Chemistry topic 4 Flashcards
(25 cards)
What is the PH scale?
Is a measure of how acidic or alkaline a solution is, the lower the ph the more acidic, the higher the ph the more alkaline. A neutral substance (pure water) has ph7.
How do you measure the PH of a solution?
Indicator is a dye which changes colour dependant on its PH. A PH probe, attached to a PH meter, will measure its ph electronically and will display it as a numerical indicator so it’s more accurate.
What is the reaction called between acids and bases?
What’s the equation?
A NEUTRALISATION REACTION.
An acid is a substance which forms an aqueous solution and forms H+ ions in water, ph of less than 7. A base has a ph of over 7 and forms OH- ions in water.
Acid + base = salt + water
H+ + OH- = H2O
What happens to acids in water?
Acids produce protons in water! They ionise in aqueous solutions and produce H+ ions - hydrogen ions (protons).
Explain why acids can be strong or weak:
- strong acids such as hydrochloric, ionise completely in water. (Forming H+ ions)
- weak acids such as citric don’t fully ionise in solutions.
- ionisation of a weak acids is a reversible reaction, which sets up an equilibrium between the undissociated and the dissociated.
- the reactions of acids involve H+ ions reacting with other substances (I’d the conc is higher the rate of reaction will be more reactive, so strong acids will be more reactive than weak in the same conc).
Ph is a measure of of the concentration of hydrogen ions, explain Why:
- the ph of an acid or alkali is the measure of conc of H+ ions in the solution.
- for every decrease of 1 on the PH scale, the conc of H+ ions increases by a factor of 10.
- the ph of a strong acid is always less than a weak one even if they have the same concentration.
(Factor H+ ion conc changes by = 10 to the power of -x)
What’s the difference between strong acids and concentrated acids?
1) acid strength tells you what proportion of acid molecules ionise in water.
2) the concentration measures the amount of acid per volume of water.
3) the larger the amount of acid in a vol of liquid, the more conc it is.
4) you can have a dilute strong acid or a concentrated weak acid.
5) PH will decrease with increasing acid conc regardless of the acids strength.
Metal oxides and hydroxides are bases EXPLAIN:
What are the equations?
- some metal oxides and hydroxides dissolve in water, these soluble compounds are called alkalis.
- bases that won’t dissolve in water still take part in neutralisation.
- all metal oxides and hydroxides react with acids to form salt and water.
-ACID + METAL OXIDE = SALT + WATER
ACID + METAL HYDROXIDE = SALT + WATER
What is a metal carbonate and the equation?
Metal carbonates are also bases, they react with acids to produce a salt, water and carbon dioxide.
ACID + METAL CARBONATE = SALT + WATER + CARBON DIOXIDE
What is the method for making soluble salts with a soluble base?
- Choose the right acid and insoluble base such as an insoluble metal oxide, hydroxide or carbonate.
- Gently warm dilute acid with a Bunsen burner.
- Add the insoluble base a little at a time until no more reacts (excess will sink).
- Filter out the excess solid to get the salt solution.
- To get pure salt crystals of salt, gently heat solution using a water bath or electric heater to evaporate some of the water. Crystals will form, should be filtered then dried. (Crystallisation).
What is the reactivity series?
(Mnemonic)
Penguins (potassium)
Sometimes (sodium)
Like (lithium)
Cuddling (calcium)
Monk seals (magnesium)
Amongst (Aliminium)
Comfy (carbon)
Zooplankton (zinc)
In (iron)
The (tin)
Lonely (lead)
Happy (hydrogen)
Crispy (copper)
Sea (silver)
Grass (gold)
Plains (platinum)
What does the reactivity series do?
- lists metals in order of their reactivity.
- the higher up the reactivity series, the easier it is to form positive ions (lost electrons)
- the higher a metal is the more easily it reacts with water or acid (more electrons)
What happens when a metal reacts with an acid? (Include equation)
ACID + METAL = SALT + HYDROGEN
1) speed of reaction is indicated by the rate at which the bubbles of hydrogen are given off.
2) the more reactive the metal, the faster the reaction will be.
3) you can also investigate the reactivity of metals by measuring the temp change of the reaction. If you use the same mass and surface area of metal each time, then the more reactive the metal, the greater the temperature change.
What happens when a metal reacts with water?
Metal + water = metal hydroxide + hydrogen
What is the type of reaction where metals are separated from their oxides?
When a metal reacts with oxygen it gains oxygen to become an oxide (oxidation). When a metal is separated from its oxide, it’s called a reduction reaction.
FORMATION OF METAL ORE = GAIN OF OXYGEN
REDUCTION = LOSS OF OXYGEN
How can a metal be extracted by carbon?
- some metals can be extracted from their ores chemically by reeducation using carbon, to become oxidised.
- the position of the metal in the reactivity series depends whether it can forgo this reaction.
(If it’s above carbon it needs to be extracted through electrolysis, if below it can forgo the reaction). - some metals, such as gold, are so unreactive that they remain in their pure form.
EXPLAIN redox reactions when electrons are transferred.
- a loss of electrons is called oxidation, but a gain of electrons is called reduction.
- OIL RIG (oxidation is loss, reduction is gain).
- these both happen at the same time (hence REDOX)
EXPLAIN how displacement reactions are also known as redox?
1) displacement reactions, involve one metal, kicking, another one out of a compound.
2) if you put a reactive metal into the solution of a dissolved metal compound, the reactive metal replace the less reactive metal in the compound.
3) in displacement reactions, it’s always the metal ion that gains electrons and is reduced. The metal atom always loses electrons and is oxidised.
What is an ionic equation?
In the ionic equation, only the particles that react in the products a form as shown. If you write out the equation, so you can see the irons, you’ll see some ions don’t change in the reaction (these are called spectator ions). The ionic equation for displacement reaction, just concentrates on the substances, which are oxidised and reduced.
What is electrolysis?
1) During electrolysis, an electric current is passed through an electrode. The ions move towards the electrodes, where they react in the compound decomposes.
2) the positive irons in the electrolyte will move towards the cathode (- ve) and gain electrons (reduced)
3) the negative ions in the electrode will move towards the anode (+ ve) and lose electrons (oxidised)
4) this creates a flow of charge through the electrolyte as ions travel to the electrodes.
5) as ions, gain or lose electrons, they form the uncharged element and are discharged from the electrolyte.
What is electrolysis of molten ionic solids and what does it form?
1) an ionic solid, can’t be electrolyse because the ions are in fixed positions and can’t move.
2) molten ionic compounds can be electrolysed, because the ions can move freely and conduct electricity.
3) molten, ionic liquids, are always broken up into their elements.
4) positive metal ions are reduced to the element at the cathode.
5) negative nonmetal ions oxidised to the element at the anode.
Explain how metals can be extracted from that was using electrolysis?
Example below:
1) aluminium is extracted from the ore bauxite by using electrolysis.
2) aluminium oxide has a very high melting temperature, so it’s mixed with cryolite to lower the melting point.
3) the molten mixture contains free ions, so will conduct electricity.
4) the positive ions are attracted to the negative electrode where they each pick up three electrons, and turn into neutral aluminium atoms where they sink to the bottom of the electrolysis tank.
5) the negative ions are attracted to the positive electrode where they each lose two electrons, the neutral oxygen atoms within combine to form oxygen molecules.
How are ions discharge through electrolysis from aqueous solutions?
1) in aqueous solutions, as well as the ions from the ionic compound that will be hydrogen ions (H+) and hydroxide ions (OH-) from the water.
2) at the cathode, if the (H+) ions and metal ions are present then hydrogen gas will be produced if the metal ions form an elemental metal that is more reactive than hydrogen. If the metal ions form elemental metal that is less reactive than hydrogen, a solid layer of pure metal will be produced instead.
3) at the anode, if the (OH-) and Halide ions are present. Molecules of chlorine, bromine, or iodine will be formed. If no Halide ions are present in the (OH-) ions are discharged and oxygen will be formed.
What is a half equation?
Half equations, show the reactions at the electrodes the important thing to remember when you combining half equations is that the number of electrons needs to be the same for each half equation. For the electrolysis of an aqueous, NaCl, the half equations are:
Neg electrode: 2H+ + 2e- = H2
Pos electrode: 2Cl- = cl2 + 2e-
Ionic equation will be: 2H+ + 2Cl- = H2 + Cl2
