Topic 3- Chemical Changes Flashcards
What does the pH scale measure
The strenght and weaknesses of acids and bases
Acids on pH scale
- pH 1-6
- Form H plus ions in water
- The higher the concentration of hydrogen ions in a solutin, the higher ot is on the pH scale
- Coloured Red, Orange and Yellow
Bases on a pH scale
- pH 8-14
- Coloured blue and purple
- Form OH minus ions in water
- The stronger the concentration of the ions, the higher the pH
Alkali
A base that is soluble in water
Base
A substance that reacts with an acid to form salt and water
Indicator
An indicator is a substance that detects whether a substance is basic or acidic
Examples include: Universal Indicator, Phenolphthalein, Litmus and Methyl Orange
Universal Indicator
Uses the PH scale
Red when acidic
Green when neutral
Purple when basic
Litmus
Red in acid
Purple in neutral
Blue when alkaline
Methyl Orange
Red in acidic
Yellow when neutral
Yellow when alkaline
Phenolphthalein
Colourless when acidic
Colourless when neutral
Pink when alkaline
Neutralisation
- Green on pH scale with a reading of 7
- Salts are neutral in the reaction of an acid and base
- H and OH ions react to form water
Acids in water
- All acids can ionise in water
- Gives off hydrogen ions
Strong Acids
- Ionise completely in water
- Have a low pH
- Sulfuric, Nitric, Hydrochloric
Weak acids
- Do not fully ionise in solution
- pHs around 2-6
- Citric, Ethanoic and Carbonic
- Reversible Reaction
- Equilibrium lies on the left
Difference between strong and concentrated acids
- Acid strength measures what proportion of the acid molecules ionise
- The concentration shows how much there is of said acid in on decimetre cubed pf water
- You can have a strong and dilute acid
Changing the concentration of acids
- If the concentration of H plus ions increase by a factor of ten, then the pH decreases by 1
Reactions of acids
- A salt is always formed in the reaction of an acid and a base
- Water is always formed
- Hydrochloric acid forms chloride salts, sulfuric acid forms sulfate salts and nitrates form nitric salts
Acid + Metal Oxide
Salt + Water
Acid + Metal Hydroxide
Salt + Water
Acid + Metal
Salt + Hydrogen
Acid + Metal Carbonate
Salt + Water + Carbon Dioxide
Test for hydrogen
Squeaky Pop test
Lit splint sill be put out with a pop
Test for carbon dioxide
Bubble it through limewater
It will turn cloudy if it is limewater
Rules of solibility
The solubility of salts can be determined using the solubility rules
Solubility of common salts of potassium, sodium and ammoniun
Soluble
Solubility of nitrates
Soluble
Solubility of common chlorides
Soluble except for silver and lead chloride
Solubility of common sulfates
Soluble except from lead, barium and calcium sulfate
Solubility of common carbonates and hydroxides
Insoluble except for sodium, potassium and ammonium
What type of reaction is the making of soluble salts?
Precipitation
Method for making insoluble salts
- Dissolve the base in water
- Dissolve the acid in water
- Mix the two together
- Filter
- Pour the contents into the filter
- Swill out the beaker with deionised water
- Rinse the contents out
- Leave it on a dry place to crystalise
How to make soluble salts
Use a titration
Titration
- Wash through the pipette and burette with acid and alkali to make sure there is no residue of other acids and bases. Do not use water
- Measure out a set amount of acid into a conical flask with a pipette
- Slowly add the base using the burette until the indicator changes to show there has been a change
Making a soluble salt
- Use the titration
- Repeat the experiment using the same values but without the indicator as with an indicator it is impure
- Slowly evaporate the water and leave it to crystalise
Electrolysis
- The breaking down of a substance using electricity
- A current is passed through an electrolyte causing it to decompose
- Oxidation and Reduction occur
- Positive ions (cations) are drawn towards the negative electrode (cathode) and are reduced
- Negative ions (anions) are drawn towards the positive electrode (anode)
- Waste products drop to the bottom of the electrolyte
Half equations
- Show what happens at each electrode
- Add electrons to balance charges (e minus)
- If there is gain, there is an attraction to the cathode as it is negative (Reduction)
- 2H+ + 2e- = H+
- If there is loss, there is an attraction to the anode as it is positive (Oxidation)
- Na = Na + + e-
How to perform electrolysis in a solution
- Get two inert electrodes such as graphite or platinum
- Clean the surfaces with emery paper
- Attach them to a circuit
- Place them into a beaker of the electrolyte
- You may need to make the substance molten
Molten Ionic Substances
- An ionic solid can not be electrolysed as the ions are in fixed positions and can not move
- Molten ionic compounds can move as they can move freely
- Positive metal ions are oxidised to metal atome at the cathode
- Negative ions are reduced to atoms or molecules at the anode
What would be seen at the cathode
A silverly liquid will from
What would be seen at the anode
A gas coloured to the halide so if it is bromide, a brown gas would form
Electrolysis of aqueous solutions
- In an aqueous solutions, hydrogen and hydroxide ions are present
- Hydrogen will be attracted to the cathode if there are no less reactive metals such as the jewellery metals
- At the anode, halide ions will be attracted unless there are none in which hydroxide will be attracted
Results of electrolysis of aqueous sodium chloride
Hydrogen gas
Chlorine
Sodium Hydroxide
The electrolyte needs to have sodium in it
Electrolysis of Copper Sulfate
- Produces oxygen
- Copper metal is produced
- No halides present so water is formed
Using Copper Electrodes
- Non-inert copper electrodes could be used to purify copper
- The electrolyte needs to contain copper
- Copped ions will be separated from the anion as it is separated in the electrolyte
- They will be attracted to the cathode as they are positivley charged and it is negatively charged
- This will cause the anode to lose mass and the cathode to gain mass
How to increase the rate of electrolysis
Increase the current
How does the electrical supply act in electrolysis
- Pulls off electrons
- Offers electrons
