Acids Flashcards
What is an acid?
A substance that forms H+ ions when they dissolve in water (aqueous solutions)
How do were measure how acidic or alkaline a substance is?
pH scale
What is an alkali?
An alkali is a substance that forms OH- ions when they dissolve in water - a soluble base
What is a weak acid?
- Only partially ionise in water, meaning not all the acid molecules break up to form H+ ions
- Most acids are weak acids
What is an example of a weak acid?
- Ethanoic acid
- CH3COOH ⇌ CH3COO- + H+
What is a strong acid?
- Fully ionise in water, meaning that all the acid molecules break up to form H+ ions
What is an example of a strong acid?
- Hydrochloric acid
- HCl -> H+ + Cl-
What is pH a measure of?
The concentration of H+ ions in a solution
How much does the concentration of H+ ions get divided by as the pH goes up by 1?
Divided by 10
What is the concentration of H+ ions at pH 0, 1 and 2?
0 - 1 mol/dm3
1 - 0.1 mol/dm3
2 - 0.01 mol/dm3
What are the 2 main ways of measuring the pH of a substance?
Universal indicator, pH probe
What are 3 ways of determining if a substance is acid or alkali?
Phenolphthalein, Methyl orange, litmus paper
Describe a method to test for change in pH when adding a solid base to a fixed volume of acid. (likely to be calcium hydroxide CaO or calcium oxide Ca(OH)2 to HCl)
1) Use a measuring cylinder to add a set volume of acid to the beaker
2) Add the amount of solid base to the beaker (number of spatulas worth)
3) Stir with a glass rod
4) Record the pH of the beakers contents by dipping in glass rod, and tapping some onto universal indicator paper
5) Match the colour to the appropriate pH on a pH colour chart after 30s
How can the experiment where you add a soild base to a set volume of acid be improved?
Use a pH probe to measure pH, as this will mean you don’t have to judge the colour, and is quicker
What is a concentrated solution?
A solution that contains a large amount of the solute in a given volume of the solvent
What is a dilute solution?
Contains a small amount of the solute in a given volume of the solvent
What is the result of Acid + Metal
Hydrogen + Salt
What is a base?
Any substance that reacts with an acid to form a salt and water.
What are examples of substances that are bases?
Metal carbonates, metal oxides, metal hydroxides
What is the result of Acid + Carbonate
Salt + water + carbon dioxide
What is the result of Acid + Hydroxide
Salt + water
What is the result of Acid + Oxide?
Salt + water
How are soluble salts made?
Reacting acid, with solid insoluble substances such as pure metals, metal oxides, metal hydroxides or metal carbonates
How do you prepare a soluble salt from an acid and a solid?
1) Use a Bunsen burner to gently warm the acid
2) Add the insoluble/soluble solid (with stirring) until the reaction no longer happens
IF USING SOLUBLE SOLIDS, USE TITRATION TO DETERMINE THE EXACT AMOUNT NEEDED
IF USING INSOLUBLE SOLIDS THEN ADD EXCESS AND FILTER
3) Filter the solution to remove excess insoluble solid if its there, leaving a solution of the salt dissolved in water
4) Heat the solution in an evaporating basin above a beaker of water, which ensures gentle heating
5) Let the solution cool and allow more water to evaporate, causing the solution to become more concentrated and the salt will begin to crystallise
What is the aim of crystallisation?
Produce a pure, dry sample of a soluble salt from an insoluble oxide or carbonte.
What salts are normally soluble in water?
- Sodium, potassium and ammonium salts
- all nitrates
- most chlorides, except silver and lead chlorides
- most sulphates, except lead, barium and calcium sulphates
- Sodium, potassium and ammonium carbonates and hydroxides
Which salts are normally insoluble in water?
- Silver and lead chlorides
- Lead, barium and calcium sulphates
- most carbonates and hydroxides, except sodium, potassium and ammonium
What is a precipitate?
Insoluble solid in a solution
What is a neutralisation reaction?
A reaction between an acid and a base
What is titration?
A technique that measures the volumes of acid and alkali solutions that are needed in a neutralisation reaction
How do indicators help with titration?
They show the point where a solution turns from acid to alkaline
Explain the method for titration
1) Using a pipette, add a set volume of a strong alkaline to a conical flask
2) Add a few drops of a suitable indicator
3) Place a strong acid (HCl, H2SO4, HNO3) of known concentration in a burette
4) Take an initial reading of the volume of acid in the burette
5) Gradually add the acid into the alkali, and ensure you are regularly swirling the conical flask as you add acid. As you get closer to the result slow the swirling.
6) As soon as the indicator changes colour stop adding acid
7) Take a reading of the final volume of acid in the burette.
8) Subtract this reading from the initial volume to find the volume of acid needed to neutralise the alkali
What is the equivalence point?
The point where the pH goes changes from acid to alkali or vice versa
Why must titration be used when preparing a soluble salt from an acid and a soluble reactant?
Because you need to know the precise amounts of reactants you need to use so that you do not have excess, because as these are soluble, there is no way of separating them from the solution, and this would contaminate the pure salt that you want to crystallise.
Why the acid and soluble solid are mixed in correct proportions?
So that there is no excess, as this would be dissolved in solution and very hard to seperate
Why after reacting an acid and a soluble solid is the solution left just salt and water?
All reactants were used up and there is no excess
