Chemistry - Unit 2b - Acids, Bases and Salts

What are 2 ways you can measure the pH of a solution?

Unviersal Indicator - A dye that changes colour depending on its pH

pH probe - Probe is placed in solution with a pH meter attached to electronically record the pH as a numerical value on a digital display (more accurate than an indicator)

Whats the colour order when using a universal indication ?

Red , organe , yellow , green , blue, purple, dark purple

Whats a base?

A compound that has a pH greater than 7 so it can react with an acid to neutralise it, forming a salt and water

What is a Alkali?

A base that can be dissolved in water to produce hydroxide ions (OH- )

What is an Acid?

Substance that forms aqueous solutions to produce hydrogen ions (H+)

What are aqueous solutions?

Solutions where the solvent is water

Whats a solvent

A substance that breaks down a solute into small particles to be evenly distributed

Whats a solute?

A substance that will be dissolved by a solvent

Whats the neautralisation equation?

H+ + OH- —> H2O

What would the pH of neutralisation products?

Always have a pH of 7(neutral)

How can we measure the concentration of a strong acid or strong alkali?

Titration

What is ionising?

When a substances is split up into its individual ions

How could you measure the strength of an acid or alkali?

By measuring how many hydrogen or hydroxide ions they produce

How do strong acids tend to ionise when in water?

Almost completely ionise in water, so a large proportion of the acid molecules ionise to release hydrogen ions

How do weak acids tend to ionise?

Dont fully ionise in the solution, so only a small proportion of the acid molecules ionise to release hydrogen ions

How do strong alkalines tend to ionise?

They ionise amlost completley when in water, so a large proportion of the alkali molecules will ionise to release hydroxide ions

How do weak alkalines tend to ionise?

They dont fully ionise in water, so only a small proportion of alkali molecules ionise to release hydroxide ions

Describe the relationship between strong acids and weak acids

They both react with metals, bases and carbonates in the same way, however weak acids just have a slower rate of reaction. Weak acids are also less exothermic

What does the acid strength tell you?

What proportion of the acid molecules ionise in water

What does the concentration of an acid tell you ?

How much acid there is in a litre of water

What type of reaction is a neutralisation reactions?

Exothermic

What are the products of a neutralisation reaction (acid+base)?

Salt + Water

Complete the table for acids producing salts:

Hydrochloric acid —→

Sulfuric acid —→

Nitric acid —→

—→ Chloride

—→ Sulfate

—→ Nitrate

Give 2 examples of different bases forms?

Metal oxides and metal hydroxides

What are the products of a Acid + Metal reaction ?

—→ Salt + Hydrogen

When you react Nitric acid with a metal, how is it different to other reactions?

Nitrate salt + mixture of water + NO +NO(2)

What factor depends on whether a metal will react with an acid?

It depends on where is it on reactivity series. If it above hydrogen then it will react but if its below then it wont

What are the products of an Acid + Metal carbonate reaction?

—→ Salt + Water + Carbon dioxide

How can you use the Acid + Metal Carbonate reaction to tsest for something?

You can use this reaction is test to see if an acid or carbonate is present. The solution will effervesces as CO(2) is produced and the gas will turn limewater milky

How can you test for sulfate ions?

Add dilute hydrochloric/nitric acid to test sample - stops any other precipitation reactions occurring. Then add barium chloride solution which will form a white precipitate of barium sulfate if sulfate ions are present?

What is effervescent?

Something fizzes

How do you make soluable salts?

Add an insoluble substance (metal oxide, metal hydroxide,) to an acid in a flask that will react to produce a salt (plus either water, hydrogen or water and carbon dioxide). Then filter of the excess insoluble solution to get only the salt and water.

To make small crystals, heat solution up really quickly to evaporate about 2/3 of the solution and leave the rest to cool to form crystals. Filter off crystals and leave them to dry.

To make big crystals leave the solution to evaporate slowing over a few days

When making soluble salts, how do you know when the acid has fully reacted?

The excess insoluble substance will sink to the bottom

Test pH by using indicator paper (should be neutral)

How can you test to see which ion an insoluble salt contains?

Flam test

What do the results of the flam test tell us?

red - lithium

yellow/organge - sodium

lilac - potassium

brick red - calcium

apple green - barium

How can we make a clean dry sample of an insoluble salt?

Precipitation reactions

How do precipitation reacts work?

You need 2 soluble salts that react to make the insoluble salt that you want.

Add a spatula of one salt to test tube with deionised water to dissolve. Do the same with other salt. Add two solution to beaker and mix together, the insoluble salt should precipitate out. Then put folded filter paper into filter funnel and put in conical flask. Pour beaker into middle of filter paper which makes the insoluble salt collect on filter paper

Whats deionised water and why do we use it in precipitation?

Water that’s been idssolved of any mineral ions so that its pure H(2)O .This means there are no other ions about during precipitation

Why do we use titrations?

To find out the concentration of a substance

When making soluble salts using an acid/alkali reaction, why do you have to use titrations?

The reaction gives no indication that its fully neutralised. However you cant just add an excess of alkali/acid as it would contaminate the salt produced as its soluble. A titration shows us the exact volume/concentration needed to neutralise the reaction

Explain a titration practical

• Using pipette, add fixed volume of alkali to conical flask w 2 drops of indicator

• Using funnel, fill burette with standard solution of acid and record initial volume

• Using burette, add acid to flask little bit at a time with a regular swirl. Go slowly at the end-point

• Once indicator changed colour, find volume needed to neutralise alkali

• Repeat 2 more times to find mean (accuracy) and make the salt

Which indicator is needed for titration reactions and why

You need an indicator with a single, clear colour change (methyl orange). Universal indicator is too gradual

Whats a standard solution?

A solution with a known concentration of solute

In titrations, what is the endpoint?

Point where acid neutralises the alkali

How do you make the salts after titrations?

Carry out reaction again with correct volume of acid and no indicator. Then let some water evaporate slowly while solution crystallise. The filter of salt and dry it

What are the units of concentration?

mol/dm(3) or g/dm(3)

What are the three calculations when finding no. moles, volume and concentration of a substance?

No. moles = Concentration(mol/dm3) x Volume(dm3)

Concentraion(mol/dm3) = No. moles/Volume(dm3)

Volume(dm3) = No. moles/concentration(mol/dm3)

How to concert mol/dm3 —→ g/dm3 ?

Mass in grams = moles x relative formula mass