Reversible Reactions and Equilibrium

Reversible Reactions

Some reactions are easily reversed. In class, we saw that blue hydrated copper sulfate crystals can be turned into anhydrous white copper sulfate crystals, by heating (which removes the crystallised water molecules). By adding a drop or two of water, this is reversed, and releases a lot of heat energy!

Equilibrium

When a reversible reaction is contained within a closed system (such as a boiling tube with a rubber/cork bung, or a bottle with a lid on it), the forward and reverse reactions "compete", until the rate of the forward reaction is equal to the rate of reaction of the reverse reaction. The concentrations of the reactants and products do not change, even though both reactions (forward and reverse) are happening. We call this dynamic equilibrium. We will explore this in more detail over the next week.

This video summarises these two concepts well (and is the one we looked at in class on Monday):

Collision Theory and Rate of Reaction

Chemical reactions occur when particles collide with enough energy and in the correct orientation. We can speed up (or slow down) reactions by changing some of the conditions that will cause reactions to occur:

• more often
• with more energy
• in the correct orientation

This part of the topic is focused on how we affect the rate of a reaction, using our knowledge of the Collision Theory.

Thermochemical Calculations

We need to remember and apply some key calculations in our thermochemistry:

ENERGY = AMOUNT (n) × ΔH

We may need to report extra information as well, such as the mass of one of the products or reactants.

Amount, Mass and Molar Mass

There is a useful relationship between amount (n), mass (m) and molar mass (M). We use this a lot in our thermochemical calculations.

Bond Enthalpy

Every chemical bond requires a specific amount of energy to be broken. The amount of energy required to break one mole of a particular chemical bond is called its Bond Enthalpy.

We can use bond enthalpy values to calculate the enthalpy change in a reaction:

Doping

Doping is not a chemical process that needs to be understood in depth in Level 2 Chemistry. However, a basic understanding of this process will help explain and discuss the properties, uses and developments of polyacetylene.

Doping is a chemical process that adds or removes electrons, changing the conductive properties of a substance (making it a better electrical conductor). It is much like rubbing a plastic ruler (or rod) with a cloth, except that the change is more long-lasting.

Doping usually involves one of two chemical processes:

OXIDATION: loss of electrons/removal of electrons

REDUCTION: gain of electrons/addition of electrons

The removal of electrons works by "making space" for electrons from a power source.

The addition of electrons works by overloading each atom with electrons. These electrons go into higher energy levels, so are easily "moved on" to the next atom by electrons from a power source.

Pi Bonds and Conjugated Systems

In Level 2 chemistry, you are not expected to understand "pi bonds" or "conjugated systems". However, a basic knowledge of these will help you explain and discuss the structure, properties and development of polyacetylene.

PI BONDS

Alkenes and alkynes have a type of bond between the carbon atoms called a "pi bond".

The first bond between the carbon atoms is called a "sigma bond" and it holds the carbon chain together.

The next bond formed between the same two carbon atoms is called a "pi bond". It is not as strong as a sigma bond, but it is what prevents rotation of the double (or triple) bond.

Alkenes have one pi bond per carbon-carbon double bond (as well as one sigma bond).

Alkynes have two pi bonds per carbon-carbon triple bond (as well as one sigma bond).

We need to understand pi bonds to understand the next concept (conjugated systems).

CONJUGATED SYSTEMS

When a molecule has alternating single and double bonds, we call this a conjugated system. This means it also has a "layer" of pi bonds, usually drawn above and/or below the sigma bonds.

In cyclic compounds (such as benzene, shown in the video), this is represented as a circle inside the cyclic structure. We do not do the same thing with non-cyclic substances (such as polymers), but we need to keep in mind that the entire structure "shares" these bonding electrons.

A polymer with conjugation will allow for movement of these pi bond electrons  You need to link this key idea to the properties and (proposed) uses of such a polymer.