Topic 4 - Chemical Changes Flashcards
What are wide range indicators and give an example of one
-A dye that changes colour depending on whether it’s above or below a certain PH
-They can contain a mixture of dyes that mean they gradually change over a broad range of pH
-Universal indicator gives the colours on a scale of 1-14
What is a pH probe and how is it used
-Attatched to a pH meter it measures pH electronically
-The problem is placed in the solution and the pH is given on a digital display as a numerical value, so it is more accurate than an indicator
Definition of an acid
-A substance that forms aqueous solutions and H+ ions in water
-Has a pH less than 7
Definition of a base/alkali
-A base is a substance with pH over 7
-An alkali is a base that dissolves in water to form a solution with a pH great than 7
-They form OH- ions in water
what is a neutralisation reaction
-A reaction between acids and bases
acid + base > salt + water
What happens to the pH of a substance during a neutralisation reaction
The products are equal
Describe how to carry out a titration experiment (e.g. to find out concentration of an alkali)
-Using a pipette and conical flask add a set volume of the alkali to a conical flask and add two or three drops of indicator
-Use a funnel to fill a burette with an acid of known concentration and record initial volume of it in the burette
-Using the burette add the acid to the alkali a bit at a time giving the conical flask a swirl and go slowly when you think the end point is about to be reached
-When the indicator changes colour all the alkali has been neutralised
-Record final volume of acid in the burette and use it along with the initial reading to calculate the volume of the acid used to neutralise the alkali
What should you do after you finish one titration experiment
-Repeat the whole thing a few times, as the first one you do should be a rough one to get an approximate idea of where the solution changes
-You should get the same answer each time after this (within 0.1cm)
-Calculate a mean ignoring any anomalies
Why should you repeat the titration experiment
-To increase the accuracy of the titration and spot any anomalous results
Why should you use a single indicator and not a universal one for calculating titrations
- Because universal indicators gradually change colour, but during a titration you want to see a sudden colour change at the end point
Name the three examples of single indicators and their colour changes
-Litmus : red as an acid, purple as an alkali
-Methyl orange : red as an acid, yellow as an alkali
-Phenolphthalein : Clear as an acid, pink as an alkali
What do strong acids do in water and what then happens to the particles
They ionise completely in water, so the particles all dissociate to release H+ ions
What do weak acids do in water and what then happens to the particles
-Do not fully ionise in solution and only a small proportion of acid particles dissociate to release H+ ions
what does the Ionisation of weak acids set up equilibrium of? And what side does it lie.
-Undissociated and dissociated acid
-Since only a few of the acid particles release H+ ions the position of equilibrium lies well to the left
Why are stronger acids more reactive than weaker ones or the same concentration
-Reactions involve H+ ions reacting with other substances, so is the concentration of H+ ions is higher, the rate of reaction will be faster, so strong acids are more reactive
What factor do H+ ions increase for every 1 decrease on the pH scale and give an example
-For every decrease of 1 on the pH scale the concentration of ions increase by a factor of 10
-E.g. for a decrease of 2 on the pH scale the concentration increase by a factor of 100
rule for factor of ions concentration in comparison to the pH factor
Factor H+ ion concentration changes by = 10 ^-x
Difference between strong and concentrated acids
-Acid strength is what proportion of the acid molecules ionise in water
-Concentration measures how much acid is in a certain volume of water
What happens to the pH of an acid when the concentration increases
The pH decreases regardless of the acids strength
Give three examples of strong acids
Sulfuric, hydrochloric and nitric acid
give three examples of weak acids
carbonic, citric and ethanoic acids
What kind of metals take part in neutralisation reactions
-Hydroxide and oxide
What do metal carbonates produce in a neutralisation reactions
They react with acids to produce salt + water + carbon dioxide
How to make soluble salts using an insoluble base (e.g Magnesium sulphate from sulphuric acid + magnesium oxide)
-Gently heat the dilute acid and add the insoluble base to the acid a bit at a time until no more reacts
-Add it in excess to guarantee we have the maximum amount of product
-It will be neutralised and all excess will sink to the bottom
-Filter out the excess to get just the salt solution
-Gently heat the solution using a water bath to evaporate the water and then stop heating it and leave the solution to cool, and it will form crystals of the salt which can be filtered out of the solution and then dried
