Chemical reactions Flashcards
general equation for metals + acids
metals + acids –> salt + hydrogen
(hydrogen gas)
Describe, in terms of electrons what is happening in this reaction:
hydrochloric acid + magnesium –> magnesium chloride + hydrogen
IS THIS A REDOX REACTION: EXPLAIN YOUR CHOICE.
The magnesium atoms react with the hydrochloric acid, they lose two electrons and form the magnesium ion Mg 2+ (Mg –> Mg2+ + 2e-)
Therefore magnesium atoms have been oxidised (oxidation is the loss of electrons)
The H+ ions (released) from the acid has gained two electrons to form hydrogen gas (2H+ +2e- —> H2), so it has been reduced
(since the H+ ions are gaining electrons) (reduction is the gain of electrons)
Both oxidation and reduction take place therefore it is a redox reaction
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The H+ ions that are released from the acid, now accept these electrons to form H2 gas
explain the rate of reactions of magnesium, zinc and iron with dilute ion
Magnesium - very rapid reaction (reacts very rapidly with dilute acid)- because it easily forms the positive ion Mg2+
Zinc - quite rapid reaction (reacts quite rapidly with dilute acids) - because it quite easily forms the positive ion Zn2+
Iron - slow reaction - reacts slowly with dilute acid - because it less easliy forms the positive ion Fe2+
iron atoms do not easily form the ions Fe2+
acid + alkali/base —>
acid + alkali —> salt + water
acid + base —-> salt + water
What does a particular salt produced in a reaction between an acid and a base or alkali depends on:
- the acid used (hydrochloric acid produces chlorides, nitric acid
produces nitrates, sulfuric acid produces sulfates) - the positive ions in the base, alkali or carbonate.
what do salts contain
Salts contain a positive ion which comes from the base or alkali
Salts also contain a negative ion which comes from the acid
describe the product of nitric acid + lithium hydroxide
In the alkali lithium hydroxide, the positive ion is the metal lithium
The nitric acid produced nitrates (the negative ion)
The salt is lithium nitrate + water (since it is a neutralisation reaction)
describe the product of sulfuric acid + zinc oxide
In the base zinc oxide, the positive ion is the metal lithium
The sulfuric acid produced sulphates (the negative ion)
The salt is zinc sulfate + water (since it is a neutralisation reaction)
acid + metal carbonate
acid + metal carbonate –> salt + water + carbon dioxide
PRACTICAL:
DESCRIBE HOW TO MAKE A SOLUBLE SALT BY USING AN ACID from an insoluble oxide or carbonate (insoluble base)
Equipment: beaker, gauze, tripod, heatproof mat, Bunsen Burner
1) Start with a fixed volume of dilute sulfuric acid. This is our limiting reactant
(the acid will run out) - we do not want any acid remaining at the end as that will contaminate our salt (make it acidic) So we need to make certain that all of the acid is reacted away
2) Gently heat the acid until it is almost boiling (we dont want it to boil, as it could bubble over when we add our other reactant and that would be dangerous)
3) Use a spatula to add small amounts of copper oxide to the acid.
4) Stir the solution with a glass rod.
5) The copper will react and seem to disappear. (the solution will turn a blue colour - the colour of copper sulfate)
6) Continue adding copper oxide if the solution continues to be clear blue
7) Stop adding copper oxide if some powder remains after stirring
At this point the reaction has stopped. All of the acid has reacted. (we have made copper sulfate, and the solution is now neutral - at the bottom of the beaker, will be excess unreacted copper oxide)
8) Use filtration to remove the unreacted copper oxide - to do this use a filter funnel and filter paper
(in the next step we are going to make crystals of the copper sulfate, but first we need to remove the unreacted copper oxide. Because the copper oxide is insoluble, we can filter it away)
9) Take the copper sulphate solution and place the solution in an evaporating basin. Heat gently over a beaker of boiling water.
10) Heat until around half the solution remains.
Leave the solution for 24 hours in a cool place for the copper sulfate crystals to form
Scrape crystals onto a paper towel and gently pat them dry
Now we have pure, dry crystals of our copper sulphate
explain why you cannot react copper and sulfuric acid acid together to make copper sulfate
Copper is not reactive enough for that reaction to take place
explain why the sulfuric acid is warmed (for practical)
to speed up the
reaction
explain why you have to keep on adding a spatula of copper oxide to the beaker and Stirring the mixture, until there is copper oxide remaining in the beaker
to make sure all the
(hydrochloric) acid reacts
explain why you need to filter the mixture
to remove the excess
copper oxide
explain why it is possible to filter out the copper oxide residue from the copper sulfate solution
Because the copper oxide is insoluble, we can filter it away)
why do you Heat until around half the solution remains.
makes the solution more saturated (the salt should start to form crystals as it becomes insoluble in the cold, highly concentrated solution)
what do acids do in aqueous solutions
In aqueous solutions, acid molecules ionise (split) and release H+ ions (hydrogen ions)
The difference between strong acids and weak acids
Strong acids fully ionise in aqueous solutions (All the acid particles dissociate to release H+ ions) — the react to give arrow, goes in one direction only, telling us that the acid has fully ionised
All molecules break into ions
Weak acids partially ionise in aqueous solutions –
the reversible reaction arrow– shows that only some of the acid molecules ionise but not all of them. — only a fraction of the acid molecules ionise
Only a small fraction of molecules break into ions
Examples of strong acids
Hydrochloric Acid
HCl (aq) —-> H+ (aq) + Cl- (aq)
Sulfuric Acid
H2SO4 (aq) —-> 2H+ (aq) + SO42- (aq)
Nitric Acid
HNO3(aq) —-> H+ (aq) + NO3- (aq)
Write an equation to show how hydrochloric acid ionises in water
(showing hydrochloric acid in aqueous solution)
Hydrochloric Acid
HCl (aq) —-> H+ (aq) + Cl- (aq)
Write an equation to show how sulfuric acid ionises in water
(showing sulfuric acid in aqueous solution)
Sulfuric Acid
H2SO4 (aq) —-> 2H+ (aq) + SO42- (aq)
Write an equation to show how nitric acid ionises in water
(showing nitric acid in aqueous solution)
Nitric Acid
HNO3(aq) —-> H+ (aq) + NO3- (aq)
H+ ions also known as
Protons (since Hydrogen ions only contain a proton)
What is the ionisation of a weak acid
The ionisation of a weak acid is a reversible reaction, which sets up an equilibrium between undissociated and dissociated acid
Since only a few of the acid particles release H+ ions, the position of equilibrium lies well to the left.
Examples of weak acids
Carbonic Acid H2CO3
Ethanoic Acid CH3COOH
Citric Acid
Write an equation to show how carbonic acid ionises in water
(showing nitric acid in aqueous solution)
H2CO3 (aq) ⇌ H+ (aq) + HCO3-(aq)
What does the pH scale show
The pH scale gives us an idea of the concentration of hydrogen ions (H+)
Describe the pH of strong acids and weak acids, for a given concentration
Strong acids have a lower pH than weak acids for a given concentration (of the acid). This is because strong acids fully ionise, producing a greater concentration of hydrogen ions than weak acids (which only partially ionise)
What happens as the pH scale decreases by one unit
As the pH scale decreases by one unit, the concentration of hydrogen ions increases by ten times
Describe the difference of two acids with a pH of 1 and pH 2
The acid at pH 1 has a 10x greater concentration of H+ ions than the acid at pH2
10x difference =
one order of magnitude
Describe the difference of two acids with a pH of 1 and pH 3
The acid at pH 1 has a 100x greater concentration of H+ ions than the acid at pH 3
100x difference =
2 orders of magnitude
What affects the pH of an acid
strength of an acid
concentration of the acid
What does the concentration of acids tell us
The concentration of acid tells us the amount of acid molecules in a given volume of solution
Describe the difference between a dilute acid and a concentrated acid
A dilute acid will have fewer proportion of acid molecules in a given volume than a concentrated acid even if the strength of the acid is the same
dilute - contains a low proportion of acids compared to the total volume of solution
concentrated - contains a high proportion of acid compared to the total volume of solution
Explain why 0.1 mol/dm3 hydrochloric acid has a lower pH than 0.1 mol/dm3 ethanoic acid
The concentration of H+ ions is greater
Explain why 0.1 mol/dm3 HCl reacts faster with magnesium and sodium than 0.1 mol/dm3 ethanoic acid does
Concentration of H+ ions is greater so there are more frequent collisions between the particles
Which method is better for finding the pH of a solution: using a pH probe or using a universal indicator
pH probe - more accurate as it gives a numerical reading
With universal indicator it is difficult to match the colour that the solution has turned (when universal indicator has been added) precisely to the colour chart
Explain why the concentratiokn of the two acids must be kept the same (experiment - investigate how fast strong and weak acids react magnesium and sodium carbonate)
To keep it a fair test. To ensure that the only factor (variable) affecting how fast the different acids react with the magnesium and sodium carbonate, is the pH of the acids (and nothing else e.g. concentration of acid)
state what happens to the H+ ion concentration when the acid is diluted by a factor 10
the h+ concentration decreases by a factor of 10
what happens to the pH of an acid when the solution is diluted by a factor of 10
the pH of the acid increases by 1 pH unit
in aqueous solution, as the concentration of H+ (aq) ions increases by a factor of 10, the pH of a solution decreases by
1 pH unit
If an acid with pH one has a concentration of H+ ions of 0.1, what will the concentration of H+ ions be with acids with pH, 2,3 and 4
pH2 = 0.01
pH 3 = 0.01
pH 4 = 0.01
A solution of 0.01 mol/dm3 hcl has a pH of 2 predict the pH of: 0.1 mol/dm3 of hcl
pH 1
A solution of 0.01 mol/dm3 hcl has a pH of 2 predict the pH of: 0.001 mol/dm3 of hcl
pH 3
