Chemical Changes Flashcards
What is the pH scale a measure of?
How acidic or alkaline a solution is
What indicates a lower pH and more acidic solution?
A higher concentration of hydrogen ions
H+ ions
What indicates a higher pH and more alkaline solution?
Lower concentration of hydrogen ions
H+ ions
What is a base?
A substance that reacts with an acid to produce a salt and water
What are water soluble bases?
Alkalis
What do alkalis form in water (aqueous solutions)?
OH- ions
Hydroxide ions
What do acids form when dissolved in water (aqueous solutions)?
H+ ions
(hydrogen ions)
What will an acid and alkali combine to form?
Salt and water
Neutralisation reaction
What happens for every increase of 1 in pH?
Concentration of H+ ions is divided by 10
What can neutralisation reactions in an aqueous solution also be shown as?
An ionic equation
H+ + OH- ——> H2O
What are the products when an acid neutralises a base?
Products are neutral and have a pH of 7
What does a pH 7 mean in terms of ions?
The concentration of hydrogen ions is equal to the concentration of hydroxide ions
Indicator
Dye that changed colour depending on if it’s above or below a certain pH
How do you use indicators?
Add a few drops to the solution you’re testing
Compare the colour the solution goes to a pH chart for that indicator
Litmus indicator
Red in acidic solutions
Purple in neutral solutions
Blue in alkaline solutions
Methyl orange indicator
Red in acidic solutions
Yellow in neutral and alkaline
Phenolphthalein indicator
Colourless in acidic or neutral solutions
Pink in alkaline
Why is a pH probe more reliable than indicators?
It shows the numerical value for pH
Less dependence on human judgement
Digital value to 2dp- more precise than universal indicator
What neutralises acids?
Bases
Dilute solution
Small amount of solute in a given volume of solvent
Small concentration of mineral ions
Concentrated solution
Large amount of solute in a given volume of solvent
What can acids do in a solution?
Ionise (dissociate)
Split up to produce a hydrogen ion (H+) and another ion
Examples of strong acids
Sulfuric
Hydrochloric
Nitric
Low pH
What do strong acids almost completely do in water? What do a large proportion of the molecules release?
Ionise
A large proportion of the acid molecules ionise and release H+ (hydrogen) ions
Tend to have low pHs
What don’t weak acids do in solutions?
Fully ionise
Only a small proportion of acid molecules dissociate to release H+ ions pH- 2-6
Examples of weak acids
Ethanoic
Citric
Carbonic
What does the dissociation/ ionisation of a weak acid set up? Why is this?
Sets up an equilibrium because it is a reversible reaction
Concentrations of reactants and products are constant- ratio doesn’t vary)
Why does the equilibrium lie well to the left when a weak acid is ionised?
Only a few of the acid particles release H+ ions
Acid strength
What proportion of acid molecules ionise in water
Concentration of an acid
How much acid is in a litre (1dm^3) of water/ amount of acid in a given volume of solution
Basically how watered down an acid is
What does concentration describe in acid molecules? What makes a more concentrated acid solution?
The total number of dissolved acid molecules
The more grams (or moles) of acid per dm3, the more concentrated the acid is
How does changing the concentration of an acid affect its pH?
If the concentration of H+ ions increases by a factor of 10, pH decreases by 1
Decreasing the H+ ion concentration by a factor of 10 means an increase of 1 on the pH scale
What type of salt forms during a neutralisation reaction?
A salt- ionic compound
In general, what does hydrochloric acid produce in a neutralisation reaction?
Chloride salts
In general, what does sulfuric acid produce in a neutralisation reaction?
Sulfate salts
In general, what does nitric acid produce in a neutralisation reaction?
Nitrate salts
Examples of bases
Metal carbonates
Metal oxides
Metal hydroxides
What happens when an acid is added to a metal oxide?
Salt and water forms
What happens when an acid is added to a metal hydroxide?
Salt and water forms
What happens when an acid is added to a metal?
Salt and hydrogen forms
What happens when an acid is added to a metal carbonate?
Salt, water and carbon dioxide forms
What happens when acids react with metals or metal carbonate bases?
Salts form
First part of salt name come from metal involved
2nd part comes from acid
What is concentration measured in?
g dm-3
mol dm-3
Test for hydrogen
Use a lighted splint
Hydrogen makes a squeaky pop with a lighted splint
Noise comes from the hydrogen burning with the oxygen in the air to form water
Gas is carbon dioxide test
Bubble the gas through limewater
If the gas is carbon dioxide, limewater will turn cloudy
What does the method for making a salt depend on?
Whether it’s soluble or insoluble
What may you need to work out when two solutions are mixed?
If a salt will form as a precipitate (insoluble salt) or whether it will form in a solution (soluble salt)
How do you know if a salt is insoluble?
If a salt forms as a precipitate when two solutions are mixed
How do you know if a salt is soluble?
If a salt forms in solution when two solutions are mixed
What do soluble salts do?
Dissociate into their anion and cation as the giant ionic lattice breaks
What do insoluble salts do?
Don’t break up- their giant ionic lattice stays intact
Are common salts of sodium potassium and ammonium soluble or insoluble?
Soluble
Are common salts of nitrates soluble or insoluble?
Soluble
Are common salts of chlorides soluble or insoluble?
Soluble (except silver chloride and lead chloride)
Are common salts of sulfates soluble or insoluble?
Soluble (except lead, barium and calcium sulfate)
Are common salts of carbonates and hydroxides soluble or insoluble?
Insoluble (except for sodium, potassium and ammonium)
What type of reaction would result in a pure, dry sample of an insoluble salt?
Precipitation reaction- solid from a solution
How do you get an insoluble salt?
Pick the right two soluble salts and react them together
When can displacement reactions occur?
If two solutions are mixed together
What can displacement reactions sometimes make?
A precipitate
If one can be made, it will be
What is a chemical that causes a precipitate to form?
Precipitant
What do symbol equations show?
If an insoluble solid is made
State symbols can show this
Precipitation reaction method
(To make an insoluble salt) eg lead chloride
Add 1 spatula of 1 soluble salt (eg lead nitrate) to a test tube. Add deionised water to dissolve, shake thoroughly to ensure all of it has dissolved
In a separate test tube, do the same with 1 spatula of the other soluble salt (eg sodium chloride)
Tip the two solutions into a small beaker and give it a good stir to sure sure it’s mixed together. The insoluble salt (lead chloride) should precipitate out
Put a folded piece of filter paper into a filter funnel and stick funnel into conical flask
Pour beaker contents into the middle of the filter paper
Swill our beaker with deionised water and tip into filter paper to get all precipitate out from beaker
Rinse filter paper contents with deionised water to make sure all soluble salt (sodium nitrate) has been washed away
Scrape insoluble precipitate (lead chloride) on fresh filter paper and leave to dry in an oven or a desiccator
How are soluble salts made?
By reacting an acid containing one of the ions needed in the salt with an insoluble base containing the other ion needed
Examples of insoluble bases to make a soluble salt
Pure metals
Metal oxides
Metal hydroxides
Metal carbonates
Method to produce a pure, dry sample of a soluble salt from an insoluble oxide, carbonate, hydroxide or metal
Acid + insoluble base
Heat acid in water bath to speed up reaction between acid and insoluble base- in fume cupboard to avoid releasing acid fumes
Add base to acid to produce a soluble salt and water. Acid is neutralised and base is in excess when excess solid sinks to bottom of flask. This is important so there is no leftover acid in the product
Filter excess solid to get solution containing salt and water
Heat solution to evaporate water, leave to cool and allow salt to crystallise
Filter off solid and dry
How can soluble salts be made using acid/ alkali reaction?
Reacting acid and alkali
The alkali will neutralise the acid
Solution left will contain soluble salt and water
Method to work out exactly the right amount of alkali to neutralise the acid
Do a titration using an indicator:
Measure acid into a conical flask using a pipette and add indicator
Add alkali slowly using a burette until the end point has been reached- the acid has been exactly neutralised and the indicator colour changes.
Carry out the reaction using the same measurements of alkali and acid but with no indicator so the salt won’t be contaminated with indicator
Solution left contains only salt and water
Slowly evaporate off some of the water and then leave solution to crystallise
Filter off solid and dry it to be left with a pure, dry salt
Why can an excess of alkali not be added in titration?
Salt is soluble and would be contaminated
Why is it impossible to tell when neutralisation reactions have finished?
There is no signal that all the acid has been neutralised
What does titration measure?
Volumes of acid and alkali solutions needed in a neutralisation reaction
What must be considered when choosing an indicator for a titration reaction?
Strength of acid and alkali
If the acids and alkalies are strong, what indicator should be used in titration?
Methyl orange or phenolphthalein
Methyl Orange
Red in acidic solutions
Yellow in neutral and alkaline
Phenolphthalein
Colourless in acidic or neutral solutions
Pink in alkaline
Litmus
Redin acidic solutions
Purple in neutral solutions
Blue in alkaline
What type of colour change should indicators give in a titration reaction?
Single clear colour change
Why is universal indicator not suitable in titration?
Colour change is too gradual
What do indicators show in titrations?
The end point
When the correct volume of alkali has been reached to neutralise the acid
What is it called when the concentration of hydrogen ions increases 10x as pH decreases by 1?
One order of magnitude
