 10ml of acid into the syringe labelled acid 3. Transfer this to a beaker 4. Add 5 drops of universal indicator and swirl 5. Slowly add alkali from the other syringe to the beaker, swirling after each addition, stopping when the colour becomes green add the alkali slowly from the syringe acid + a few drops of universal indicator 9 acid + alkali + a few drops of universal indicator

10 Acid + Metal Hydroxide acid + metal hydroxide salt + water Acid + Metal Oxide acid + metal oxide salt + water Acid + Metal Carbonate acid + metal carbonate salt + water + carbon dioxide Naming Salts The name of the salt formed in a reaction can be easily worked out as shown in the following tables. Acid Salt sulphuric acid nitric acid hydrochloric acid metal sulphate metal nitrate metal chloride Acid Metal Hydroxide/Oxide/Carbonate Salt sulphuric acid sodium hydroxide sodium sulphate hydrochloric acid zinc oxide zinc chloride nitric acid calcium carbonate calcium nitrate 10

11 Full Examples 1. sulphuric + sodium sodium sulphate + water acid hydroxide 2. hydrochloric + zinc zinc chloride + water acid oxide 3. nitric + calcium calcium + water + carbon dioxide acid carbonate nitrate Formula of Acids H 2 SO 4 sulphuric acid (hydrogen sulphate) HNO 3 nitric acid (hydrogen nitrate) HCl hydrochloric acid (hydrogen chloride) Everyday Examples of Neutralisation Reactions Adding lime, an alkali, neutralises the excess acid when soil is too acidic Adding lime to lakes to reduce acidity Indigestion is caused by too much acid in the stomach. Treat by taking acid indigestion remedies, these are alkaline and therefore neutralise the excess acid in the stomach 11

12 LI 8 Making Salts To find out if the salt to be made is soluble or insoluble look at page 5 of the data book. Soluble Salts The easiest way to make a soluble salt is to neutralise the relevant acid with an excess of an insoluble metal oxide or carbonate. An indicator is not required as the following will occur when the reaction has finished: some of the insoluble base will be left at the bottom of the beaker no more fizzing will be seen The excess metal oxide or carbonate can then be filtered off and the salt can then be obtained by evaporating the solution left. No accurate masses or volumes are required since the excess metal oxide or carbonate is easily removed by filtering. Experiment Preparation of A Salt Insoluble Salts remember to read over the notes you have made on this experiment in your jotter The easiest way to make an insoluble salt is by a precipitation reaction. Precipitation is the reaction in which two solutions react to form an insoluble product. The insoluble product is called a precipitate. Example To make the insoluble salt, barium sulphate, just look at page 5 of the data book and react together two soluble compounds, one containing barium and the other containing sulphate. For example, barium chloride + sodium sulphate barium sulphate + sodium chloride (vs) (vs) (i) (vs) Notice the metals just swap places. The insoluble barium sulphate can be removed from the sodium chloride by filtration and then dried. Note the other product formed must be soluble. 12

13 E LI 9 Spectator Ions Spectator ions do not take part in the reaction. Example 1 hydrochloric + lithium lithium + water (word eq) acid hydroxide chloride H + + Cl - + Li + + OH - Li + + Cl - + H 2 O (ionic eq) (when state symbols are included in an ionic equation this is known as an ionic state equation) The lithium and chloride ions are exactly the same on both sides of the equation and therefore have not taken part in the reaction, i.e. they are spectator ions. The reacting species are determined by the omission of the spectator ions. H + + Cl - + Li + + OH - Li + + Cl - + H 2 O In this reaction the hydrogen and hydroxide ions are the reacting species and react together to form water. i.e. H + + OH - H 2 O Example 2 hydrochloric + sodium sodium chloride + water + carbon dioxide acid carbonate 2H + + 2Cl - + 2Na + + CO 3 2-2Na + + 2Cl - + H 2 O + CO 2 The chloride and sodium ions are the spectator ions leaving the hydrogen ions and carbonate ions to form water and carbon dioxide. 2H + + 2Cl - + 2Na + + CO 3 2-2Na + + 2Cl - + H 2 O + CO 2 13

14 E LI 10 Titration Calculations Titration is an analytical technique used to determine the accurate volumes involved in chemical reactions such as neutralisation. An indicator is used to show the end-point of the reaction, i.e. when the reaction is finished. When carrying out titration calculations use the following formula Acid Alkali V x C x H + = V x C x OH - V= volume of acid or alkali C= concentration of acid or alkali H + = number of hydrogen ions in acid formula OH - = number of hydroxide ions in alkali formula Example What volume of a sodium hydroxide solution, with a concentration of 4moll -1, is neutralised by 40cm 3 of 1moll -1 sulphuric acid? Acid H 2 SO 4 Alkali NaOH VxCxH + = VxCxOH - 40x1x2 = Vx4x1 80 = 4V 80/4 = V 20cm 3 = V In titration calculations the volume does not need to be converted to litres as long as it is the same unit for both the acid and the alkali The volume of the sodium hydroxide solution = 20cm 3 14

15 Rough Titres Whenever doing titrations always ignore the rough titre to get the volume of acid required and average the rest. Titre Volume of Acid (ml) Rough st nd 25.2 The volume of acid required in this titration is 25.3ml, i.e. the average of the 1 st and 2 nd titres. Experiment - Titration 15

16 LI 11 Energy Changes of Chemical Reactions All chemical reactions involve energy changes. Sometimes these signs of a chemical reaction are obvious like an explosion or flames, sometimes they are more subtle like a small temperature change. Experiments 1. Adding 50 ml of 1 mol/l hydrochloric acid to 50 ml of 1 mol/l sodium hydroxide. Measuring temperature before and after. 2. Dissolving 5g of ammonium nitrate in 50 ml of water. Measuring temperature before and after. Results Reaction Temperature before ( 0 C) Temperature after ( 0 C) Temperature change ( 0 C) Neutralisation Average of acid and alkali Dissolving ammonium nitrate Endothermic or Exothermic? Some reactions, like the neutralisation reaction above, release heat to their surroundings. These reactions are described as being exothermic. Some reactions, like dissolving ammonium nitrate, take in heat from their surroundings. These reactions are described as being endothermic. You may not have been aware of any reactions that made their surroundings colder but you have met reactions that produce heat. Can you name any other exothermic reactions? Write them below: 16

17 Topic 5 Pupil Self Evaluation Acids and Alkalis If there is an E in any part of the notes or the success criteria is in italics, then this is extension level work Number Learning Intention Success Criteria 1 I will find out about the ph scale I can state if a ph number represents: An acid, alkali or neutral substance A more or less acidic or alkaline substance 2 I will test some substances with universal indicator to find out their ph 3 I will find out the names of some common laboratory and household acids and alkalis I can: Test the ph of a substance State the colours observed when an acid, alkali or neutral substance is tested using universal indicator I can name some common laboratory and household acids and alkalis 16

18 4 I will find out: why acid is added to some food and drink one health problem caused by this addition 5E I will find out: The most concentrated ion in acids and alkalis About the dissociation of pure water about dilution (adding water) of acids and alkalis 6 I will find out about: the solubility of metal and nonmetal oxides the effects soluble non-metal oxides have on the environment the term carbon foootprint I can state: the three main reasons acids are added to some food and drink one health problem caused by this addition I can: I can: state the ion there is a high concentration of in an acidic or alkaline solution State the concentration of the ions present in pure water and other neutral solutions State what happens to the ph number and acidity and alkalinity of an acidic or alkaline solution when they are diluted explain why this happens state whether a metal oxide or non-metal oxide produces an acidic or an alkaline solution if soluble in water state what happens to the hydrogen and hydroxide ion concentrations when metal oxide or non-metal oxides are soluble in water explain how non-metal oxides produce acid rain explain the damaging effects of acid rain on buildings, steel structures, animal life and soil and plant life 18

19 7 I will find out about neutralisation reactions state the gas responsible for ocean acidification and mainly responsible for Global warming and state the various ways this gas can be produced give the definition of the term carbon footprint I can: State the definition of a neutralisation reaction State how the course of a neutralisation reaction can be followed Give examples of a base State the products formed when an acid reacts with a base Name the salt formed in a neutralisation reaction 8 I will find out the best way to make I can describe the best way to make a soluble or insoluble salt soluble and insoluble salts 9E I will find out about spectator ions I can, given a reaction, state the reacting species 10E I will find out how to carry out: the titration technique titration calculations 11 I will find out that reactions can take in heat as well as release heat I can carry out: the titration technique titration calculations I can: state that a reaction which gives out heat is called an exothermic reaction state that a reaction that takes in heat is called an endothermic reaction 19

20 20