Post on 23-Dec-2015
Acids“ A substance that can dissolve in water form
hydronium ions (H3O+)”• Hydrogen is found in all acids• Can be solid , liquid or gas• Can neutralise bases (eg fizz neutralises jelly fish
stings)• Corrosive, burn skin, taste sour• Can be synthetic (made from chemicals) & natural
/ organic (found in nature, contain carbon)• Dilute Acid a little acid, lots of water• Concentrated Acid lots of acid, a little water
Bases
“Substances which can form hydroxide ions (OH-) in solution”
• Can be solids, liquids or gases• They neutralise acids (eg toothpaste
neutralises plaque acid)• Dissolve biological material “caustic” (eg
grease, dirt oven cleaner)• Bases soluble in water are called alkalis
(feel soapy – turn skin oil into soap)
Common AcidsFormula Name Use
H2SO4 Sulfuric acid Car batteries
HCl Hydrochloric acid Stomach acid
HNO3 Nitric acid Making explosives
CH3COOH Acetic acid Vinegar
C6H8O7 Citric acid In citrus fruits
Tartaric acid In grapes
Carbonic acids Fizzy drinks
Lactic acid Makes yoghurt
DNA Genetic code
Formic acid In ant/bee stings
Common BasesFormula Name Use
NaOH Sodium hydroxide Oven cleaner
NH3 Ammonia Household cleaner
NaHCO3 Sodium bicarbonate Baking
Ca(OH)2 Calcium hydroxide Fertiliser
Explaining Acids & Bases
• The special properties of acids are due to hydronium ions H3O+ in aqueous solutions.
HCl + H2O H3O+ + Cl-
• Whereas basic solutions contain hydroxide ions:NaOH + H2O Na+ + OH-
NH3 + H2O NH4+ + OH-
Aqueous Solution: Particles dissolved in water
Effect of Acid on Browning on Apples
• How was this experiment set up?
• What happened?
• What do the results show?
Explaining Strength of Acids & bases
Original idea: acid particles burn skin because they have sharp stingy spikes. Now we know:•Strong Acids become H3O+ ions easily
– Eg HCl becomes mostly H3O+ ( & not much HCl)
•Weak Acids don’t become H3O+ ions easily – Eg Acetic acid stays mostly acetic acid ( & not much H3O+)
(H3O+ accounts for the properties of acids)
So, conversely:• Strong Bases become OH- ions easily
– Eg NaOH becomes mostly OH- ( & not much NaOH)
• Weak Bases don’t become OH- ions easily – Eg NH3 stays mostly NH3 ( & not much OH-)
(OH- accounts for the properties of bases)
Disappearing InkDone for you…
1 Make a dilute ammonia solution by adding 4mL of concentrated ammonia to 96mL H2O.
2 Divide into 10mL samples
You do…
3 Add a few drops of phenolphthalein indicator.
4 Use this to “write” on some paper with a matchstick dabbed in the solution
Disappearing Ink - Questions1) What pH is the ammonia?
2) Why does the “ink” disappear?
…spray with NaOH.
http://sciencewithtoys.wikispaces.com/Disappearing+ink
pH• Scale from 0-14 indicating acidity or basicity:
0-3 = strong acid, react fast (eg sulfuric acid)
4-6 = weak acid, react slowly (eg urine)
7 = neutral (eg pure water)
8-11 = weak base, react slowly (eg sea water)
12-14 = strong base, react fast (eg sodium hydroxide)
• Most acids found in living things are weak
*The truth about pH
• Measures the “power of hydrogen”
• Measures how much H3O+ there is
• A pH of 3 has 10 times the H3O+ of a pH of 4
• This is why strong acids have a low pH (they have loads of H3O+)
Indicators
“Substances which change colour in solutions of different pH”
• Sources:– Plant pigments (eg litmus comes from lichen)– Synthetic chemicals (eg bromothymol blue)
• Most indicators have two colours, colour change is reversible
• Universal Indicator: a mixture of indicators so that it changes colour many times at many different pHs
Common Indicators(& their colour changes)
• Blue litmus– Turns red in acids
• Red litmus– Turns blue in bases
• Phenolphthalein– Pink above pH 10.0, clear below pH 8.2
• Universal Indicator:Colour Red Orange Yellow Green Blue Purple
pH 0-1 1-3 3-7 7-8 9-12 12-14
Purple Cabbage IndicatorBackground: The purple pigment found in purple cabbage is one example of a substance that changes colour in solutions of different pHAim: To investigate its use as an indicator of pH.Method:
1. Rip up a small amount of cabbage.2. Heat in 100mL of water until water is dark purple3. While heating test the supplied acids/bases with pH
paper to find their pH4. Add small samples (a few drops) of the supplied
acids/bases to a few mL of purple water in a test tube and note the colour change.
5. Take a photo of your test tube rack clearly showing the range of colours and their pH
Acid & Base Reactions(neutralisation)
• Neutralisation: when acid and base react & bring their pHs closer to 7 and a salt and water are produced.
• If complete neutralisation occurs:– Sour acid taste disappears
– Salty taste increases
– Indicators show a pH of 7
• Reaction:General: acid + base salt + water
Word: Hydrochloric acid + sodium hydroxide sodium chloride + water
Symbol: HCl + NaOH NaCl + H2O
• How could you prove a salt was made?• Evaporate the water off, look for crystallisation salt crystals (other
chemical tests needed to prove the type of salt)
Acid + Metal Salt + Hydrogen Gas• Example
Nitric acid + potassium potassium nitrate + hydrogen gas
• More ExamplesSulfuric acid + magnesium magnesium sulfate + hydrogen gas
Hydrochloric acid + sodium sodium chloride + hydrogen gas
• Chemical Equation Examples:HNO3 + K KNO3 + H2
H2SO4 + Mg MgSO4 + H2
HCl + Na NaCl + H2
pH becomes neutral!
Naming salts: 1st part from the metal, 2nd part from the acid ending
Test for H2: pop! test
Acid & Carbonate Reactions(a type of acid & base reaction)
Acid + Carbonate Salt + Carbon dioxide + water
•ExampleNitric acid + calcium carbonate calcium nitrate + carbon dioxide + water
•More examplesSulfuric acid + copper carbonate copper sulfate + carbon dioxide + water
•Chemical equation examples:HCl + MgCO3 MgCl2 + CO2 + H2O
H2SO4 + PbCO3 PbSO4 + CO2 + H2O
Naming salts: 1st part from the metal, 2nd part from the acid ending
pH becomes neutral!
Test for CO2: bubble through
limewater (cloudy = CO2) or
lit match extinguishes
?What is this slide?
• Al2(CO3)3 + 6HNO3 2Al(NO3)3 + 3H2CO3
• Al2(CO3)3+ 3H2SO4 Al2(SO4)3 + 3CO2 + 3H2O
Balanced Chemical Equations1 Can’t change element type (we’re doing chemical reactions, not nuclear so same types of atoms must be on each side of the reaction) AND Can’t get something from nothing (the total numbers of atoms on each side of the reaction must be the same)
2 Problem as 2 x H on the right and 1 x H on left. Can’t make HNO3 become H2NO3 as that’s not nitric acid anymore!
3 But can have extras of any whole molecule by putting numbers IN FRONT
4 For example, the red two fixes the hydrogen problem (now two on each side, but now have a NO3 imbalance.
5 The green two fixes NO3, and introduces a K imbalance which is solved with the blue two and we now have the same number of each type of atom on both sides. The equation is balanced!
HNO3 + K KNO3 + H2
2HNO3 + K KNO3 + H2
2HNO3 + K 2KNO3 + H2
2HNO3 + 2K 2KNO3 + H2
Try balancing these:
The reaction of sodium in water (makes sodium hydroxide and hydrogen gas)
Na + H2O -> NaOH + H2
Baking Soda + Vinegar
• Word:
Acetic acid + sodium bicarbonate sodium acetate + water + carbon dioxide
• Symbols:
CH3COOH + NaHCO3 NaCOOH + H2O + CO2
• Balanced:
CH3COOH + 2NaHCO3 2NaCOOH + H2O + 2CO2
Sherbet(an acid / base reaction)
Recipe• 1T raro• 1t citric acid• 1t tartaric acid• 1t sodium bicarbonate
(baking soda)• 1C icing sugar• Mix, serve
Recipe Using Formulae• 1T raro• 1t ____________
• 1t C4H6O6
• 1t ____________
• 1C ____________• Mix, serve
T = tablespoon, t = teaspoon, C = cup
Sherbet Questions
1) Rewrite recipe using chemical formulae2) Explain why you can safely eat the acids3) Describe what happens in your mouth4) What is the name of this type of reaction5) Why don’t the acids/base react as soon as
they are mixed in the cup?6) Why is so much sugar needed?7) *Write a word equation for the reaction8) **Write a chemical equation for the reaction9) ***Write a balanced chemical equation
Sherbet Questions
1) .2) They are weak acids3) The acids and base react, fizzing4) Neutralisation5) When dry they can’t react, need water (from saliva)6) Because the acids taste sour7) Citric Acid + Tartaric Acid + Baking Soda Carbon
Dioxide + Water + Sodium citrate + Sodium tartate
8) C6H8O7 + ??? + H2O + NaHCO3 CO2 + H2O + Na??? + Na ???
9) .