Characteristics of Acids and Bases Flashcards
What pH values do acids have? What happens when they are added to a base
Acids have pH values of below 7, have a sour taste (when edible) and are corrosive
Acids are substances that can neutralise a base, forming a salt and water
Acids can be ….. or …..
Dilute or concentrated
Why must acids be handled carefully?
They must be handled carefully, especially the concentrated solutions because they are corrosive, they can eat metals away, skin and cloth
What are the acids that are not corrosive, even when they’re concentrated called?
Weak acids
What do acids form when added to water?
When acids are added to water, they form positively charged hydrogen ions (H+)
The presence of H+ ions is what makes a solution acidic
4 examples of common acids
Hydrochloric acid, Sulfuric acid, Nitric acid, Ethanoic acid
HCl (aq) → H+ (aq) + Cl- (aq)
What colour does blue litmus paper turn in an acidic solution?
Red
Only which metals in the reactivity series react with dilute acids?
Only metals above hydrogen in the reactivity series will react with dilute acids.
What products are formed when acids react with metals?
When acids react with metals they form a salt and hydrogen gas:
acid + metal → salt + hydrogen
Colour of purple litmus dye/ paper in an acid
Red
How does an alkali solution affect litmus?
Alkali turns litmus blue
What happens when an acid reacts with a base?
Metal oxides and metal hydroxides (alkalis) can act as bases
When they react with acid, a neutralisation reaction occurs
In all acid-base neutralisation reactions, salt and water are produced
acid + base → salt + water
Why do acids conduct electricity?
They contain ions, which makes them conduct electricity. This can be measured with a conductivity meter
Acids fall into two groups, how is this related to their conductivity?
The first group has high conductivity and low pH (strong acids). The second group does not conduct electricity well, it has a higher pH, these are weak acids
Reaction between acids and metal carbonates
Acids will react with metal carbonates to form the corresponding metal salt, carbon dioxide and water:
acid + metal carbonate → salt + carbon dioxide + water
Where is litmus extracted from?
Many plants contain substances that can act as indicators and the most common one is litmus which is extracted from lichens
What colour is phenolphthalein in acid?
Colourless
What colour is phenolphthalein in alkali solutions?
Pink
What colour is methyl orange in acid?
Red
What colour is methyl orange in alkali soloutions ?
Yellow
Name 3 neutral substances
Many substances are not acids nor alkalis. They are neutral.
- Pure water
- Aqueous solutions of NaCl
- Sugar
Name 4 common alkalis
Pure alkalis are solid except for ammonia which is gas
- Sodium hydroxide NaOH (aq)
- Potassium hydroxide KOH (aq)
- Calcium hydroxide Ca(OH)2 (aq)
(lime water)
- Ammonia NH3 (aq)
These are used in the lab as aqueous solutions.
How many numbers are on the pH scale?
14,
- An acidic solution has pH less than 7
- An alkali solution has a pH greater than 7
- A neutral solution has a pH of exactly 7
What happens to the molecules in a strong acid?
All molecules become ions in strong acids, acids conduct much better electricity
What happens to molecules in weak acids?
Only some molecules become ions
What are synthetic indicators?
Synthetic indicators are organic compounds that are sensitive to changes in acidity and appear different colours in acids and alkalis
Thymolphthalein and methyl orange are synthetic indicators frequently used in acid-alkali titrations.
Synthetic indicators are used to show the endpoint in titrations as they have a very sharp change of colour when an acid has been neutralised by alkali and vice-versa
pH of a base
Bases have pH values of above 7
A base which is water-soluble is referred to as an alkali
Bases are usually oxides or hydroxides of…..
Bases are usually oxides or hydroxides of metals
What happens when an alkali is added to water?
When alkalis are added to water, they form negative hydroxide ions (OH–)
The presence of the OH- ions is what makes the aqueous solution an alkali
Sodium hydroxide
NaOH (s) → Na+ (aq) + OH- (aq)
What happens when bases react with acids?
When bases react with an acid, a neutralisation reaction occurs
Acids and bases react together in a neutralisation reaction and produce a salt and water:
Acid + base → salt + water
Hydrochloric acid gives…
chlorides
Sulfuric acid gives…
Sulfates
Nitric acid gives…
Nitrates
What happens to ammonium salts when they are warmed with an alkali?
Ammonium salts undergo decomposition when warmed with an alkali
What is formed when a base reacts with ammonium salt?
Ammonia gas is formed
Why can ammonia be easily displaces from the salt by another alkali?
Even though ammonia is itself a weak base, it is very volatile and can easily be displaced from the salt by another alkali
A salt, water and ammonia are produced
For example:
NH4Cl + NaOH → NaCl + H2O + NH3
Describe the test for the presence of ammonia
Alkali is added to the substance with gentle warming followed by the test for ammonia gas using damp red litmus paper
The damp litmus paper will turn from red to blue if ammonia is present
Why is the reaction of acids with metals redox?
Because electrons are transferred
Why are neutralisation reactions not redox?
In neutralisation reactions electrons are not transferred. Elements dont chnage oxidation state
Are acids or bases proton donors or acceptors?
Acids are proton donors, bases are proton acceptors
Ionic equations can show what is going on in a neutralisation reaction. Describe how to write one
- Write all ions present in the equation
- Cross out ions that appear unchanged on both sides
(The crossed out ions are present in the solution but do not take part in the reaction. So they are called spectator ions) - what is left is the ionic equation
During neutralisation reactions the H+ ion combines with OH- ion to produce a water molecule
How can an insoluble base such as magnesium oxide react to form a salt?
Magnesium oxide is a lattice of mg and o ions. It is insoluble in water, but when HCL is added, the acid donates protons to the oxide ions. The oxide accepts them forming water molecules, so the lattice breaks down. The mg ions joins the CL ions in the solution. If the water is evaporated, Magnesium Chloride can be obtained
What is a neutralisation reaction?
Acids are a source of hydrogen ions, H+
Bases (or alkalis) are a sources of hydroxide ions, OH–
When they react together in a neutralisation reaction, the H+ ions react with the OH– ions to produce water
This is the net ionic equation of all acid-base neutralisations and is what leads to a neutral solution, since water has a pH of 7:
H+ (aq) + OH– (aq)⟶ H2O (
pH range of strong and weak acids
The lower the pH then the more acidic the solution is:
pH 0-2 = strong acid
Extremely acidic substances can have values of below 1
pH 3-6 = weak acid
pH range of strong and weak alkali
The lower the pH then the more acidic the solution is:
pH 0-2 = strong acid
Extremely acidic substances can have values of below 1
pH 3-6 = weak acid
What is the relationship between the amount of hydrogen/hydroxide ions and the strength of the acid/base?
The more hydrogen ions the stronger the acid, but the lower the pH
The higher the concentration of hydroxide ions in a solution the higher the pH
So pH is a measure of the concentration of H+ ions in solution, but they have an inverse relationship
Each change of 1 on the scale represents a change of concentration by a factor of how much?
The pH scale is logarithmic, meaning that each change of 1 on the scale represents a change in concentration by a factor of 10
What happens to the equilibrium in weak acids?
For weak acids, there is usually an equilibrium set-up between the molecules and their ions once they have been added to water
Example of a weak acid: propanoic acid
CH3CH2COOH ⇌ H+ + CH3CH2COO-
The equilibrium lies to the left, indicating a high concentration of intact acid molecules, with a low concentration of H+ ions in the solution
What are oxides?
Oxides are compounds made from one or more atoms of oxygen combined with one other element
Metal oxides act as bases
When are acidic oxides formed? What happens when they react with a base?
Products when they dissolve in water.
Common examples
Acidic oxides are formed when a non-metal element combines with oxygen
They react with bases to form a salt and water
When dissolved in water they produce an acidic solution with a low pH
Common examples include CO2, SO2, NO2 and SiO2
When are basic oxides formed? What happens when they react with an acid?
Products when they dissolve in water.
Common examples
Basic oxides are formed when a metal element combines with oxygen
They react with acids to form a salt and water
When dissolved in water they produce a basic solution with a high pH
Common examples include CuO and CaO
Why are neutral oxides “neutral”?
Some oxides do not react with either acids or bases and thus are said to be neutral
Examples include N2O, NO and CO
What are amphoteric oxides? what is formed when they react with an acid/alkali?
Common examples of amphoteric oxides
Amphoteric oxides can behave both as a base and an acid, depending on whether the other substance is acidic or alkali. In both cases salt and water are formed.
Two of the most common amphoteric oxides are zinc oxide, ZnO and aluminium oxide, Al2O3
The hydroxides of both of these elements also behave amphoterically
Amphoteric oxide examples
Example of aluminium oxide behaving as a base:
Al2O3 + 6HCl → 2AlCl3 + 3H2O
Example of aluminium oxide behaving as an acid:
Al2O3 + 2NaOH → 2NaAlO2 + H2O
