Higher Chemistry

Front 1

How do you calculate the average rate of reactions?

Back 1

The change in mass over the change in time.

Front 2

How do you calculate the relative rate of reaction?

Back 2

1 over t.

Front 3

How does particle size affect a reaction?

Back 3

More collisions occur if the particle size of a solid reactant is decreased, since the overall surface area is increased. Since it is only the particles on the surface of a solid that can react initially, breaking up a solid into smaller pieces exposes more surfaces and hence more particles are available to react.

Front 4

How does concentration affect the reaction rate?

Back 4

If the concentration of a reactant is increased, more collisions between particles will take place since there are now more particles occupying the same volume of space.

Front 5

How does temperature affect the rate of reaction?

Back 5

Raising the temperature at which the reaction occurs does more than merely increase the number of collisions between particles. Temperature can be regarded as a measure of the average kinetic energy of the particles in a substance. Hence, at a higher temperature the particles have a greater kinetic energy and will collide with greater force.

Front 6

Explain collision theory.

Back 6

For a chemical reaction to occur, the reactants must be brought together in some way so that their particles will collide. However, not all collisions result in a successful reaction.

Front 7

What is activation energy?

Back 7

For a reaction to occur, particles must collide with the right geometry and with enough energy. This means that particles must have a minimum amount of kinetic energy, called the activation energy. The activation energy required varies from one reaction to another.

Front 8

Explain the distribution of energy.

Back 8

The distribution of energy changes when the temperature changes. At a higher temperature there are many more molecules with energy equal to or greater than the activation energy. This is the real reason why a small change in temperature can have such a marked effect on the rate of a reaction.

Front 9

What does exothermic mean?

Back 9

An exothermic reaction is a chemical change which gives out heat energy. The products always have less energy enthalpy than the reactants.

Front 10

What is an endothermic reaction?

Back 10

An endothermic reaction is a chemical change which takes in heat energy. The products always have a greater enthalpy than the reactants.

Front 11

What is the activated complex?

Back 11

As a reaction proceeds from reactants to products, an intermediate stage is reached at the top of the activation energy barrier at which the activated complex is formed. They are very unstable and only exist for a short time.

Front 12

What is a catalyst?

Back 12

A catalyst is a substance that alters the rate of a reaction without being used up in the reaction. Catalysis occurs on the surface of the catalyst at certain points called active sites. At these sites molecules of at least one of the reactants are temporarily adsorbed (bonded to the surface).

Front 13

What does a catalyst do to the activation energy?

Back 13

A catalyst provides an alternative route for a reaction with lowered activation energy. Without the catalyst, more energy would be required to break the bonds of the reactants. When the catalyst is used, less energy is required as the catalyst helps to weaken the reactant bonds.

Front 14

Describe the contrasting ways in which a catalyst and heat affect the rate of a reaction.

Back 14

Heating speeds up a reaction by increasing the number of molecules that have energy greater than the activation energy. A catalyst speeds up a reaction by lowering the activation energy. The former provides energy to overcome the energy barrier; the latter lowers the barrier.

Front 15

What happens when you cross a period in the Periodic Table?

Back 15

The atomic size decreases. Moving from one element to the next, electrons are being added to the same energy level and protons are being added to the nucleus. The electrons in the outer energy level are attracted more strongly and pulled closer to the nucleus because of its increasing positive charge.

Front 16

What happens when you go down a grouo on the Periodic Table?

Back 16

The atomic size increases. This is because on moving from one element to the next, the number of occupied energy shells increases.

Front 17

Describe metallic bonding.

Back 17

Metals do not have enough electons to allow them to achieve a full outer shell of electrons by covalent bonding. Instead, the outer electrons of metallic atoms are loosely held which allows them to become delocalised. Metallic bonding consists of the atoms losing their outer electrons to a common 'pool' of delocalised electrons. The atoms become positively charged ions and and are attracted to the pool of electrons. As these electrons are free to move, they conduct electricity.

Front 18

What is covalent bonding?

Back 18

Atoms are held together by the attraction between their positive nuclei and the shared pairs of electrons. This attraction holds the atoms together. A huge amount of energy is required to overcome these forces of attraction.

Front 19

What is ionic bonding?

Back 19

This is a force of attraction between oppositely charged ions. Since these forces of attraction are strong, large amounts of energy are required to overcome them.

Front 20

Explain what is meant by electronegativity.

Back 20

It is a measure of the attraction which an atom involved in a covalent bond has for the bonding electrons. Atoms of different elements have different electronegativities. On crossing a period in the periodic table, the electronegativity value increases. On descending a group in the periodic table, the electronegativity values decrease.

Front 21

What are intermolecular forces?

Back 21

Intermolecular forces are the forces of attraction between molecules.

Front 22

What is a polar covalent bond?

Back 22

In most covalent bonds, the bonding electrons are pulled closer to one of the atoms. This is due to the different electronegativities. The atom with the greater share of electrons will have a slight negative charge by comparison with the other atom. Covalent bonds with unequal electron sharing are called polar covalent bonds.

Front 23

Describe the difference between polar bonds and polar molecules.

Back 23

Differences in attractions for the bonding electrons allow the polarity of the bond to be predicted, it is necessary to look at the symmetry of a molecule before predicting whether a molecule is polar.

Front 24

What is london dispersion forces?

Back 24

They are weak forces of attraction that can operate between all atoms and molecules. They are caused by the uneven distribution of the constantly moving electrons around the nuclei of the atoms. This causes temporary dipoles on the atoms. Atoms then attract each other.

Front 25

What is hydrogen bonding?

Back 25

They are relatively strong intermolecular forces. In water the oxygen and hydrogen have large differences in their electronegative values, and this results in a highly polar covalent bond.

Front 26

Describe solubity.

Back 26

Many ionic compounds are soluble in water and this is a result of the polar nature of the bonds in water molecules. The electrostatic attractions between the water molecules and the ions result in an increase in energy. This energy is used to overcome ionic bonds.

Front 27

What is pure covalent bonding?

Back 27

Sometimes called non polar covalent bonds, this occurs when there is equal sharing of electrons. Both atoms have the same atrraction for bonded electrons electrons, i.e. have the same electronegativity.

Front 28

Explain permanent dipole dipole interactions.

Back 28

These occur between polar molecules. Some molecules have permanent dipoles when electrons are pulled to one side of the molecule. In other words, one side of the molecule has a slight positive charge while the other side has a slight negative charge.