topic 3 - chemical changes

state what acids are sources of

hydrogen ions

state what alkalis are sources of

hydroxide ions

state the pH of a neutral solution

7

state the general pH of acidic solutions

lower than 7

state the general pH of alkaline solutions

higher than 7

state the effect of acids on litmus indicator

red

state the effect of acids on methyl orange indicator

red

state the effect of acids on phenolphthalein indicator

colourless

state the effect of alkalis on litmus indicator

blue

state the effect of alkalis on methyl orange indicator

yellow

state the effect of alkalis on phenolphthalein indicator

pink

state what effect the concentration of hydrogen ions has on pH

increasing concentration of hydrogen ions decreases pH

state what effect the concentration of hydroxide ions has on pH

increasing concentration of hydroxide ions increases pH

state what effect increasing the concentration of hydrogen ions by factors of 10 has on pH

pH will decrease by 1

explain the method of how to investigate the change in pH of adding calcium hydroxide to hydrochloric acid

1. use a volumetric pipette to measure a fixed volume of dilute hydrochloric acid into a conical flask

2. add 1 spatula of calcium hydroxide into the flask and swirl

3. when the base has reacted, record the pH of the solution using a pH meter

4. repeat for different spatula amounts of calcium hydroxide but the same volume of hydrochloric acid

analyse the expected results from the experiment investigating the change in pH of adding calcium hydroxide to hydrochloric acid

• the graph indicates a sudden change in pH which corresponds to the vertical section of the graph

• this indicates that as the amount of solid base increases

• the higher the pH is

• therefore the base is neutralising the acid

explain what the term dilute means

• dilute solutions contain a small amount of solute

• in a large volume of solution

explain what the term concentrated means

• concentrated solutions contain a large amount of solute

• in a small volume of solution

explain what the term strong acid means

• strong acids dissociate completely in water

• to produce the maximum number of hydrogen ions

explain what the term weak acid means

• weak acids don't fully dissociate in water

• to only produce some hydrogen ions

state what a base is

• a substance that only reacts with an acid

• to form a salt and water only

state what alkalis are

soluble bases

state the general reaction of metals and acids

metal + acid → metal salt + hydrogen

state the general reaction of metal oxides and acids

metal oxide + acid → metal salt + water

state the general reaction of metal hydroxides and acids

metal hydroxide + acid → metal salt + water

state the general reaction of metal carbonates and acids

metal carbonate + acid → metal salt + water + carbon dioxide

describe the chemical test for hydrogen

• place a lighted splint into a test tube containing the gas

• if the gas is hydrogen, a squeaky pop will be produced

describe the chemical test for carbon dioxide

• bubble the unknown gas through limewater

• if a white precipitate is produced, the gas is carbon dioxide

state what a neutralisation reaction is

• a reaction between

• an acid and a base

state what an acid-alkali neutralisation is

• a reaction in which hydrogen ions from the acid

• react with hydroxide ions from the alkali

• to form water

explain why, if soluble salts are prepared from an acid and an insoluble reactant, excess reactant is added

• to ensure all of the acid has reacted

• with the insoluble reactant

explain why, if soluble salts are prepared from an acid and an insoluble reactant, the excess reactant is removed

to yield a solution of only salt and water

explain why, if soluble salts are prepared from an acid and an insoluble reactant, the solution remaining is only salt and water

• as all the acid has reacted

• and the excess insoluble reactant has been removed

• leaving only salt and water

• due to a neutralisation reaction occurring

explain why, if soluble salts are prepared from an acid and a soluble reactant, a titration must be used

• both the acid and reactant are soluble

• so if one is in excess

• it cannot easily be removed

• meaning you need exact volumes of reactants

• which can be done using titration

explain why, if soluble salts are prepared from an acid and a soluble reactant, the acid and soluble reactant are then mixed in correct proportions

so that the solution remaining is only salt and water

explain why, if soluble salts are prepared from an acid and a soluble reactant, the solution remaining is only salt and water

• as a neutralisation reaction has occurred

• leaving only salt and water

state the method of how to investigate the preparation of copper sulfate crystals from copper oxide

1. heat 50cm³ of dilute sulfuric acid in a beaker and warm gently with a bunsen burner

2. add the copper oxide slowly to the dilute acid until the copper oxide is in excess

3. filter the mixture into an evaporating basin to remove the excess base

4. gently heat the solution in a water bath to evaporate the water and saturate the solution

5. check the solution is saturated by dipping a cold glass rod into the solution and see if crystals form on the end

6. leave the filtrate in a warm place to dry and crystallise

7. decant the excess solution and dry the crystals

state the results of the investigation of the preparation of copper sulfate crystals from copper oxide

• bright blue

• regularly shaped

• hydrated copper sulfate crystals

state the method of an acid-alkali titration to create a pure, dry salt

1. add 25cm³ of base to a conical flask using a pipette and a pipette filler

2. add a few drops of phenolphthalein indicator into the conical flask

3. place the conical flask on a white tile

4. setup the burette in the clamp and stand

5. close the tap and use the funnel to pour approximately 10cm³ of acid into the burette

6. place a beaker under the burette and open the tap, allowing the tip of the burette to fill with acid and displace any air bubbles

7. close the tap before the burette empties

8. use a funnel to fill the burette with acid

9. record the initial volume of acid to the nearest 0.05cm³

10. place the burette above the conical flask and carry out a rough titration, swirling the flask constantly

11. close the burette tap as soon as the solution decolourises

12. record the final volume of the acid

13. use the rough titre as a guide and repeat the titration until two concordant titres are obtained

14. use the concordant results to calculate a mean titre

15. pour the solution in the conical flask into an evaporating basin and heat over a bunsen burner

16. heat the solution until the water evaporates

17. leave the crystals in the basin to dry at room temperature

state which sodium salts are soluble

all

state which potassium salts are soluble

all

state which ammonium salts are soluble

all

state which nitrates are soluble

all

state which chlorides are insoluble

• silver

• lead

state which sulfates are insoluble

• lead

• barium

• calcium

state which carbonates are soluble

• sodium

• potassium

• ammonium

state the method to prepare a pure, dry insoluble salt

1. measure out 25cm³ of one soluble salt into a beaker

2. measure out 25cm³ of another soluble salt into the beaker

3. add water to the beaker and stir with a glass rod

4. filter to remove the precipitate from the mixture

5. wash the filtrate with distilled water

6. leave in an oven to dry

state what electrolytes are in molten state

ionic compounds

state what electrolytes are when dissolved in water

ionic compounds

state what electrolysis is

• a process in which

• electrical energy from a direct current supply

• decomposes electrolytes

explain the movement of cations during electrolysis

• positively charged cations migrate

• to the negatively charged cathode

explain the movement of anions during electrolysis

• negatively charged anions migrate

• to the positively charged anode

explain the formation of copper chloride solution using electrolysis

1. copper is below hydrogen in the reactivity series

2. so copper cations are preferentially discharged at the cathode

3. chlorine is a halogen, so preferentially discharged at the anode

state what product is formed on the cathode when copper chloride is formed by electrolysis

copper metal is discharged

state what product is formed on the anode when copper chloride is formed by electrolysis

chlorine gas is released

explain the formation of sodium chloride solution using electrolysis

1. sodium is above hydrogen in the reactivity series

2. so hydrogen cations are preferentially discharged at the cathode

3. chlorine is a halogen, so chlorine anions are preferentially discharged at the anode

state what product is formed on the cathode when sodium chloride is formed by electrolysis

hydrogen gas is released

state what product is formed on the anode when copper chloride is formed by electrolysis

chlorine gas is released

explain the formation of sodium sulfate solution using electrolysis

1. sodium is above hydrogen in the reactivity series

2. so hydrogen cations are preferentially discharged at the cathode

3. hydroxide anions are preferentially discharged over sulfate anions

4. so oxygen is produced at the anode

state what product is formed on the cathode when sodium chloride is formed by electrolysis

hydrogen gas is released

state what product is formed on the anode when sodium chloride is formed by electrolysis

chlorine gas is released

explain the formation of acidified water solution using electrolysis

1. hydrogen cations are discharged at the cathode

2. oxygen from water molecules is preferentially discharged at the anode

explain the formation of a molten lead bromide solution using electrolysis

1. lead cations gain 1 electron at the cathode

2. to become lead atoms

3. bromide anions lose 1 electron at the anode

4. to become bromine atoms

5. which covalently bond

6. to become bromide molecules

state what oxidation is in relation to electrons

the loss of electrons

state what reduction is in relation to electrons

the gain of electrons

state the half equation occurring at the anode during electrolysis

• oxidation

• 4OH^-(aq) → O₂ (g) + 4e^- + 2H₂O (l)

state the half equation occurring at the cathode during electrolysis

• reduction

• H₂O → H⁺ + OH^-

state which electrode oxidation occurs at

anode

state which electrode reduction occurs at

cathode

explain what product is formed at the anode during the electrolysis of copper sulfate solution

• oxygen gas (O₂)

• as sulfate is being oxidised

• causing it to lose electrons

• and form oxygen gas