chemical changes Flashcards
What does the pH scale range from and what pH is an acid, alkali and neutral ?
pH scale ranges from 0-14
-acid ranges from 0-6
-alkali ranges from 8-14
-neutral = 7
What is the pH of a strong and weak acid and alkali and give examples.
Strong acid = pH 0-2 (red) e.g. HCL, HN0₃, H₂S0₄
weak acid = pH 4-6 (orange/ yellow) e.g. citric acid,
ethanoic acid
Strong alkali = pH 12-14 (purple) e.g. NaOH, LiOH,
KOH
weak alkali = pH 8-10 (blue) e.g. NH₃
Give two ways to measure the pH of a solution and explain how it works.
1) Universal indicator - a dye that changes colour depending on the pH of the solution and is useful for estimating the pH as they are wide range.
2) pH probe - measures the pH electronically by placing the probe in the solution giving a digital display as a numerical value ; more accurate than universal indicator.
What ions do alkali’s and acids form when dissolved in water ?
acids = H+ (up script +)
alkalis = OH- (up script -)
what is the simplest ionic equation for neutralisation between acids and alkalis including state symbols ?
H+(aq) + OH-(aq) → H₂O(l)
What is the equation for neutralisation between acids and bases ?
acid + base –> salt + water
-when an acid neutralises a base or vice versa the products are neutral (pH 7)
How can we tell whether an acid is strong or weak ?
strong acid - when the acid is put in an aqueous solution the acid will completely ionise and produce H+ ions.
weak acids - when the acid is put in an aqueous solution less H+ ions are dissolved and the acid only partially ionises
How can we tell whether an alkali is strong or weak ?
strong alkali - when the alkali is put into an aqueous solution the alkali will completely ionise and produce OH- ions.
Weak alkali - When the alkali is put in an aqueous solution less OH- ions are dissolved and the alkali only partially ionises.
What happens to the concentration of H+ ions in an acid when the pH decreases by one ?
if the pH decreases by 1 then the concentration of the H+ ions increases by 10.
i.e pH 5 = 1x 10 (-5) (standard from) mol/dm3 (cubed)
pH 4 = 1x 10 (-4) mol/dm3
How is the concentration of an acid different to the strength of the acid ?
The concentration of an acid measures how much acid there is in a certain volume of water. The larger amount of acid in volume of liquid = more concentrated.
So you can have a dilute concentration but strong acid or a concentrated but weak acid.
What do bases, metal oxides and metal hydroxides, form when they react with acids ?
They both form salt + water
(Acid + metal oxide –> salt + water)
(Acid + metal Hydroxide –> salt +water)
What does the base, metal carbonate form when it reacts with acid that is different what what bases, metal oxides and metal hydroxide produce ?
forms carbon dioxide as well as salt and water .
(Acid + metal carbonate –> salt + water + carbon )
dioxide
What are the steps to the practical of making soluble salts using an insoluble base ?
1) You need to pick the right acid and insoluble base e.g. an insoluble hydroxide, metal carbonate or oxide.
2) Gently warm the dilute acid using a Bunsen burner, then turn off the Bunsen burner.
3) add the insoluble base to the acid a bit at a time until no more reacts (the base is in excess). The acid has been neutralised when even after stirring, the excess solid will sink to the bottom.
4) then filter out the excess solid to get the salt solution.
5) to get pure, solid crystals of the salt, gently heat the solution using an electric heater to evaporate some of the water
What are the steps to the practical of making soluble salts using an insoluble base ?
1) pick out an insoluble base e.g. an insoluble hydroxide, metal carbonate or oxide. You then measure out the volume of the acid in a a measuring cylinder and mix it together with the excess insoluble base in a beaker.
2) then heat the solution to speed up the reaction with a Bunsen burner and tripod with the beaker on top.
3) The filter the solution to remove the excess insoluble base as it has now all reacted with the acid. Do this by using a funnel and filter paper to filter out the remaining reacted acid into a conical flask.
4) Then use a Bunsen burner again to heat the reacted acid. the water will then evaporate out of the solution and into an evaporating basin. You are then left with salt crystals in the beaker and leave to dry off.
What are metals reactivity determined by ?
How easily they lose electrons and form positive ions.
