3. Chemical Changes Flashcards
Which ions make aqueous solutions acidic?
Hydrogen ions (H⁺)
Which ions make aqueous solutions alkaline?
Hydroxide ions (OH⁻)
What is the pH scale?
The pH scale ranges from pH 0 to pH 14 and measures the acidity alkalinity of a solution.
What is the pH range for acids?
Less than pH 7 (1 is the strongest)
What is the pH range for alkalis?
Greater than pH 7 (14 is the strongest)
What is the pH of a neutral solution?
pH 7
What can be used to measure pH?
- Universal indicator
- pH probe
What colour is phenolphthalein in acid and alkali?
In an acid = colourless
In an alkali = pink
What colour is methyl orange in an acid and alkali?
In an acid = red
In an alkali = yellow
What colour is blue litmus paper in an acid and alkali?
In an acid = turns red
In an alkali = stays blue
What colour is red litmus paper in an acid and alkali?
In an acid = stays red
In an alkali = turns blue
Suggest a problem with using universal indicator to test the pH of a solution
The colour of the solution is matched to a pH colour chart. This is quite subjective as people may disagree with what colour the solution matches.
It doesn’t provide an exact pH value.
Acid X has a pH of 1. What can you say about the concentration of hydrogen ions in acid X?
There is a high concentration of hydrogen ions in the acid, making it a strong acid.
The lower the pH of the acid, the higher the concentration of H⁺ ions.
Alkali Y has a pH of 8.5. What can you say about the concentration of hydroxide ions in alkali Y?
There is a low concentration of hydroxide ions in the alkali, making it a weak alkali.
The lower the pH of the alkali, the lower the concentration of OH⁻ ions.
If pH decreases by one unit, what happens to the concentration of hydrogen ions?
The hydrogen ion concentration increases by a factor of 10.
What is a neutralisation reaction?
A reaction between an acid and a base.
During an acid-alkali neutralisation reaction, what happens?
H⁺ ions from the acid react with OH⁻ ions from the alkali to form water.
What is the ionic equation for a neutralisation reaction?
H⁺ (aq) + OH⁻ (aq) –> H₂O (l)
What do the terms concentrated and dilute mean when talking about acids?
Concentrated acids have more moles of acid per unit volume of water than dilute acids.
Is concentration of an acid the same as the strength of an acid?
The concentration of an acid is not the same as the strength.
What does strength of an acid refer to?
Strength refers to whether the acid has been completely dissociated in water or not.
What can be said about the strength of an acid that partially dissociates in water?
Weak acid
What is a base?
Any substance that reacts with an acid to form salt and water only.
TRUE OR FALSE: Alkalis are insoluble bases
False - alkalis are soluble
What are the products when an acid reacts with a metal?
Salt and hydrogen
What are the products when an acid reacts with a metal oxide?
Salt and water
What are the products when an acid reacts with a metal hydroxide?
Salt and water
What are the products when an acid reacts with a metal carbonate?
Salt, water, and carbon dioxide
Why are metal oxides normally bases rather than alkalis?
Metal oxides are normally insoluble whereas alkalis are soluble.
What is the name of the salt formed from magnesium and sulfuric acid?
Magnesium sulfate
What is the name of the salt formed from zinc oxide and nitric acid?
Zinc nitrate
What is the name of the salt formed from calcium carbonate and hydrochloric acid?
Calcium chloride
Describe the chemical test for hydrogen
Insert a lit splint into a test tube of gas.
A ‘squeaky pop’ will be heard if hydrogen is present.
Describe the chemical test for carbon dioxide
Bubble the gas through limewater (calcium hydroxide).
Limewater turns cloudy if carbon dioxide is present.
When a soluble salt is prepared from an acid and an insoluble reactant, why is excess of the insoluble reactant added?
To ensure all the acid reacts.
When a soluble salt is prepared from an acid and an insoluble reactant, how and why is the excess reactant removed?
By filtration.
It is removed to leave a pure solution of the salt.
What method must be used to prepare a salt from an acid and a soluble reactant? Why?
Titration.
Since both the reactants are soluble, a titration allows you to combine the reactants exactly and avoid adding an excess of either reactant as this would be hard to remove.
Name the method that could be used to prepare a sample of soluble copper sulfate from insoluble copper oxide and sulfuric acid
Filtration
What 3 steps are required when producing a pure dry salt from an acid and alkali?
Complete a titration to find the volume of acid that reacts exactly with a set volume of alkali.
Use the results from the titration to mix the acid and alkali in the correct proportions.
Evaporate the water from the solution, leaving pure dry salt crystals.
Describe how to carry out an acid-alkali titration
1) Use a pipette to add a measured volume of acid to the conical flask then add a few drops of indicator. Place on a white tile.
2) Fill the burette with the alkali, noting the initial volume.
3) Add the alkali to the conical flask. First complete a rough trial to find the end point (the point at which the indicator first changes colour).
4) Repeat, adding the alkali drop by drop near the end point and swirling the the flask constantly to mix.
5) Record the final volume in the burette. Repeat until you have concordant results.
Most common chlorides are soluble. What are the two that are not?
Silver chloride and lead chloride are insoluble.
TRUE OR FALSE: All nitrates are soluble.
True
Fill in the gap: ‘All common sodium, potassium and ammonium salts are _______’
Soluble
Most common sulfates are soluble. What are the three that are not?
Lead sulfate, calcium sulfate and barium sulfate are insoluble.
Most common carbonates and hydroxides are insoluble. What are the three that are not?
The carbonate / hydroxides of sodium, potassium and ammonium are soluble.
What salt is produced when lead reacts with sulfuric acid? Will a precipitate form?
Lead sulfate.
A precipitate will form because lead sulfate is insoluble.
How could you prepare a pure, dry sample of an insoluble salt?
1) Mix the two solutions required to form the salt.
2) Filter the mixture using filter paper.
3) The residue on the filter paper is the insoluble salt.
4) Wash the salt with distilled water and leave to dry.
What is an electrolyte?
An ionic compound in its molten or aqueous state.
What does aqueous mean?
Dissolved in water.
Why can an electrolyte carry charge?
An ionic compound in its molten or aqueous state has mobile ions which can carry charge.
What is electrolysis?
A process which uses electrical energy (from a direct current supply) to decompose electrolytes.
What is a cathode?
A negatively charged electrolyte.
What is an anode?
A positively charged electrolyte.
What is a cation?
A positively charged ion.
What is an anion?
A negatively charged ion.
Where do charged ions in the electrolyte move to during electrolysis?
Cations (positive ions) move towards the cathode (negative electrode).
Anions (negative ions) move towards the anode (positive electrode).
What happens at the anode during electrolysis?
The anions (negatively charged ions) lose electrons to form their elements.
What happens at the cathode during electrolysis?
The cations (positively charged ions) gain electrons to form their elements.
At which electrode during electrolysis does oxidation occur?
Anode
At which electrode during electrolysis does reduction occur?
Cathode
What type of element is formed at the positive anode in electrolysis?
Non-metal
What type of element is formed at the negative cathode in electrolysis?
Metal or hydrogen.
How can you predict whether a metal or a hydrogen will form at the negative cathode in electrolysis?
If hydrogen is above the metal in the reactivity series then the metal will form.
If the metal is more reactive than hydrogen then hydrogen will form.
What is formed at each electrode during the electrolysis of copper chloride solution?
Cathode : copper
Anode : chlorine
What is formed at each electrode during the electrolysis of sodium sulfate solution?
Cathode : hydrogen
Anode : oxygen
What is formed at each electrode during the electrolysis of molten lead bromide?
Cathode : hydrogen
Anode : bromine
Predict what will be formed at each electrode during the electrolysis of molten zinc chloride?
Cathode : zinc
Anode : chlorine
What is formed at each electrode during the electrolysis of sodium chloride solution?
Cathode : hydrogen
Anode : chlorine
What is formed at each electrode during the electrolysis of water acidified with sulfuric acid?
Cathode : hydrogen
Anode : oxygen
What are the half equations for the reactions occuring at the cathode and anode during the electrolysis of copper chloride?
Cathode:
Cu²⁺ + 2e⁻ —> Cu
Anode:
2Cl⁻ —> Cl₂ + 2e⁻
What does oxidation mean in terms of electrons?
Loss of electrons
What does reduction mean in terms of electrons?
Gain of electrons
Describe how electrolysis of copper sulfate can be used to purify copper
Place 2 copper electrodes into copper sulfate solution. The anode should be impure copper and the cathode should be pure copper. Connect to a power supply.
The copper in the impure anode is pulled towards the cathode where they gain electrons to form pure copper. Impurities form as sludge below the anode. Cu²⁺ ions from copper sulfate remain in solution.
The cathode will increase in mass as it gains pure copper, whilst the anode will lose mass as copper ions are lost.
