Chemical changes Flashcards
Explain 2 ways you can measure the pH of a solution.
- An indicator Used to estimate pH.
- A pH probe attached to a pH meter that measures pH electronically. Probe is placed in solution and the pH is given on a digital display.
Why would a pH probe be used over an indicator?
The pH is given on a digital display as a numerical value. This is more accurate.
What is an acid?
A substance that forms H+ ions when dissolved in water.
What is a base?
A substance with a pH over 7.
What is an alkali?
A substance that forms OH- ions in water.
What are bases usually?
A metal oxide.
What is an alkali the same thing as?
A metal hydroxide.
What is the symbol equation for neutralisation of acids and alkalis?
H+ + OH- -> H20
What is a strong acid?
An acid that fully dissociates/ ionises in water.
What is a weak acid?
An acid that only partially dissociates in water.
Give 3 examples of strong acids.
Hydrochloric acid
Sulfuric acid
Nitric acid
Give 3 examples of weak acids
Ethanoic acid
Citric acid
Carbonic acid
What is the word equation for neutralisation involving a base?
Acid + metal oxide -> salt+ water
What is the word equation for neutralisation involving an alkali?
Acid + metal hydroxide -> salt+ water
True or false:
The ionisation of a weak acid is a reversible reaction
True
Are strong or weak acids more reactive? Why?
Strong
because if the concentration of H+ is higher, the rate of reaction will be higher.
What is the relationship between acidity and pH?
Every increase by one pH, the acid gets 10x weaker.
What is the difference between a strong acid and a concentrated acid?
A strong acid is an acid that fully dissociates in water.
A concentrated acid is how much acid is in a certain volume of water. It’s one that has little to no water molecules mixed with the acid molecules meaning the CONCENTRATION OF H+ IONS IS HIGH.
What will be the products when an acid and metal carbonate react?
Salt+ water+ carbon dioxide
List the reactivity series.
Potassium
Sodium
Lithium
Calcium
Magnesium
(carbon)
Zinc
Iron
(hydrogen)
Lead
Copper
Silver
Gold
Do metals lose or gain ions in reactions.
Lose ions.
What would be the products for the reaction of acid and metal?
Salt + hydrogen
How can you tell how reactive a metal is? Explain 2 ways.
React it with an acid. The more reactive the metal is, the faster the rate at which the bubbles of hydrogen gas are given off.
You measure the temperature of the reaction with an acid or water over time. The more reactive the metal, the greater the temperature change.
What products would be formed from the reactions of alkali metal and water?
Metal hydroxide +hydrogen
Which two processes can be used to extract metals from their oxides? When do you which?
Reduction: when metal is below carbon on reactivity series.
Electrolysis: when metal is above carbon in reactivity series.
Name a disadvantage of electrolysis.
It’s expensive.
For metals, what is oxidation?
The gain of oxygen.
For metals, what is reduction?
The loss of oxygen.
What metals are found in the earth as the metal itself?
Unreactive metals like gold.
What is oxidation in terms of electrons?
The loss of electrons.
What is reduction in terms of electrons?
The gain of electrons.
True or false: displacement reactions are redox reactions.
True
What equipment is needed for electrolysis?
.beaker
.electrolyte (a molten or dissolved ionic compound)
.electrodes (cathode and anode)
.battery
.wires
Explain the process of electrolysis?
1) An electric current is passed through an electrolyte. The ions move towards the electrodes, where they react, and the compound decomposes.
2) The positive ions move towards the cathode and gain electrons (they are reduced)
3)The negative ions move to the anode and lose electrons (they are 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 uncharged element and are discharged from the electrolyte.
Why can’t ionic solids be electrolysed?
The ions are in fixed position and can’t move.
When can ionic compounds be electrolysed?
When molten or dissolved because the ions are free to move through the structure and conduct electricity.
Why is carbon used in an electrode?
It’s a good conductor and it’s cheap
Explain the method of electrolysis for extracting metals from their ore. Use aluminium from it’s ore bauxite. Al2O3
1)Aluminium has a very high melting point so it’s mixed to cryolite to lower it.
2) The molten mixture contains free electrons so it’ll conduct electricity.
3)The AL+ ions are attracted to the cathode where they pick up 3 electrons to turn into neutral aluminium atoms then sink to the bottom
4) The O2- ions are attracted to the positive electrode where they lose two electrons.
The neutral oxygen atoms will combine to form O2 MOLECULES.
In electrolysis of a certain aqueous solution, oxygen was produced from water. Explain how.
The water separates in H+ ions and OH- ions. The H+ ions are attracted to the cathode. The OH- ions are attracted to the anode. OH- ions are discharges to form neutral hydrogen atoms and oxygen atoms (the oxygen atoms combine).
Why does the positive electrode have to be replaced?
Oxygen is produced at anode. The anode is made of carbon so it reacts with oxygen to produce carbon dioxide.
In aqueous solutions solutions what is the other equation in electrolysis?
H20 ->(reversible) H+ + OH-
What is the rule for the cathode?
If the element is more reactive than hydrogen, hydrogen will be produced.
If the element is less reactive than hydrogen, a solid layer of the pure metal is produced instead.
What is the rule for the anode?
If the element is a halide, the halogen will be formed. If no halide present, the OH- ions are discharged and oxygen will be formed.
What is titration used for?
TO find out how much acid is needed to neutralise a quantity of alkali or vice versa.
How do you make sure titration experiment is accurate?
. Repeat tests and calculate a mean result
. Use concordant results
Explain how you would use titration to find out the concentration of some alkali?
1)Using a pipette and a pipette filler add a set volume of alkali to conical flask.
2)Add two to three drops of indicator
3)Use a funnel to fill a burette with some acid of known concentration. Record initial volume of acid- do below eye level to avoid getting acid in eye
4)Using burette, add acid to alkali bit at a time giving the conical flask a regular twirl
5)Slow near end and stop when indicator changes colour.
6)Record final volume and use initial volume to calculate volume needed of acid to neutralise.
Why use a burette in titration?
Can add in small increments
.More accurate than a measuring cylinder.
What indicators can be used in titration? What colours are they in acid and alkali.
Methyl orange: acid- red alkali- yellow
Phenolphthalein: acid- colourless alkali-pink
Litmus: acid- red alkali- purple
Explain method of making soluble salts.
1) Gently warm dilute solution using Bunsen burner. Then turn off Bunsen burner.
2)Add the insoluble base to the acid bit at a time until no more reacts (if stirred excess solid will just sink to bottom of flask)
3) Then filter out the excess solid to get the salt solution.
4) To get pure solid crystals, do crystallisation- heat gently using water bath to make it more concentrated then stop heating and leave solution to cool. Filter out solution then leave to dry.
