Chemical changes Flashcards
What do metals react with oxygen to produce?
Metal oxides.
What happens in an oxidation reaction?
A substance (metal) gains oxygen.
Oxidation reaction word equation
Metal + Oxygen → Metal Oxide
What are oxides often?
The ores that metals need to be extracted from.
What is a reaction that separates a metal from its oxide called?
A reduction reaction.
Reduction of a Metal Oxide using carbon equation
Metal Oxide + Carbon → Pure Metal + Carbon Dioxide
What is reduction?
-The loss of oxygen
-The gaining of electrons
What is oxidation?
-The gaining of oxygen
-The loss of electrons
Give the symbol equation for the oxidation of magnesium oxide.
2Mg + O₂ → 2MgO
2Mg + O₂ → 2MgO
What happens to magnesium in this reaction?
It is oxidised to make magnesium oxide.
Give the symbol equation for the reduction of copper oxide.
2CuO + C → 2Cu + CO₂
How are unreactive metals such as gold found in the Earth?
As a pure metal
How are most metals found?
As compounds that require chemical reactions to extract the metal.
How can metals less reactive than carbon be extracted from their oxides?
Reduction with carbon.
Why is reduction with carbon a redox reaction?
-The ore is reduced as oxygen is removed from it
-Carbon gains oxygen so is oxidised
Why do metals more reactive than carbon have to be extracted using electrolysis?
Carbon is less reactive than the metals so cannot displace them from their oxides.
How is iron oxide reduced?
In a blast furnace to make iron.
What are ionic equations?
Equations showing only the substances being oxidised or reduced (losing or gaining electrons).
What must ionic equations include?
State symbols!
What do we call ions that don’t change in an equation?
Spectator ions.
What is reduction with carbon a special kind of?
Displacement reaction.
What are species (chemistry)
A set of molecular entities which are chemically identical
(basically if an atom is identical to another atom, they are defined as being the same chemical species).
What is the reactivity of a metal related to?
Its tendency to lose electrons and form positive ions.
What does the reactivity series show?
Metals, arranged in order of their reactivity (with other substances).
Are metals higher up the reactivity series more or less likely to form positive ions?
More.
What happens in a displacement reaction?
A more reactive metal displaces a less reactive one from a compound.
Reactivity series
Potassium
Sodium
Lithium
Calcium
Magnesium
Aluminium
(Carbon)
Zinc
Iron
Tin
Lead
(Hydrogen)
Copper
Silver
Gold
Platinum
How can you can get the order of the reactivity series?
Comparing the relative reactivity of different metals with either an acid or water and putting them in the order of most to least reactive.
Why are hydrogen and carbon in brackets on the reactivity series?
They are non-metals.
Reaction of a metal with an acid word equation
Metal + Acid → Salt + Hydrogen
What state is the hydrogen produced from the reaction of a metal with an acid in?
Gas
What is the speed of reaction of a metal with an acid indicated by?
The rate at which the bubbles of hydrogen gas are given off.
The more reactive the metal, the ______ the reaction will go.
faster
Which metals react explosively when reacted with an acid?
Very reactive ones - potassium, sodium, lithium, calcium.
In general, what won’t copper react with?
Cold, dilute acids.
Which metals react less violently with acids?
Less reactive ones - magnesium, zinc, iron.
How can we make the reaction of a metal with an acid stronger?
Heating.
What is the fancy word for lots of bubbles in a liquid?
Effervescence
How can we investigate the reactivity of metals?
-Look for vigorous bubbles
-Measure the temperature change of the reaction with acid or water over a set time period
In a reaction of an acid with a metal, there will be a greater temperature change for…
…a more reactive metal.
Controls in investigating reactivity experiment?
-Mass of metal
-Surface area of metal
-Initial temperature of acid/water
(-Room temperature)
How is the reaction of a metal with an acid a redox reaction?
The metal atoms lose electrons
-The hydrogen ions in the acid gain electrons.
Reaction of a metal with water word equation?
Metal + Water → metal hydroxide + Hydrogen (g)
What is the symbol equation for the reaction of calcium with water?
Ca (s) + 2H₂O (l) → Ca(OH)₂ (aq) + H₂
The substance becoming part of a compound is…
…oxidised as it loses electrons to become part of the compound.
The substance becoming separate from the compound, becomes an atom and is…
…reduced as it gains electrons from the element joining the compound.
What do elements not in a compound exist as?
Atoms (or molecules)
Which metals will react with water?
Potassium, sodium, lithium, calcium.
Which metals won’t react with water?
Zinc, iron, copper.
What do elements in a compound exist as?
Ions (+ for metals)
What is the symbol equation for the reaction of magnesium and hydrochloric acid?
Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)
What is the symbol equation for the reaction of magnesium and sulfuric acid?
Mg (s) + H₂SO₄ (aq) → MgSO₄ (aq) + H₂ (g)
What is the symbol equation for the reaction of zinc and hydrochloric acid?
Zn (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂ (g)
What is the symbol equation for the reaction of zinc and sulfuric acid?
Zn (s) + H₂SO₄ (aq) → ZnSO₄ (aq) + H₂ (g)
What is the symbol equation for the reaction of iron and hydrochloric acid?
Fe (s) + 2HCl (aq) → FeCl₂ (aq) + H₂ (g)
What is the symbol equation for the reaction of iron and sulfuric acid?
Fe (s) + H₂SO₄ (aq) → FeSO₄ (aq) + H₂ (g)
What can acids be neutralised by?
-alkalis (e.g. soluble metal hydroxides)
-bases (e.g. insoluble metal hydroxides and metal oxides)
What does the particular salt produced in any reaction between an acid and a base/alkali depend on?
-The acid used
-The positive ions in the base, alkali, or carbonate
What type of salt does hydrochloric acid produce?
Chlorides
What type of salt does nitric acid produce?
Nitrates
What type of salt does sulfuric acid produce?
Sulfates
Pure metals are in the _____ state
solid
Compounds are in the _______ state
aqueous
Acid + metal carbonate → what products? (neutralisation rection)
Salt + Water + Carbon Dioxide
Acid + metal hydroxide → what products? (neutralisation rection)
Salt + Water
Acid + metal oxide → what products? (neutralisation rection)
Salt + Water
Neutralisation reaction ionic equation
H+ (aq) + OH- (aq) → H₂O (l)
What are the rules for naming salts?
-The first name of the salt will always be the metal
-The second name of the salt will depend on the acid used
What MUSTMUSTMUSTTT we do when writing symbol equations?
balance them!
Common acids
HCl with H+ ion and Cl- ion (hydrochloric acid)
H₂SO₄ with 2H+ions and an SO₄- ion (sulfuric acid)
HNO₃ with H+ ion and NO₃- ion (nitric acid)
Common bases
NaOH - sodium hydroxide
CaCO₃ - calcium carbonate
Carbonate ion
CO₃ ²⁻
Hydroxide ion
OH-
How can soluble salts be made from acids?
By reacting them with solid substances.
These include metals, metal oxides, metal hydroxides, and metal carbonates.
This solid is added to the acid until no more reacts and the excess solid is filtered off to produce a solution of the salt.
Salt solutions can be _________ to produce solid salts.
crystallised
What is an acid?
A chemical substance that dissociates into hydrogen ions (H+, a proton) when dissolved in water/ in aqueous solutions.
What is the pH of aqueous solutions of acids?
Less than 7.
What is an alkali?
A substance that produces hydroxide ions when dissolved in water (a soluble base)
What is the pH of aqueous solutions of alkalis?
Over 7
What is the pH scale?
A measure of the acidity or alkalinity of a solution.
From 0 to 14, 0 being the most acidic.
How can we measure the pH of a solution?
-Using a chemical indicator
-Using a pH probe
What is a chemical indicator?
A dye that will change colour based on the pH of substance it’s mixed with, usually referred to as an Indicator solution.
How is a pH probe used to measure the pH of a solution?
The probe is connected to a meter that will give a numerical reading showing pH when probe is placed in a solution.
Universal indicator
Made up of several different indicator solutions.
This means that it undergoes a smooth colour change over a wide range of pH values.
What colour does universal indicator become in acidic solutions?
Red/Orange
What colour does universal indicator become in alkaline solutions?
Blue/Purple
What colour does universal indicator become in neutral solutions?
Yellow/Green
Why might using a probe be more reliable than using an indicator?
-Determining the colour of an indicator is subjective (requires human judgement so less accurate).
-More accurate results
What colours does Litmus turn?
Blue in alkaline solutions and red in acidic solutions.
What colours does Phenolphthalein turn?
Pink in alkaline solutions and colourless in acidic solutions.
What colour does Methyl Orange turn?
Yellow in alkaline solutions and red in acidic solutions.
Symbol equation for any neutralisation reaction.
H⁺ (aq) + OH⁻ (aq) → H₂O (l)
How can we reduce the risk of harm from acids burns?
Wear gloves
What happens in a neutralisation reaction?
In neutralisations between an acid and an alkali, hydrogen ions in the acid react with hydroxide ions in the alkali to produce water.
A solution with pH 7 is…?
neutral.
How can the volumes of acid and alkali solutions that react with each other be measured?
By titration using a suitable indicator.
How do we convert from mol/dm³ to g/dm³?
Multiply by the Mr
How do we convert from g/dm³ to mol/dm³
Divide by the Mr
Hydroxide vs oxide
Hydroxide and oxide are both chemical compounds that contain oxygen.
However, hydroxide contains one oxygen atom and one hydrogen atom, (OH-) while oxide contains only oxygen atoms (O)
What determines the strength of an acid?
The proportion of acid molecules which dissociate(ionise) into hydrogen ions in water.
What is the difference between a strong and weak acid?
A strong acid is completely ionised in an aqueous solution, whereas weak acids only partially ionise.
Name some strong acids.
-Hydrochloric acid
-Sulfuric acid
-Nitric acid
Name some weak acids
-Ethanoic acid
-Citric acid
-Carbonic acids
When acid molecules are added to water and split apart, what do we say they do?
‘ionise’ or ‘dissociate’ (same thing).
The dissociation of weak acids is a reversible reaction, which means that…
… the products can react together to reform the acid.
For a weak acid, does the position of equilibrium lie to the left or the right?
Left
As the concentration of hydrogen ions in a solution increases, what happens to the pH?
The pH decreases
A solution decreases from pH 6 to pH 5.
By what factor has the concentration of hydrogen ions increased?
10 x
A solution increases from pH 5 to pH 9.
By what factor has the concentration of hydrogen ions decreased?
÷ 10000
At any given concentration of an aqueous solution, will the stronger acid have a higher or lower pH?
Lower, as a higher proportion of the strong acid molecules will have dissociated into hydrogen ions (this is the only factor that determines the pH).
Acid strength vs concentration.
Acid strength refers to the proportion of acid molecules which dissociate into hydrogen ions in water.
Whereas acid concentration refers to the number of moles of acid molecules per unit of volume. (usually dm³).
What does dilute mean?
There is a lesser amount of substance in a given volume of a solution.
What does concentrated mean?
There is a greater amount of substance in a given volume of a solution.
What happens in a Redox reaction?
One substance gains electrons (is reduced), whilst another loses electrons (is oxidised)
What do half equations show?
The gain/loss of electrons for each individual element involved in a (displacement) reaction.
In a half equation, what must the charges on either side of the arrow do?
balance.
The electrodes in an electrolysis cell are normally made of inert carbon. What does the term ‘inert’ mean?
It is unreactive, so will not take place in the reaction.
Practice your…
HALF EQUATIONSSSS
What happens to the ions of an ionic compound when the compound is melted or dissolved in water?
They are free to move around within the liquid or solution and carry charge.
In what form can ionic compounds conduct electricity in?
-Molten
-Aqueous
What is a binary compound?
A chemical compound consisting of exactly two different elements.
What is electrolysis?
Splitting up ionic compounds using electricity.
What happens to metal ions during electrolysis?
They are reduced to metal atoms forming clumps of pure metals.
What is the name for the positive electrode?
Anode
What is the name for the negative electrode?
Cathode
What happens during electrolysis?
Passing an electric current through an electrolyte causes the ions to move to the electrodes.
Ions are discharged at the electrodes producing elements.
In electrolysis, where do positively charged ions move?
To the cathode
In electrolysis, where do negatively charged ions move?
To the anode.
What happens to negative ions at the anode?
They lose electrons (are oxidised).
What happens to positive ions at the cathode?
They gain electrons (are reduced).
Why can’t an ionic solid be electrolysed?
The ions are in a fixed position and can’t move.
As ions travel through the electrolyte to the electrodes, this creates…
…a flow of charge.
What are molten ionic liquids always broken up into in electrolysis?
Their elements.
Half equation for the reduction of lead ions at the cathode.
Pb²⁺ + 2e⁻→ Pb
Half equation for the oxidation of bromine ions at the anode.
2Br⁻ - 2e⁻ → Br₂
Cons of electrolysis
Extracting metals this way is expensive as lots of energy is required to melt the ore and produce the required current .
Cryolite
A substance added to aluminium oxide before electrolysis to reduce the melting point.
Anion
negative ion
Cation
positive ion
Electrolyte
The solution which is being broken down during electrolysis.
Where is aluminium extracted from?
The ore bauxite, which contains aluminium oxide. (Al₂O₃)
Why is electrolysis used to extract aluminium from its ores?
It is more reactive than carbon so carbon will not displace aluminium.
What is the symbol equation for the separation of aluminium from oxygen?
2Al₂O₃ ➔ 4Al + 3O₂
During the electrolysis of molten aluminium oxide, are the O2- ions oxidised or reduced?
The O2- ions each lose 2 electrons, so are oxidised.
Electrolysis of aluminium oxide (steps 1 and 2)
-Aluminium oxide has a very high melting point so is mixed with cryolite, lowering the melting point
-The molten mixture contains free ions so can conduct electricity
Electrolysis of aluminium oxide (steps 3 and 4)
-The positive Al ³⁺ ions are attracted to the cathode, where they gain 3 electrons
- neutral aluminium atoms are formed and discharged
-They then sink to the bottom of the electrolysis tank
Electrolysis of aluminium oxide (steps 5 and 6)
-The negative O²⁻ (oxide) ions are attracted to the positive electrode where they each lose two electrons
-The neutral oxygen atoms will then combine to form O₂ molecules.
What is the half equation for the oxidation of oxygen ions?
2O²⁻ → O₂ + 4e⁻
What is the half equation for the reduction of aluminium ions?
Al³⁺ 3e⁻→ Al
What is the equation for the electrolysis of aluminium oxide?
2Al₂O₃ (l) → 4Al (l) + 3O₂ (g)
When a simple ionic compound (e.g. lead bromide) is electrolysed in the molten state using inert electrodes, what is produced at the cathode?
The metal (in this case, lead).
When a simple ionic compound (e.g. lead bromide) is electrolysed in the molten state using inert electrodes, what is produced at the anode?
The non-metal (in this case, bromine gas)
What is a safer alternative to lead bromide for practical work?
Anhydrous zinc chloride
Anhydrous
Contains no water.
In the electrolysis of a solution, which two ions are present from the water molecules themselves?
H+
OH-
In the electrolysis of aqueous solutions, what is the rule for determining which ion will be oxidised at the anode ?
It will always be the OH-, unless there is a halide ion (such as Cl-, or Br-) present.
If OH- ions are oxidised, what is the product discharged at the anode?
Oxygen gas
What products will form at the anode if there is a halide ion (such as Cl-, or Br-) present?
Molecules of chlorine, bromine, or iodine.
In the electrolysis of aluminium oxide, what is used as the electrodes?
Carbon
Why must the positive electrode be continually replaced in the electrolysis of aluminium oxide?
The oxygen produced reacts with the carbon of the positive electrodes, forming carbon dioxide, so they gradually burn away.
What do the ions discharged when an aqueous solution is electrolysed using inert electrodes depend on?
The relative reactivity of the elements involved.
In the electrolysis of aqueous solutions, what product is formed at the cathode?
Hydrogen, if the metal is more reactive than hydrogen.
The pure metal, if the metal is less reactive than hydrogen.
In the electrolysis of aqueous solutions, what product is formed at the anode?
Oxygen, unless the solution contains halide ions.
Then the halogen is produced.
Why is oxygen formed at the anode? (unless halide ions are present)
In an aqueous solution, water molecules break down producing hydrogen ions and hydroxide ions that are discharged.
At the anode, are the reactions oxidation or reduction?
At the anode, negatively charged ions lose electrons and so the reactions are oxidations.
At the cathode, are the reactions oxidation or reduction?
At the cathode, positively charged ions gain electrons and so the reactions are reductions.
How can the reactions at electrodes be represented?
Half equations.
2H+ + 2e- → what products?
H₂
4OH- - 4e- → what products?
O₂ + 2H₂O
(this is for when a halide isn’t present in an aqueous solution)
What is the first step to working out which products will form at the electrodes in the electrolysis of an aqueous solution?
Write out the ions present in the solution (include H+ and OH-)
How can we test for chlorine gas?
Damp blue litmus paper → bleached white
How can we test for hydrogen gas?
Lit splint → squeaky pop
How can we test for oxygen gas?
It will relight a glowing splint.
When drawing the apparatus for an electrolysis experiment, don’t forget to include…
-A dc power supply
-Wires
-Labels for the anode (on the longer side of the cell symbol) and cathode
What is the equation linking concentration and volume?
Concentration = mass ÷ volume
or
Concentration = moles ÷ volume
More concentrated solutions contain more what per unit of volume?
acid
What does 2 mol/dm³ mean?
2 moles of atoms/molecules per dm³ of solution.
