Chemistry Edexcel topics 1 & 2 (Calculations involving masses…
Chemistry Edexcel topics 1 & 2
Atomic structure and The Periodic Table
Dalton's model of the atom ( atoms as solid spheres) changed when Thompson discovered sub atomic particles which he called electrons. Then the plum pudding model was formed which was the model of positively charged space with floating electrons.
An atom has a nucleus made of protons and neutrons and electrons hang in shells that span from the nucleus to the end of the particle. Most of the mass of the atom is concentrated in the centre because the relative mass of a proton and neutron is both 1 but it's 1/1820 for an electron.
Atoms contain an equal number of protons and electrons because atoms have no overall charge. The relative charge of a proton is +1 and neutron is 0 and an electron is -1.
Isotopes are different atoms of the same element with the same number of protons but differing numbers of neutrons. The existence of isotopes and their isotopic abundance results in the Ar of elements not always being an integer.
Dmitri Mendeleev arranged the elements in periods (row) and groups (columns) in order of increasing atomic number and the elements properties. Mendeleev was able predict the properties of elements that hadn't been discovered yet and left gaps for them in the table.
Metals are located on the left side of the table from a jagged line down from Boron to Astatine. Non-metals are on the right and transition metals are located in the middle.
Non metals make anions and metals make cations. The period an element is in is the number of shells and the group is the same number of electrons on the outer shell.
Types of substances and bonding
Bonding- Ionic and Covalent
An ionic bond forms when a metal and a non metal react together. The structure of an ionic compound is a regular lattice held together by strong electrostatic forces of attraction between the opposites charged ions.
Common endings of ionic substances:
halides- F-, Cl-, I-, As- or Br-
The ending -ide is on names of negative ions that only have one element present.
The ending -ate is on names of negative ions where oxygen and at least one other element is present.
Elements in group 6 & 7 form anions because they are non metals. Atoms in group 6 gain two electrons and atoms in group 7 gain one electron.
Elements in group 1 & 2 form cations because they are metals and atoms in group 1 lose one electron and atoms in group 2 lose 2 electrons
Ions are formed when an atom loses or gains electron(s). Cations are positively charged ions and are formed when an atom
electrons and anions are negatively charged ions and are formed when an atom
A covalent bond is a shared pair of electrons between two non metal atoms. This results in the formation of molecules.
Types of substances and their properties
Elements or compounds can be classed as ionic, simple molecular, giant covalent or metallic depending on their structure.
Ionic compounds are made of a metal and a non-metal.
Covalent compounds are made of two non-metals.
Metallic compounds are made of two metals.
Ionic compounds have high melting and boiling points due to the strong forces of attraction between the particles and it takes a lot of energy to overcome. They only conduct electricity when molten or in solution as then the electrons are free to carry a current. Many dissolve easily in water.
Simple molecular compounds have very low melting and boiling points as the forces of attraction is very low while the covalent bonds are very very strong. Most molecular substances are liquids or gases at room temperature. Molecular compounds do not conduct electricity as they do not have any free electrons. Some are soluable and some are not.
Giant covalent compounds have very very high melting points as it takes a lot of energy to overcome the strong covalent bonds. They don't generally contain charged particles so they can't conduct electricity. They are not soluble in water.
Metallic compounds have very high melting and boiling points because of the strong forces of attraction between the delocalised electrons and the positive ions. They are good conductors of heat and electricity but aren't soluble in water. The layers of atoms in a pure metals can slide over each other making metals malleable.
Graphite, Graphene, Diamond and C60 or Buckminsterfullerenes are allotropes of carbon and are giant covalent compounds.
Graphite has 3 covalent bonds and is arranged in hexagonal layers. There are no bonds between the layers so they can slide over each other making it an ideal lubricant. Diamond has 4 covalent bonds meaning every electron is used in bonding making it the strongest substance in existence. This makes it ideal for cutting tools.
C60 is a fullerene which is either a closed tube or a hollow ball.It is made of 20 hexagons and 12 pentagons and can be used to cage other molecules or in drug delivery systems in the body.
Graphene is one layer of graphite and is only one atom thick so it can be rolled. The atoms are joined together in hexagons.
Calculations involving masses
Avogadro's constant is the number of particles and is 6.02x10^23 or it is the mass of the relative particle mass in grams.
The equation for moles is: no of moles= mass/relative formula mass
Concentration= mass/volume or moles/volume
To work out the relative formula mass of a substance simply add up the Ars of each element in the compound taking into account their abundance, for example: CaCO3= 100
In a closed system, the total mass of the system does not change due to the law of conservation of mass. However in an open system, it can increase or decrease due to the system taking in or expelling gas,
The empirical formula of a compound tells you the smallest whole number ratio of the atoms in the compound. You can use the empirical formula and the Mr to find a compound's molecular formula.
Reactions stop when one reactant is used up therefore the amount of product is dependent on the reactant that is limited rather than the one in excess.
The amount of product formed is directly proportional to the amount of limiting reactant used.
There are limitations to models used to show what substances look like.
2D representations don't show the shape of the substance or the size of the particles.
3D models only show the outer layer of the substance,
Dot and cross diagrams don't show the size of the atoms or ions or how they're arranged
Ball and stick models are misleading as they make it look like there are big gaps between the ions when in reality this is where electron clouds interact. They also don't show the correct size of the atoms or ions as they can be different sizes to each other but the model doesn't reflect this.
States of matter and separating substances
Solid to gas= sublimation
Gas to solid= deposition
Solid to liquid= melting
Liquid to solid= freezing
Liquid to gas= boiling
Liquid to gas= evaporation
Gas to liquid= condensing
Solids have a regular lattice structure and the particles vibrate around a point as they are in fixed positions.
The particles in a liquid have more energy than those in a solid and they are free to move over each other so they don't keep a definite shape.
There are next to no forces of attractions between the particles so they move to fill any container they were in. The gas state has more energy than the liquid or solid state.
Pure in chemistry means a substance that only has one type of atom whereas usually pure is used to mean clean or natural. More than one type of substance would make it a mixture like air.
A pure substance would have a sharp fixed melting point but a mixture would melt over a serous of temperatures.
Simple distillation is used for separating a liquid from a solution.
Fractional distillation is used to separate a mixture of liquids.
Filtration is used to separate an insoluble solid from a liquid.
Crystallisation is used to separate a soluble solid from a solution.
Paper chromatography is used to separate a mixture of soluble substances and identify them. It has a mobile phase (the solvent) and a stationary phase (the paper). The equation for Rf values is distance travelled by solute/ distance travelled by solvent. It should always work out to e less than 1.
Water can be made potable through sedimentation which is where you add iron sulfate to the water to made any fine particles clump together and sink to the bottom. Then the water is filtered to ensure that all dirt and gravel etc is removed from the water. Then it undergoes chlorination which kills any bacteria and harmful chemicals.
Water can be made potable through distillation but it is very expensive so we don't use it in the UK, however it is used in dry countries eg, Kuwait. Water that is used in analysis has to be deionised or pure so it doesn't produce fake results.