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GCSE Science Chemistry Module5
GCSE Chemistry Module 5
What is a mole?
6.023 x 10
When you get precisely this number of atoms or molecules, they weigh exactly the same number of grams as the relative atomic mass/relative formula mass of the element or compound
What is the formula for finding the mole?
mass / M
What is relative atomic mass?
The relative atomic mass of an element is the average mass of an atom of the element compared to the mass of 1/12th of an atom of carbon-12
How do you calculate the percentage composition by mass?
x No. of atoms) / M
] x 100
What is empirical formula?
The simplest ratio of atoms in a compound
What is the empirical formula of ethane?
What is the empirical formula of glucose?
What is concentration a measure of?
How crowded things are
The more solute you dissolve in a given volume, the more crowded the solute molecules are and the more concentrated the solution
Translate 1 litre into cm
How do you work out concentration?
Concentration = No. of moles / volume
What are Guideline Daily Amounts?
The amounts of nutrients that an average adult should eat each day in a healthy diet
Why aren't GDAs always accurate?
The amounts are given per 100 g (or ml) of the food but we may eat more or less than this
We may add other things (e.g. milk) which may increase some of the quantities
How can we estimate the amount of salt in a product?
Use the sodium content
Find the ratio of sodium chloride's M
to sodium's A
Multiply this by the amount of sodium
This is probably an over estimate though as the sodium probably won't all come from the sodium chloride
What are titrations?
Used to find out concentrations
Allow you to find out exactly how much acid is needed to neutralise a quantity of alkali (or vice versa)
How do we use titrations?
Using a pipette and pipette filter, add some alkali (usually 25 cm
) to a conical flask, along with two or three drops of indicator
Fill a burette with acid
Using a burette, add the acid to the alkali a bit at a time - giving the conical flask a regular swirl
The indicator changes colour when all the alkali has been neutralised
What types of indicators should be used for titrations?
Universal indicator is used to estimate the pH of a solution because it can turn a variety of colours
Made from a mixture of different indicators so is a sudden colour change
In a titration we need a sudden colour change so we need to use a single indicator such as litmus
How can we use a gas syringe?
Gas syringes usually give gas volumes accurate to the nearest cm
If the reaction is too vigorous, the plunger can blow out of the syringe
How can we use an upturned measuring cylinder or burette?
Use a delivery tube to bubble the gas into an unpside-down measuring cylinder or gas jar filled with water
Not good for collecting things such as hydrogen chloride or ammonia (because they just dissolve in water)
An upturned burette is more accurate
What volume does 1 mole of gas occupy?
One mole of any gas always occupies 24 dm
at room temperature and pressure
When does a reaction stop?
When all of one of the reactants is used up (limiting reactant)
Amount of product formed is directly proportional to the amount of limiting reactant
If you add more reactant, there will be more reactant particles to take part in the reaction, which means more product particles
What is a reversible reaction?
A reversible reaction is one where the products of the reaction can themselves react to produce the original reactants
What is equilibrium?
As reactants react, their concentrations fall so the forward reaction will slow down
As more and more products are made and their concentrations rise, the backward reaction will speed up
After a while, the forwards and backwards reactions will be the same rate
This is equilibrium
At equilibrium both reactions are still happening but there's no overall effect
Only takes place in a closed system (so none of the products or reactants can escape)
When does the position of equilibrium lie to the right?
Lots of products and not many reactants
When does the equilibrium lie to the left?
Lots of reactants and not many products
What things can change the position of equilibrium?
What doesn't change the position of equilibrium?
Catalysts speed up the forward and backward reaction by the same amount
Adding a catalyst means the reaction reaches equilibrium faster but you end up with the same amount of product as you would without the catalyst
How does temperature change the position of equilibrium?
All reactions are exothermic in one direction and endothermic in the other
If you decrease the temperature the equilibrium will move to try and increase the temperature so it moves in the exothermic direction to produce more heat
If you raise the temperature, the equilibrium will move to try and decrease it so the equilibrium moves in the endothermic direction
How does pressure change the position of equilibrium?
Only affects equilibrium involving gases
If pressure increased, equilibrium tried to reduce it. Moves in the direction where there are fewer moles
If pressure decreased, equilibrium tries to increase it. Moves where there are more moles
How does concentration change the position of equilibrium?
If concentration increased, equilibrium tries to decrease it by shifting to the right
If concentration decreased, equilibrium tries to increase it by shifting to the left
What is the contact process?
Used to make sulfuric acid
Make sulfur dioxide by burning sulfur in air
Sulfur dioxide oxidised (with a catalyst) to make sulfur trioxide
Sulfur trioxide used to make sulfuric acid
What conditions are used to make SO
What is the perfect temperature for the contact process?
Oxidising sulfur dioxide to form sulfur trioxide is exothermic (gives out heat)
To get more product you'd think the temperature should be reduced (equilibrium shifted to the right)
But reducing the temperature slows the reaction down
So a compromise temperature of 450
is used to get a high yield quite quickly
What is the perfect pressure for the contact process?
There are two moles of product compared to three moles of reactant
To get more product you'd think the pressure should be increased (equilibrium shift to right)
But increasing the pressure is expensive and as equilibrium is already on the right it's not necessary
Atmospheric pressure (1 atmosphere) is used
What is the perfect catalyst for the contact process?
To increase the rate of reaction a vanadium pentoxide catalyst (V
) is used
It doesn't change the position of equilibrium
What do acids produce in water?
What are strong acids?
Ionise completely in water
E.g. sulfuric, hydrochloric and nitric
What are weak acids?
Do not fully ionise in water
E.g. ethanoic, citiric and carbonic
Is the ionisation of weak acids reversible or irreversible?
Equilibrium lies to the left
What is the pH of an acid a measure of?
What is a typical pH of a strong acid?
1 or 2
What is a typical pH of a weak acid?
4, 5 or 6
How can we measure the pH of an acid?
A pH meter or with universal indicator paper
What is the difference between strong acids and concentrated acids?
Acid strength tells you whats proportion of the acid molecules ionise in water
Concentration measures how many moles of acid there are in 1 litre of water
Concentration is basically how watered down your acid is
The concentration describes the total number of dissolved acid molecules - not the number of molecules that produce hydrogen ions
The more moles the more concentrated the acid is
What type of acid is a better electrical conductor?
Strong are better than weak
Ethanoic acid has a much lower electrical conductivity than the same concentration of hydrochloric acid
The ions carry the charge through the acid solutions as they move
The lower concentration of ions in the weak acid means less charge can be carried
Electrolisis of hydrochloric acid or ethanoic acid produces H
because they both produce H
Which type of acid reacts faster?
What products react to make hydrogen?
Hydrochloric acid (strong) and magnesium
Ethanoic acid (weak) and magnesium
What products react to make carbon dioxide?
Hydrochloric acid (strong) and calcium carbonate
Ethanoic acid (weak) and calcium carbonate
What does the volume of gas produced depend on?
The amount of acid
What is a precipitation reaction?
Normally involve two solutions reacting together to make an insoluble substance
Insoluble substance is the precipitate and it makes the solution turn cloudy
Most precipitation reactions involve ions
These ions need to collide, so they need to be able to move
The ionic substances have to be in solution of molten
Usually the reactions are extremely quick because there is a high collision frequency between the ions
What is the state symbol for a solid?
What is the state symbol for a liquid?
What is the state symbol for a gas?
What is the state symbol for aqueous?
Dissolved in water
What does an ionic equation show?
The useful bits of reactions
How can we test for sulfate ions?
Add dilute hydrochloric acid followed by barium chloride
A white precipitate of barium sulfate means the original compound was a sulfate
How can we test for chloride ions?
Add dilute nitric acid followed by lead nitrate
A white precipitate of lead chloride should form
How can we test for bromide ions?
Add dilute nitric acid followed by lead nitrate
A cream precipitate of lead bromide should form
How can we test for iodide ions?
Add dilute nitric acid followed by lead nitrate
A yellow precipitate of lead iodide should form
What is stage one of preparing insoluble salts?
Add 1 spatula of lead nitrate to a test tube and fill it with distilled water
Shake thoroughly to ensure that all the lead nitrate has dissolved
Do the same with 1 spatula of potassium iodide
Tip the solutions into a smaller beaker and give a good stir
The salt should precipitate out
What is stage two of preparing insoluble salts?
Put a folded piece of paper into a filter funnel and stick the funnel into a conical flask
Pour the contents of the beaker into the middle of the filter paper
Swill out the beaker with more distilled water and tip this into the filter paper to make sure you get all the product form the beaker
What is stage three of preparing insoluble salts?
Rinse the content of the filter paper with distilled water to make sure than all the soluble salts have been washed away
The just scrape the lead iodide on to some fresh filter paper and leave it to dry