C2 Elements, Compounds and Mixtures

From Physics and Maths Tutor

What is meant when a substance is described as pure?

A pure substance is made up of a single element or compound.

How does the scientific definition of 'pure' differ from the everyday definition?

In everyday terms, 'pure' describes something with nothing added, but scientifically, it refers to a single element or compound.

How can melting points be used to identify a pure substance?

A pure substance will have an exact sharp melting point, while impure substances melt across a range of temperatures.

What apparatus could be used to measure temperature?

A thermometer or temperature probe.

Which temperature measuring apparatus is the most precise?

The temperature probe is the most precise, recording temperature to 2 decimal places.

What is the meaning of relative atomic mass?

The average mass of an atom of an element compared to 1/12th the mass of an atom of carbon-12.

What is the meaning of relative formula mass?

The weighted mean average masses of the formula units compared to 1/12th the mass of an atom of carbon-12.

What is the meaning of relative molecular mass?

The mean average mass of one molecule of an element or compound compared to 1/12th the mass of one atom of carbon-12.

How is relative formula mass calculated?

By adding together the relative atomic masses of each of the elements in the chemical formula.

What is the relative formula mass of Ca(OH)2?

The formula mass is 74.

What is the empirical formula?

The smallest whole number ratio of atoms of each element in a compound.

What is the molecular formula?

The formula that shows the actual number of atoms of each element in the compound.

What is the empirical formula of CH4?

CH4 is already in the smallest possible ratio.

What is the empirical formula of C4H10?

C2H5.

What are the molecular and empirical formulae of C2H4Br2?

Molecular formula: C2H4Br2; Empirical formula: CH2Br.

What is an alloy?

A mixture of two or more metals.

Why are alloys commonly used instead of pure metals?

Alloys can be engineered to have more desirable properties, often being harder and stronger than pure metals.

What is a formulation?

A mixture containing exact quantities of different substances refined for optimum properties.

Give examples of materials which are formulations of mixtures.

Sunscreen, medicine, perfume, drinks, and many more.

What process can be used to separate an insoluble salt from a solution?

Filtration.

How does filtration work?

Filter paper is used to separate the insoluble salt from the solution.

What process can be used to separate a soluble salt from a solution?

Crystallisation.

How does crystallisation work?

Gently heating the solution increases concentration, allowing salt crystals to form as water evaporates.

When is simple distillation used?

To separate one liquid from a mixture of liquids with different boiling points.

Describe the process of separating ethanol from a mixture of ethanol and water using simple distillation.

Heat the mixture in a flask; ethanol evaporates first due to its lower boiling point, condenses in the condenser, and drips into a beaker.

What substance is fractional distillation commonly used to separate?

Crude oil.

How does fractional distillation work to separate crude oil?

The oil is heated, vapours enter a column with a temperature gradient, and different compounds condense at different fractions.

What is chromatography?

A process used to separate a mixture of soluble substances using a stationary and a mobile phase.

What are the specific phases used in paper chromatography?

Stationary phase: Paper; Mobile phase: Solvent.

What are the specific phases used in thin layer chromatography (TLC)?

Stationary phase: Thin layer of an inert substance; Mobile phase: Solvent.

Describe how to carry out paper or thin layer chromatography.

Draw a pencil line, place a dot of the mixture, immerse in solvent, and allow separation based on solubility.

Why must the solvent level be below the pencil line in chromatography?

To prevent the mixture from dissolving directly into the solvent instead of traveling up the stationary phase.

Why must the solvent level be below the pencil line in paper chromatography?

To prevent the solvent from submerging the substance being tested and washing it away.

Why is pencil used to draw the baseline in paper chromatography?

Pencil is insoluble and will not affect the results of the experiment.

What is an Rf value?

The Rf value is a ratio between the distance travelled by the dissolved substance (the solute) and the distance travelled by the solvent.

How do you calculate Rf from a chromatogram?

Rf = Distance travelled by substance / Distance travelled by solvent.

When is gas chromatography used?

Gas chromatography is used to separate mixtures of volatile liquids.

What are the specific phases used in gas chromatography?

Stationary phase: Solid / liquid on solid support; Mobile phase: Inert carrier gas.

How does gas chromatography separate a mixture of compounds?

A gas (mobile phase) carries substances through a column packed with a solid (stationary phase), separating them based on different speeds.

How can chromatography be used to distinguish between pure and impure substances?

Pure substances show one spot on a chromatogram or one peak on a graph; impure substances show multiple spots or peaks.

What are metals and where are they found in the periodic table?

Metals are elements that react to form positive ions and are found on the left side of the periodic table.

What are non-metals and where are they found in the periodic table?

Non-metals are elements that react to form negative ions and are found towards the top right of the periodic table.

What are the general properties of metals?

Metals are shiny, good conductors, dense, malleable, ductile, and have high melting and boiling points.

What are the general properties of non-metals?

Non-metals have a dull appearance, are poor conductors, have lower density than metals, low melting and boiling points, and are brittle.

What is formed when a metal reacts with oxygen?

Metal oxide.

How are positive and negative ions formed?

Positive ions are formed when a metal loses an electron; negative ions are formed when a non-metal gains an electron.

What is the chemical equation for the reaction between magnesium and oxygen?

2Mg + O2 → 2MgO.

How are elements arranged in the periodic table?

Elements are arranged in order of increasing atomic number, with elements in the same group having similar properties.

Why do elements in the same column have similar properties?

They have the same number of outer shell electrons, which determines their reactivity.

What does the period (row) number tell you about all the elements in that row?

They all have the same number of electron shells.

What does group (column) number tell you about all the elements in that group?

They all have the same number of outer electrons.

Describe the difference between a covalent and ionic bond.

A covalent bond forms when two non-metals share a pair of electrons; an ionic bond forms between a positive metal ion and a negative non-metal ion.

Describe the bonding in an ionic compound.

Ionic bonds form between positive metal ions and negative non-metal ions, held together by electrostatic attraction.

Why do ionic compounds have high melting and boiling points?

The strong electrostatic forces of attraction between oppositely charged ions require a lot of energy to overcome.

When do ionic compounds conduct electricity? Why?

Ionic compounds conduct electricity when molten or aqueous because the ions are free to move.

Describe the bonding in simple molecules.

Covalent bonds form when two non-metals share a pair of electrons.

Why do simple molecules have low boiling points despite containing strong covalent bonds?

They need to overcome weak intermolecular forces, not the covalent bonds, to change state.

Why are simple molecules unable to conduct electricity?

They have no overall charge.

How and why do boiling points of simple molecules change as the size of the molecules increases?

As the size of the molecule increases, the strength of the intermolecular forces increases, requiring more energy to overcome.

Describe the bonding in giant covalent structures.

Giant covalent structures have many strong covalent bonds formed by shared pairs of electrons.

Why do giant covalent structures have very high melting points?

All atoms are covalently bonded, requiring a lot of energy to break these strong bonds.

What type of bonds are found in polymers?

Covalent bonds.

Why are polymers solids at room temperature?

Polymers have strong intermolecular forces due to their large size, requiring a lot of energy to melt.

Describe the structure and bonding in metals.

Metallic bonding involves a giant structure with positive metal ions held in a sea of delocalised electrons.

Why are metals able to conduct electricity?

Metals can conduct electricity due to the presence of delocalised electrons that are free to move.

Why are metals typically very malleable?

The atoms in metals are arranged in uniform rows that can easily slide over one another, allowing metals to be bent and shaped.

Why do metals have relatively high melting points?

They have very strong metallic bonding, requiring a lot of energy to overcome the electrostatic attraction between positive ions and negative electrons.

What is a limitation of dot and cross diagrams?

They don't show the 3D arrangement of molecules.

What is a benefit of using ball and stick models to represent molecules?

They show the 3D shape and how atoms are bonded.

What is a limitation of ball and stick models?

They don't model electrons.

What is the highest electron configuration when looking at the first three shells?

2, 8, 8

What is the most desirable electron configuration?

All atoms want to have a full outer shell, which would be 8 electrons in the outer shell (or 2 if the atom only has one shell).

Why are the noble gases (group 0) very unreactive?

They have very stable electron configurations due to their full outer shell of electrons, meaning they don't want to lose or gain electrons.

Why might an element with the electron arrangement of 2, 8, 1 be very reactive?

It's very reactive because it can lose an electron (becoming a positive ion) to obtain the stable configuration 2, 8.

How did Mendeleev order his early periodic table?

In order of increasing atomic mass, with some adjustments to group similar properties.

Why did Mendeleev leave gaps in his periodic table?

For undiscovered elements, using surrounding elements to predict properties of the missing elements.

How has the modern periodic table modified Mendeleev's table?

The elements are now ordered by increasing atomic number rather than increasing atomic mass.

How many covalent bonds can carbon form?

Four

What are organic compounds?

Compounds containing carbon covalently bonded to other atoms.

Why is there such an array of natural and synthetic organic compounds?

Because carbon can form families of similar compounds, chains, and rings.

Describe the structure of graphite.

Each carbon atom is bonded to 3 other carbon atoms, forming layers of hexagonal rings with one delocalised electron per carbon atom.

Describe the properties of graphite.

Soft and slippery due to weak intermolecular forces between layers, and an electrical conductor because it contains delocalised electrons.

Describe the structure of diamond.

Each carbon atom is covalently bonded to four other carbon atoms, with no charged particles.

Describe the properties of diamond.

Very hard and has a very high melting point due to strong covalent bonds; does not conduct electricity because there are no charged particles.

What is a fullerene?

A molecule made up of carbon atoms, shaped like a closed tube or hollow ball.

Name two fullerenes.

Graphene and C60 (buckminsterfullerene).

What are the properties and uses of fullerenes?

They have a large surface area useful for trapping catalysts and a hollow structure for targeted drug delivery systems.

Why is graphene useful in electronics?

It is extremely strong, has free electrons to conduct electricity, and is only one atom thick.

During which changes of state is energy transferred to the surroundings?

Condensing and freezing.

During which changes of state is energy transferred to a substance?

Evaporating and melting.

What state is Substance A at -174°C if it melts at -200°C and boils at -183°C?

Liquid.

What state is Substance B at -7°C if it melts at -5°C and boils at 23°C?

Solid.

Do individual atoms have the same physical properties as the substances that contain them?

No, physical properties depend on the bonds and their arrangement.

How do nanoparticles compare to atoms in size?

Nanoparticles are 1-100 nanometres across and are approximately 100 times larger than atoms.

Compare the surface area to volume ratio of particles and nanoparticles.

Nanoparticles have a much larger surface area to volume ratio.

Why are nanoparticles very reactive?

They have a very high surface area to volume ratio, providing more reaction sites.

Why are nanoparticles useful catalysts?

They have a very high surface area to volume ratio, providing more reaction sites.

How can you calculate the surface area to volume ratio?

Surface area to volume ratio = Surface area ÷ Volume.

Why might nanotubes be suitable for making electrical circuits for computers?

They are electrical conductors, very small, and lightweight.

Why might nanoparticles be used in sunscreen?

They block UV light without leaving visible white marks on the skin.