Topic 4 - chemical changes (topics from Jan assessments) Flashcards
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What is the range of the PH scale?
0-14
What is the ph scale a measure of?
How acidity or alkalinity
The lower the ph of a solution …
the more acidic it is
The higher the ph of a solution…
the more alkaline it is
A substance of ph7 is …
neutral
What is the ph of a neutral substance?
7
What two things can be used to measure ph?
- universal indicator
- ph probe
What is the ph of aqueous solutions of acids?
ph values less than 7
What is the ph of aqueous solutions of alkalis?
- ph values greater than 7
What happens in a neutralisation reaction between an acid and an alkali?
hydrogen ions react with hydroxide ions to produce water
What ion is contained in aqueous solutions of alkalis?
hydroxide ions
What do acids produce in aqueous solutions?
hydrogen ions
What is an indicator?
a dye that changes colour depending on whether it is above or below a certain ph?
Why do some indicators contain a mixture of dyes?
So they gradually change colour over a broad range of ph
What is the name for indicators that contain a mixture of dyes?
What are these useful for?
a wide range indicator
they are useful for estimating the ph of a solution
Give an example of a wide range indicator?
universal indicator
How is a ph probe used?
How is it more useful than an indicator?
- A ph probe attached to a ph meter can be used to measure ph electronically
- the probe is placed into the solution you are measuring and the ph is given as a numerical value on a digital display - meaning it is more useful than an indicator
Acids and bases ………….. each other?
neutralise
What neutalises an acid?
A base
What neutralises a base?
an acid
What is an acid? -2
an acid is a substance that forms aqueous solutions of a pH less than 7
Acids form H+ (hydrogen) ions in water
- they are also proton donors
What is a base?
a base is a subsance with a pH greater than 7
What is an alkali?-2`
a soluble base that dissolves in waterto form a soution of a pH hreater than 7
for OH- (hydroxide) ions in water
What is the ph of the products of a neutralisation reaction?
when an acid and base neutralise each other, the producst are neutral (pH 7)
- an indicator can be used to show a neutralisation rection is over
What can the neutralisation of strong acids and alkalis be used to calculate?
the concentration of an acid or alkali by titration
Some metal oxides and metal hydroxides react with water?
What are these soluble compounds called?
alkalis (also bases)
Will bases that don’t dissolve in water (insoluble) still take part in neutralisaton reactions with acids?
they will
When reacting a metal oxide/hydroxide, what two things determine the salt that is produced?
- the acid
- the metal ion in the oxide or hydroxide
What do you get when you react an acid with a metal oxide?
a salt + water
What do you get when you react an acid with a metal hydroxide?
a salt + water
What do you get when you react an acid with a metal carbonate?
a salt +water + carbon dioxide
What are metal carbonates?
What do they produce when they react wth an acid?
bases
a salt + water + carbon dioxide
What are metal oxides and metal hydroxides that dissolve in water?
What are metal carbonates?
- alkalis (also bases)
- bases
How can soluble salts be made from acids?
By reacting then with solid insoluble substances (such as metals, metal oxides, hydroxides or carbonates)
How can you produce solid salts from salt solutions?
they can be crystallised
What do acids do in aqueous solutions?
- they ionise in aqueous solutions (they produce hydrogen ions H+)
- they produce protons in water (a hydrogen ion is just a proton)
What is a hydrogen ion, H+?
a proton
Why is hydrochloric acidic but hydrogen chloride gas isn’t?
acids don’t produce hydrogen ions until they meet water
Give 3 examples of strong acids?
- sulfuric acid
- hydrochloric acid
- nitric acid
What are strong acids?
acids that ionise completely in water
(all acid particles dissociate to release H+ ions)
What happens to strong acids in water?
- they ionise completely
- all acid particles dissociate to release H+ ions
What are 3 examples of weak acids?
- ethanoic acid
- citric acid
- carbonic acid
What are weak acids?
- acids that do not fully ionise in solution
- only a small proportion of acid particles dissociate to release H+ ions
What happens to weak acids in solution (water)?
- it does not fully ionise
- only a small proportion of acid particles dissociate to release H+ ions
What do reactions of acids involve/ what will always happen?
the H+ (hydrogen ions) react with other substances
What do stronger acids have a higher concentration of?
H+, hydrogen ions
Explain why a strong acids will be more reactive than a weak acid of the same concentration?
- the concentration of H+ ions is higher
- the rate of reaction will be faster
(reactions of acids involve the H+ ions reacting with other substances)
What is the pH of an acid or alkali a measure of?
the concentration of H+ ions in the solution
What happens to the concentration of H+ ions in a solution for every decrease of 1 on the ph scale?
concentration of H+ ions increases by a factor of 10
How does the concentration of H+ ions compare for an acis of pH4 and an acid of pH5?
the acid of pH4 has 10 times the H+ ions than an acid of pH5
For a decrease of 2 on the ph scale, what does the concentration of H+ ions increase by?
a factor of 100
When at the same concentration, how will the pH of a strong acid compare to that of a weak acid?
- the pH of a strong acid is always less than that of a weaker acid if they have the same concentration
What does acid strength tell you?
(i.e. strong or weak) tells you what proportion of the acid molecules ionise in water
REMEMBER:
The concentration of an acid is NOT the same as its strength
What is the concentration of an acid?
concentration measures how much acid there is for a certain volume of water (basically how watered down your acid is)
Remember:
concentration describes the total number of dissolved acid molecules- not the number of molecules that are ionised to produce hydrogen ions at any given moment
Increasing the amount of acid in a certain volume of liquid will…
increase the concentration of the acid
REMEMBER:
- you can have a dilute but strong acid
- or a concentrated but weak acid
;)
How does pH of an acid change with increasing acid concentration?
pH will decrease with increasing acid concentration regardless of whether it’s a strong or weak acid
