Studied by 10 people
What is an element
substances made from one kind of atom
What is a compound
two or more elements chemically combined
Elements chemical reactions
Elements take part in chemical reactions in which new substances are made
These processes most often involve an energy change
In these reactions, atoms combine together in fixed ratios that will give them full outer shells of electrons, producing compounds
What happens to elements / compounds in mixtures
In a mixture, elements and compounds are mixed with each other, but are not chemically combined
this means that the components of a mixture (the original elements / compounds) retain the same characteristic properties as when they are in their pure form
Homogenous mixture
A homogeneous mixture has uniform composition and properties throughout
Therefore, seperate components are not able to be seen
Heterogenous mixture
A heterogeneous mixture has non-uniform composition, so its properties are not the same throughout
It is often possible to see the seperate components (e.g bubbles in a soda)
Separating mixtures
Since the original components of mixtures retain their individual properties - we can often separate them pretty easily
Image below: mixtures & seperation techniques
Neutralisation reaction
acid + base → salt + water
1 hydrogen from the acid, 1 hydrogen + 1 oxygen from the base to form water
the other elements form the salt
Complete + incomplete combustion reaction
Incomplete → substance + O2 → products
Complete → substance + O2 → CO2 + H2O
What are state changes
physical changes that are reversible
these changes do not change the chemical properties or chemical makeup of the substances involved
What does vaporization include
evaporation and boiling
What does evaporation include
the change of liquid to gas
unlike boiling, evaporation occurs only at the surface and takes place at temperatures below the boiling point
Solid → Gas name
sublimation
Gas → Solid name
deposition
Overview of state changes drawing
Draw and explain a graph of the relationship between temperature and energy during state changes
Between 1 & 2, the particles are vibrating and gaining kinetic energy and the temperature rises
Between 2 & 3, all the energy goes into breaking bonds – there is no increase in kinetic energy or temperature
Between 3 & 4, the particles are moving around and gaining in kinetic energy
Between 4 & 5, the substance is boiling, so bonds are breaking and there is no increase in kinetic energy or temperature
From 5 & 6, the particles are moving around rapidly and increasing in kinetic energy
Define the avogadro constant
the number of particles in one mole of a substance
applies to atoms, molecules and ions
the value of the Avogadro constant is 6.02 x 1023 g mol-
What is the relative atomic mass / molar mass
The relative molecular mass (Mr) is the weighted average mass of a molecule compared to one twelfth the mass of a carbon-12 atom
Has no units
Draw the moles and particles formula triangle
Draw the moles, mass and molar mass, formula triangle
What is the molecular formula
the formula that shows the number and type of each atom in a molecule
e.g: ethanoic acid: C2H4O2
What is the empirical formula
the simplest whole number ratio of the atoms of each element present in one molecule or formula unit of the compound
e.g: ethanoic acid: CH2O
How to calculate empirical formula - example: find the empirical formula of a compound that contains 10 g of hydrogen and 80 g of oxygen
How to calculate molecular formula from empirical formula - example: The empirical formula of X is C4H10S and the relative molecular mass of X is 180.42. What is the molecular formula of X? Relative Atomic Mass Carbon: 12.01 Hydrogen: 1.01 Sulfur: 32.07
Calculate the total molar mass of the empirical formula
Divide the given molecular molar mass of the compound by the molar mass calculated for the empirical formula
Multiply each subscript by the whole number that resulted from step 2 - This is now the molecular formula.
ANSWER: The molecular formula of X is C8H20S2
What are the masses of reactants useful for
They are useful to determine exactly how much of the reactants react with each other to prevent waste
Reasons for reactants not forming products
Other reactions take place simultaneously
The reaction does not go to completion
Products are lost during separation and purification
What does the percentage yield show / what is the equation
how much of a particular product you get from the reactants (actual yield) compared to the maximum theoretical amount (theoretical) that you can get
Av
The actual yield is the number of moles or mass of product obtained experimentally
The theoretical yield is the number of moles or mass obtained by a reacting mass calculation
Avogadro’s law and gases
Avogadro’s law allows the mole ratio of reacting gases to be determined from volumes of the gases
He deduced that equal volumes of gases must contain the same number of molecules
At standard temperature and pressure(STP) one mole of any gas has a volume of 22.7 dm3
The units are normally written as dm3 mol-1(since it is 'per mole')
What are the conditions of STP
a temperature of 0° C (273 K)
pressure of 100 kPa
Worked example of gases and avogadro’s law - Example:
What is the total volume of gases remaining when 70 cm3 of ammonia is combusted completely with 50 cm3 of oxygen according to the equation shown?
4NH3 (g) + 5O2 (g) → 4NO (g) + 6H2O (l)
Step 1: From the equation deduce the molar ratio of the gases, which is NH3 :O2 :NO or 4:5:4 (water is not included as it is in the liquid state)
Step 2: We can see that oxygen will run out first (the limiting reactant) and so 50 cm3 of O2 requires 4/5 x 50 cm3 of NH3 to react = 40 cm3
Step 3: Using Avogadro's Law, we can say 40 cm3 of NO will be produced
Step 4: There will be of 70-40 = 30 cm3 of NH3 left over
Therefore the total remaining volume will be 40 + 30 = 70 cm3 of gases
Molar gas volume, volume of gas, and moles formula triangle
What does the kinetic theory of gases state
that molecules in gases are constantly moving
What are the 5 assumptions that the kinetic theory of gases is making
The gas molecules are moving very fast and randomly
The molecules hardly have any volume
The gas molecules do not attract or repel each other (no intermolecular forces)
No kinetic energy is lost when the gas molecules collide with each other (elastic collisions)
The temperature of the gas is directly proportional to the average kinetic energy of the molecules - as temperature increases so does kinetic energy
What is the name of gases that follow the kinetic theory
Ideal gases
What does the volume that a gas occupies depend on (2)
pressure
temperature
Ideal gas equation
Why do gases in a container exert a pressure?
as the gas molecules are constantly colliding with the walls of the container
Describe Boyle’s law
As volume decreases, pressure increases (at constant temperature)
this is because it causes the molecules to be squashed together which results in more frequent collisions with the container wall
Describe Charle’s Law
as volume increases, so does temperature (at constant pressure)
when a gas is heated - the particles gain more kinetic energy and undergo more frequent collisions with the container walls
To keep the pressure constant, the molecules must get further apart and therefore the volume increases
Describe Gay-Lussac’s law
As temperature increases, so does pressure (at constant volume)
Increasing the temperature of the gas causes the molecules to gain more kinetic energy
This means that the particles will move faster and collide with the container walls more frequently - increasing the pressure
What is the equation when you change the conditions of a fixed amount of gas and why?
For a fixed amount of gas, n and R will be constant, so if you change the conditions of a gas we can ignore n and R in the ideal gas equation
So the new equation is:
(p1v1/t1 = p2v2/t2)
What are standard solutions
solutions which are used for analysis whose concentrations are known precisely
made as accurately and precisely as possible
Formula triangle for moles, concentration, and volume
Worked example of concentration calculation:
Calculate the volume of hydrochloric acid of concentration 1.0 mol dm-3 that is required to react completely with 2.5 g of calcium carbonate.
Answer:
Step 1: Write the balanced equation for the reaction
CaCO3 + 2HCl → CaCl2 + H2O + CO2
Step 2: Determine the moles of the known substance, calcium carbonate
Step 3: Use the balanced equation to deduce the mole ratio of calcium carbonate to hydrochloric acid:
1 mol of CaCO3 requires 2 mol of HCl
So 0.025 mol of CaCO3 requires 0.050 mol of HCl
Step 4: Calculate the volume of HCl required
What is volumetric analysis (titration)
a process that uses the volume and concentration of one chemical reactant (a standard solution) to determine the concentration of another unknown solution
The technique most commonly used is a titration
What is the end / equivalence point in a titration
The end point or equivalence point occurs when the two solutions have reacted completely and is shown with the use of an indicator.
What are the steps of a titration
Measure a known volume of solution 1 (with unknown concentration)
The other solution (solution) (with a known concentration) is placed in the burette
A few drops of the indicator are added to solution 1
The tap on the burette is carefully opened and the solution added, portion by portion, to solution 1 until the indicator just changes colour
Multiple trials are carried out until concordant results are obtained
Note 
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