User:Isma/Chemistry Resources:Acids and Bases
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At the end of this topic you should be able to:
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Contents
ACIDS AND BASES
Acids
Acids are substances, which ionize in water solutions or in aqueous medium) to produce hydrogen ions. Some ionize more than others. The more H+ ions the acid releases, the stronger is the acid. The following equations show the ionization of sulphuric acid and phosphoric acid.
H2SO4 (aq) ===> 2 H+ (aq) + SO4 2- (aq) H3PO4 (aq) ===> 3 H+ (aq) + PO4 3- (aq)
Bases
Bases are substances, which ionize to produce hydroxide ions in water solutions (or in aqueous medium)
NaOH (aq) ===> Na+ + OH-
| COMMON ACIDS | COMMON BASES |
|---|---|
| Hydrochloric acid, HCl | Sodium hydroxide, NaOH |
| Methanoic acid, HCOOH | Potassium hydroxide, KOH |
PROPERTIES OF ACIDS
- Acids taste sour. Citric acid is responsible for the sour taste of lemons, limes, grapefruits, and oranges. Acetic acid is responsible for the sour taste of vinegar.
- Acids turn litmus (or indicator papers) red. Litmus is a vegetable dye that may be either red or blue, depending on the acidity. When a sample of an acid is placed on red litmus paper, the color of the litmus does not change. Red litmus has been previously treated with acid. Adding more acid does not change the red color. However, when the same acid is placed on blue litmus paper, the color turns from blue to red. (Blue litmus has been treated with a base).
- Certain acids react with metals to form hydrogen gas and a salt. (when a metal takes the place of hydrogen in an acid, the compound that forms is called a salt)
Zn (s) + 2HCl (aq) ===> H2 (g) + ZnCl2 (aq)
Zn (s) + H2SO4 (aq) ===> H2 (g) + ZnSO4 (aq)
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Answer the following question in your exercise book Write down the equation for the reactions below
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NOTE:
The salts of sulphuric acid are always called sulphates (anion is SO4 2- )
The salts of hydrochloric acid are called chlorides (anion is Cl-)
The salts of nitric acid are called nitrates (NO32-)
- Acids react with bases (alkalis) to form a salt and water. The salt is made from the anion of the acid and the cation of the base. The hydrogen ion of the acid and the hydroxide ion of the base unite to form water.
e.g NaOH + HCl ===> NaCl + H2O
- They have pH numbers less than 7
The concentration of hydrogen ions is commonly expressed in terms of the pH scale. The pH scale measures how acidic or basic a substance is. The pH scale ranges from 0 to 14. The lower the concentration of H+ ions, the higher the pH
A pH of 7 is neutral.
A pH less than 7 is acidic.
A pH greater than 7 is basic.
The pH Scale - Acids and Alkalis
The colours of solutions with universal indicator
An indicator is a substance or mixture of substances that when added to the solution gives a different colour depending on the pH of the solution.
Other common indicators used in the laboratory (often used in titrations)
| Indicator | colour in acidic solutions pH<7 | colour in neutral solutions pH=7 | colour in alkaline solutions pH >7 |
|---|---|---|---|
| Litmus solution | red | purple | blue |
| phenolphthalein | colourless | colourless | pink |
| methyl orange | Red/orange | yellow | yellow |
- Acids react with carbonates to form a salt, carbon dioxide and water
e.g CaCO3(s) + 2HNO3(aq)===> Ca(NO3)2(aq) + H2O(l) + CO2 (g)
- Acids react with metal oxides to form a salt and water.
e.g ZnO(s) + 2HCl(aq)===> ZnCl2(aq) + H2O(l)
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Mini-rocket launch This fun activity can be carried out using common household products and materials. Adult supervision is recommended.
3. Put 1/2 of an Alka Seltzer tablet into the container and 1/3 fill with warm water
(If you want to use vinegar and bicarbonate of soda instead, you'll want to dissolve the bicarbonate of soda in around 1/4 of water and add an equal volume of vinegar to start the reaction)
You may be wondering what acids and bases have to do with Alka Seltzer - well, the tablets are in fact made from sodium bicarbonate(base) and dehydrated citric acid, this makes them fizz when you drop them in water. This is why you can replace the tablet with vinegar (or lemon juice) and baking powder in the experiment. The reaction produces carbon dioxide gas and it is this that launches the rocket. The reaction is faster if you use warm water and this will make the rocket go higher.
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Strong acids
A strong acid is an acid, which is completely dissociated into its ions in aqueous medium e.g sulphuric acid
e.g H2SO4 (aq)===> 2 H+ (aq) + SO42- (aq)
A weak acid is one, which is not completely dissociated into its ions in aqueous medium e.g ethanoic acid
e.g CH3COOH ===> CH3COO- + H+
Question
A strong acid and weak acid would indicate low pH values. In which case would the pH be lower?
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Discuss whether all acids are dangerous. |
PROPERTIES OF BASES
Bases which are soluble in water are called alkalis e.g. NaOH sodium hydroxide, KOH potassium hydroxide or Ca(OH)2 calcium hydroxide.
- Alkaline solutions are slippery or soapy to the touch
- All alkalis have a pH greater than seven
- They turn red litmus blue - this is how you test for an alkali
- They will react with acids to form a salt and water. This reaction is called a neutralization reaction. During a neutralization reaction equal amounts of H+ ions from the acid combine with equal amounts of OH- from the alkali to form H2O
e.g NaCl + NaOH ===> NaCl + H2O
- All the alkalis except ammonia will react with ammonium compounds driving ammonia out.
e.g Ca(OH)2 (s) + 2NH4Cl (s)===> CaCl2(s) + H2O(g)
+2NH3(g)
Weak and strong alkalis
A weak alkali is one, which is not completely dissociated into hydroxide ions in aqueous medium
NH3 + H2O ===> NH4+ + OH-
A strong alkali is one, which dissociates completely into hydroxide ions in aqueous medium.
NaOH ===> Na+ + OH-
Tracking a neutralization
When you add an alkali to an acid, you can monitor the neutalization process in several ways
- pH
As H+ ions are removed the pH of the solution rises. You can follow the changes in pH using either universal indicator or a pH meter.
- Conductivity
As H+ ions get removed, the solution is less able to conduct electricity. Conductivity reaches its lowest when neutralization is complete.
- Temperature
Neutralization is exothermic. So the temperature of the solution rises until the reaction is complete. This can be observed using a thermometer.
Other bases include Metal oxides, metal hydroxides, metal carbonates, metal hydrogen carbonates, ammonia solution
The following bases produce a salt and water when reacted with acids
- Metal oxide + acid ===> metal salt + water
e.g CuO(s) + H2SO4(aq) ===>CuSO4(aq) + H2O(l)
- Metal hydroxide + acid à metal salt + water
e.g Ca(OH)2(s) + 2HCl(aq)===>CaCl2(aq) + 2H2O(l)
- Ammonia solution + acid à ammonium salt + water
e.g NH3(aq) + HCl(aq)===>NH4Cl(aq)
The following bases produce a salt, water and carbon dioxide
- Metal carbonates + acid ===>metal salt + water + carbon dioxide
MgCO3(s) + H2SO4(aq)===> MgSO4(aq) + H2O(l) + CO2
Metal hydrogen carbonates+ acid ===>metal salt + water + carbon dioxide
NaHCO3(s) + HNO3(aq) ===> NaNO3(aq) + H2O(l) + CO2
ACIDS AND BASES IN TERMS OF PROTON TRANSFER
The bronsted-Lowry theory of acids and bases
An acid is a "proton donor" (Hydrogen ion, H+ donor). A base is a "proton acceptor" (Hydrogen ion, H+ acceptor)
COMMON USES of acids and bases
Bases
- To neutralize acidic soils
CaO, calcium oxide that is alkaline in nature is put on soil that is too acidic for healthy plant growth. A neutral pH is desirable for plant growth. Powdered limestone (CaCO3, calcium carbonate) is slower and less effective. Both are bases and react to neutralise acids in soils. They can be used on a larger scale in farming and rivers and lakes.
- Indigestion tablets containing magnesium oxide or calcium carbonate are mild alkalis that react by neutralizing excess stomach acidic (hydrochloric acid)
- Bicarbonate powder or baking powder can be used with sour milk (acidic) for raising action in baking.
The acidic milk reacts with the sodium hydrogen carbonate (NaHCO3) to form carbon dioxide gas to give the rising action
- Ants sting venom often contains methanoic acid ('formic acid') which can have a pH of 3 and is presumably 'soothed' by mild alkalis.
Wasp stings can be neutralized with vinegar. Why? Bee stings can be neutralized with baking soda. Why?
Acids
Sulphuric acid is used in car batteries Citric acid is used in beverages
USES of bases in industry
- Sodium hydroxide is used in the manufacture of soap (saponification)
- Ammonia is used in the manufacture of fertilizers such as ammonium sulphate,
NH4)2SO4 and ammonium nitrate, NH4NO3
- Neutralising harmful sulphur dioxide gas (acidic, irritating and toxic SO2) in power station smoke from burning fossil fuels, by absorbing it in alkaline calcium hydroxide solution (limewater)
Types of oxides
There are four types of oxides:
- BASIC OXIDES
- ACIDIC OXIDES
- NEUTRAL OXIDES
- AMPHOTERIC OXIDES
BASIC OXIDES
Oxides formed between metals and oxygen are known as Basic Oxides (metallic oxides). These oxides have basic character.
4Na + O2+ ===> 2Na2O
ACIDIC OXIDES
Oxides formed between non-metals and oxygen are known as acidic oxides. These oxides have acidic character
S + O2 ===> SO2
C + O2 ===> CO2
NEUTRAL OXIDES
Neutral oxides are those oxides, which in contact with water produce neither an acid nor a base. The aqueous solution of these oxides have a neutral character.
For e.g:
H2O, NO, N2O
AMPHOTERIC OXIDES
These are the oxides of certain metals such aluminium, zinc and tin which possesses dual properties of an acid and a base.
2Zn + O2 ===> 2ZnO
4Al + 3O2 ===> 2Al2O3
PREPARATION OF SALTS
Metal + acid
Zinc + sulphuric acid
Excess metal is added to some acid (until some of the metal remains unreacted)
The excess metal is removed by filtration.
The filtrate is heated to evaporate some of the water. Then it is left to cool. Crystals of salt will start forming.
Question
This method is suitable for metals like magnesium, aluminium, iron and zinc but not for sodium or gold. Explain why.
Insoluble base (e.g copper oxide) + acid
The method is the same as the one described above
Alkali (soluble base) + acid
Sodium hydroxide + hydrochloric acid
It is difficult to know when the reaction is over. Hence a titration is performed using a suitable indicator to find the volume of alkali required to neutralize a certain volume of acid. This can be obtained from the burette readings. The reaction is carried out again but this time there is no need for an indicator. The rite volume of acid and alkali are mixed to form the salt. The solution is heated to evaporate some of the water and left to cool. Crystals of salt are thus obtained.
Making insoluble salts
Soluble salts can be crystallized out from aqueous solutions. However not all salts are soluble:
Notice that nitrates and most chlorides are soluble. If we want to make an insoluble salt, we can react together two soluble salts in a precipitation reaction
e.g
AgNO3 + NaCl===> AgCl + NaNO3
(soluble)(soluble) (insoluble) (soluble)
The steps for obtaining the precipitate are described below
Solutions of the 2 soluble salts are mixed
The precipitate is filtered out
It is rinsed with distilled water
Then it is put in a warm oven to dry
To precipitate an insoluble salt you must mix a solution that contains its positive ions with one that contains its negative ions.
e.g To precipitate barium sulphate a solution of barium chloride and magnesium sulphate can be mixed.
Making insoluble salts
Salts containing two elements can be made by direct chemical reaction. E.g if iron is heated in chlorine, FeCl3 is formed. Write the equation for the reaction.
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