Flashcards in Topic 4: Chemical Changes Deck (46):
The pH scale goes from 0 to
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 neutral substance has pH
What are wide range indicators? Are they useful for estimating the pH of solutions?
Indicators that contain a mixture of dyes meaning that that they gradually change colour over a broad range of pH
e.g. universal indicator
How can the pH of a substance be measured electronically?
By a pH probe attached to a pH meter
The probe is placed in a solution you are measuring and the pH is given on a digital display as a numerical value, meaning it's more accurate than an indicator
A compound that releases hydrogen ions in water and forms aqueous solutions with a pH of less than 7
Any compound that can neutralise an acid (pH bigger than 7)
A soluble base which releases hydroxide ions in water (pH bigger than 7)
Reaction between acids and bases (neutralisation)
acid + base = salt + water
Example reaction of neutralisation
H+ + OH- -> H2O
Hydrogen H+ ions react with hydroxide OH- ions to produce water
When an acid neutralises a base (or vice versa) the products have a pH of 7 meaning it is?
neutral- an indicator can be used to show the neutralisation reaction is over
Neutralisation reactions of strong acids and alkalis can be used to calculate the concentration of alkali or acid by what practical?
Acids ionise in aqueous solutions meaning that they produce
hydrogen ions (H+)
Acids do not produce hydrogen ions until they meet water. Is hydrogen chloride gas acidic?
H+ + Cl-
H+ + NO3-
How do strong acids such as sulfuric, hydrochloric and nitric acids ionise in water?
Completely- all acid particles dissociate to release H+ ions
How do weak acids such as ethanoic, citric and carbonic acids ionise in water?
Not fully- only a small proportion of acid particles dissociate to release H+ ions
The ionisation of a weak acid, unlike a strong acid, is what kind of a reaction?
A reversible reaction- which sets up an equilibrium between the dissociated and dissociated acid.Since only a few of the acid particles release H+ ions, the position of equilibrium lies well to the left.
e.g. CH3COOH ->
Reactions of acids involve the H+ ions reacting with other substances. If the concentration of H+ ions is higher, what happens rate of reaction wise?
The rate of reaction will be faster, so strong acids will be more reactive than weak acids of the same concentration
pH of an acid or alkali is a measure of the concentration of what in a solution?
Concentration of H+ ions
For every decrease of 1 on the pH scale, the concentration of H+ ions increases by a factor of what?
So an acid that has a pH of 4 has 10 times the concentration of H+ ions of an acid that has a pH of 5
For a decrease of 2 on the pH scale, the concentration of H+ ions increases by a factor of x100
The general rule for pH being a measure of the concentration of hydrogen ions:
Factor H+ ion concentration changes by= 10(to the power of -x)
x being the difference in pH: if the pH falls from 7 to 4 the difference is -3 and the factor of H+ ion concentration has increased by 10-(to the power of -3) which equals 10(3)= 1000
If a strong acid and a weak acid had the same concentration which would have a higher pH?
The weak acid
Acid strength (i.e. strong or weak) is...
what proportion of the acid molecules ionise in water
Concentration of an acid measures...
measures how much acid there are in a certain volume of water (NOT the number of molecules that are ionised to produce hydrogen ions at any given moment)
the larger the amount of acid there is in that certain volume, the more concentrated it is
You can have a dilute (not very concentrated) strong acid which could be the same as
a concentrated but weak acid
pH will decrease with what regardless of whether it is a weak or strong acid?
decrease with an increasing pH concentration
Metal oxides and metal hydroxides are
What soluble compounds that are alkalis dissolve in water and react with acids in neutralisation reactions?
Metal oxides and metal hydroxides
Acid + metal oxide ->
salt + water
Acid + metal hydroxide ->
salt + water
Hydrochloric acid + copper oxide ->
2HCl + CuO ->
copper chloride + water
CuCl2 + H2O
The reactivity series lists metals in order of their reactivity towards other substances. If Potassium is above Sodium what is more reactive?
For metals, their reactivity is determined how?
By how easily they lose electrons- forming positive ions. If it reacts more easily with a water or acid it is very reactive
What is the reactivity series?
A comparison of the relative reactivity of different metals with either an acid or water and put in order of most reactive to least reactive
Some metals react with gases to produce a salt and?
Acid + metal -> salt + hydrogen
The speed of reaction is indicated by?
By the rate of which bubbles of hydrogen are given off *(the more reactive the metal, the faster the reaction will go- very reactive metals like potassium, sodium, lithium and calcium react explosively, but less reactive metals like zinc/iron react less violently)
What metal on the reactivity series generally DOES NOT react with cold, dilute acids?
Magnesium reacts vigorously with cold, dilute acids such as HCl or H2SO4 and produces lots of bubbles
Mg(s)+ 2HCl(aq) -> MgCl2(aq)+H2(g)
Mg(s)+ H2SO4(aq) -> MgSO4(aq)+H2(g)
Zinc reacts slowly with dilute acids but more strongly if heated up
Zn(s)+ 2HCl(aq) -> ZnCl2(aq)+H2(g)
Zn(s)+ H2SO4(aq) -> ZnSO4(aq)+H2(g)
By measuring the temperature of metals by measuring the temperature change of the reaction with an acid or water over a set time period what is investigated?
The reactivity of of metals
If you use the same mass and surface area of metal each time, then the more reactive the metal, the greater the temperature change should be
The reaction of metals with water also shows?
The reactivity of metals
Metal and water
Metal + water -> metal hydroxide + hydrogen
Ca(s) +2H2O(l) -> Ca(OH)2(aq) + H2(g)