Topic 3 - Chemical changes Flashcards
Learn about acid, alkali and salts
What are the sources of acid and alkali?
Acids in a solution are sources of hydrogen ions
Alkalis in a solution are sources of hydroxide ions
What is an acid?
An acid is a substance with a pH of below 7. They form H+ ions.
Give examples of acid with their symbols. (5)
Nitric acid - HNO3 Ethanoic acid - CH3CO2H Hydrochloric acid - HCl Sulfuric acid - H2SO4 Phosphoric acid - H3PO4
What is a strong acid?examples?
A strong acid is a substance that completely splits up into ions. They release all hydrogen ions. Hydrochloric, Nitric and Sulfuric.
What is a weak acid?
A weak acid is one where it only partially splits up into ions. They release some hydrogen ions. Ethanoic, lactic and citric.
What is the difference of concentrated vs dilute?
A concentrated solution has a greater amount of dissolved solute particles than dilute.
What does a high concentration of hydrogen ions mean?
It means that the solution is more acidic which means that the pH will be lower.
What happens as the hydrogen ion concentration in a solution increases by a factor of 10?
The pH of the solution decreases by 1
Describe the reactions of acids with metals.
Reactive metals react with acids to produce a salt and hydrogen only.
Metal + acid = salt + hydrogen
Describe the reaction of acid with metal hydroxides.
A salt and water are produced when acids react with metal hydroxides. Metal hydroxides are bases because they neutralise acids.
acid + metal hydroxide → salt + water
Describe the reaction of acid with metal oxides.
A salt and water are produced when acids react with metal oxides. Metal oxides are bases, because they neutralise acids.
acid + metal oxide → salt + water
Describe the reaction of acid with metal carbonates.
Metal carbonates react with acid to produce a salt, water and carbon dioxide.
Metal carbonate + acid = salt + water + carbon dioxide
What is a base?
A base is any substance that reacts with an acid to produce salt and water only. An alkali is soluble base.
What are other bases?
Metal oxides
What does high concentration of hydroxide ions mean?
This means the solution is more alkaline and therefore a higher pH.
What is neutralisation?
The reaction between an acid and a base which produces salt and water.
What is the ionic equation for neutralisation?
H+ + OH- = H20
What colour is litmus at acidic, alkaline and neutral?
Acidic - Red
Neutral - Purple
Alkaline - Blue
What colour is methyl orange at acidic, neutral and alkaline?
Acidic - Red
Neutral - Yellow
Alkaline - Yellow
What colour is phenolphthalein at acidic, neutral and alkaline?
Acidic - Colourless
Neutral - Colourless
Alkaline - Pink
How is a salt formed?
A salt forms when hydrogen ions in an acid are replaced by metal ions or ammonium ions. The name of a salt consists of two parts: First part - the metal in the base; Second part - from the acid used.
Describe the investigation of the change in pH for neutralisation. (4)
1) Add some dilute hydrochloric acid to the beaker.
2) Measure and record the pH of the contents of the beakers.
3) Add a small mass of calcium hydroxide powder, stir, and then measure and record the pH again.
4) Repeat until the pH no longer changes.
How do you make a soluble salt from an insoluble base? (6)
1) Warm the acid to speed up the reaction.
2) Then add the base to the acid. Ensure that the base in excess so there is no leftover acid in the product. You will know that the reaction is complete if there is excess solid at the bottom of the flask.
3) Filter off the excess solid to get a solution containing only the salt and water.
4) Heat the solution using a Bunsen burner to evaporate the water.
5) Leave the solution to cool and allow the salt to crystallise.
6) Filter off the solid salt and leave it to dry.
How do you make a soluble salt from a soluble base using titration? (6)
1) Measure out a set amount of acid into a conical flask using a pipette. Add a few drops of indicator.
2) Slowly add alkali to the acid, using a burette, until you reach the end point - this is when the acid’s been exactly neutralised and the indicator changes colour.
3) Repeat until you get concordant results
4) Then, carry out reaction using exactly the same volumes of alkali and acid but with no indicator, so the salt won’t be contaminated with indicator. The solution that remains when the reaction is complete contains only salt and water.
5) Slowly evaporate off some of the water and then leave the solution to crystallise.
6) Filter off the solid and dry it - you will be left with a pure, dry salt.