Higher Chemistry - Unit 1

How is the periodic table arranged?

In order of increasing atomic number

What are 2 features of the periodic table?

• Groups - vertical columns with the same number of electrons in their outer shell, meaning they have similar chemical properties

• Periods - horizontal rows arranged in increasing atomic number and increasing electron shells

What is the description of covalent radius?

A measure of the size of an atom (half the distance between nuclei of two identical bonded atoms)

What is the trend in covalent radius across a period?

Decreases because another proton is added to the nucleus

This means the nuclear charge increases which attracts the electrons more strongly, bringing them closer to the nucleus, which decreases the size of the atom

What is the trend in covalent radius down a group?

Increases because another electron shell is added

This means the outer electrons become further from the nucleus and are less strongly attracted to the nucleus because of the screening effect which increases the size of the atom because the electrons are not pulled so tightly towards the nucleus

What is the description of the first ionisation energy?

The energy needed to remove one mole of electrons from one mole of gaseous atoms of an element

What is the trend in ionisation energy across a period?

Increases because another proton is added to the nucleus

This means the nuclear charge increases which attracts the electrons more strongly to the nucleus meaning more energy is needed to remove them

What is the trend in ionisation energy down a group?

Decreases because another electron shell is added

This means the outer electrons become further from the nucleus and are less strongly attracted to the nucleus because of the screening effect, so they can be removed with less energy

What is the definition of electronegativity and what is it used to predict?

The measure of an atom’s attraction for the shared pair of electrons in a bond

It is used to predict the type of bonding and therefore properties of substances

What is the trend in electronegativity across a period?

Increases because another proton is added to the nucleus

This means the nuclear charge increases which increases the atom’s attraction for the bonded electrons

What is the trend in electronegativity down a group?

Decreases because another electron shell is added

This means the outer electrons become further from the nucleus and the screening effect decreases the attraction of the electrons for the nucleus

What is a covalent bond?

The mutual attraction of the positive nuclei of two atoms for a shared pair of electrons

What is an ionic bond?

The strong electrostatic attractions between positive and negative ions

Why do giant structures have very high melting points?

Because the particles are held together by strong bonds which need a lot of energy to be broken

What are intramolecular forces?

Atoms within the molecules are held together by strong covalent bonds which do not break even when the element boils and turns into a gas

What are intermolecular forces?

Weak forces between molecules

What is the definition of London Dispersion Forces?

• A short-lived weak interaction between a temporary dipole and an induced dipole

• They are intermolecular forces between non-polar molecules

Why do London Dispersion Forces have low boiling points?

Because the weak Van der Waals forces are broken when the substance boils

When do temporary and induced dipoles form?

A temporary dipole forms when, for a small moment in time, more electrons exist on one side of the atom than the other

An induced dipole forms when the temporary dipole induces another dipole in a neighbouring atom

What effect does the number of electrons have on London Dispersion Forces?

The more electrons there are in a molecule, the stronger the London Dispersion Forces are

This is because the more electron shells an atom has, the bigger the temporary and induced dipoles can be

What is the definition of non-polar?

The equal sharing of electrons due to the electrostatic attraction from the nuclei of both atoms

What is the definition of polar?

The unequal sharing of electrons, resulting in a slightly positive end and a slightly negative end of the atom (the electrons are pulled towards the more electronegative atom)

Explain why covalent bonding becomes ionic when the difference in electronegativity is more than 1.5:

When the difference in electronegativity is more than 1.5, the bonding electrons are much more strongly attracted to one atom than the other

This means that the electrons are no longer shared because the bonding electrons have been transferred to the more electronegative atom, forming ions with full positive and negative charges

What is a general rule for the solubility of polar and non-polar substances and substances with similar types of bonding?

• Polar substances generally dissolve in polar substances

• Non-polar substances generally dissolve in non-polar substances

• Substances with similar types of bonding and Van der Waals forces generally mix or dissolve

Why are boiling points a good measure of the Van der Waals forces in a substance?

Because for a molecular substance to boil and change from a liquid to a gas, it is the Van der Waals forces which must be broken

What does the polarity of a molecule have a big effect on?

The strength of the Van der Waals forces and therefore the boiling point and other properties too

Why do polar substances have higher boiling points than non-polar substances?

• Polar molecules have permanent fixed dipoles

• Molecules with permanent fixed dipoles have higher boiling points than molecules with London Dispersion Forces because permanent fixed dipoles are a stronger type of Van der Waals force than London Dispersion Forces

What is hydrogen bonding?

• A particularly strong type of permanent dipole-permanent dipole interaction between molecules

• A hydrogen atom must be bonded to nitrogen, oxygen or fluorine, the three most electronegative, therefore substances containing hydrogen bonding are very polar

Describe the hydration of ions:

• Most ionic compounds are soluble in water

• When an ionic substance dissolves, the ions undergo a process called ‘hydration‘

• Water molecules, because they are very polar, are attracted to the ions

• Hydration is highly exothermic which provides the energy needed to break up the ionic lattice

What is meant by a ‘pure covalent bond‘?

Both atoms have about the same electronegativity, (less than 0.5) so the bonding electrons are equally shared

What is the definition of viscosity?

The resistance of a liquid to flow

How do Van der Waals forces affect viscosity?

The stronger the Van der Waals forces are, the higher the viscosity is

This is because the molecules are more tightly packed together due to the stronger intermolecular force

Why is ice less dense than water?

Because there is space in the crystal lattice structure of ice caused by hydrogen bonding

What is the displacement rule for redox reactions?

A metal that is higher up in the electrochemical series will displace a metal that is lower down from a solution of its ions

Oxidising agents…

are electron acceptors and are reduced themselves

Reducing agents…

are electron donors and are oxidised themselves

In redox reactions which reaction is above the other in the electrochemical series?

The oxidation reaction is always above the reduction reaction

What does H+(aq) mean when in an ion-electron equation in the electrochemical series?

The solution is acidified

What is a biocide?

A strong oxidising agent which can kill fungi, bacteria, and even viruses

What are the rules for working out ion-electron equations? (In the correct order)

• Balance atoms other than H or O if needed

• Balance O atoms by adding H₂O as needed

• Balance H atoms by adding H⁺ (aq) as needed

• Balance the charge (multiply equations to balance electrons if needed)