Chemical reactions

Physical and chemical changes

Chemical vs physical changes:

Rate of reaction

The speed at which reactants are converted into product in a chemical reaction.

Generic formula: rate = (change in mass of product/reactant) / time

Factors that increase the rate of reaction:

• Increasing concentration of the reactants
• Increasing the pressure used
• Increasing the temperature used
• Increasing the surface area of the reactants
• Using a catalyst

Catalyst — a substance that increases the rate of reaction while remaining unchanged (not used up) at the end of the reaction.

Collision theory

For a reaction to take place, the particles must:

1. Collide with each other
2. Collide with enough energy (equal to or more than the activation energy) to be successful

Thus, to increase the chances of successful collisions you can:

• Increase the number of particles colliding (by increasing the concentration of reactants or increasing pressure)
• Increase the kinetic energy of the particles (by increasing the temperature)
• Increase the frequency of collision (by increasing the surface area of the reactants)
• Add a catalyst (which would lower the activation energy required of the particles)

Same as 'factors that increase rate of reaction', but explained by collision theory.

Methods for investigating rate of reaction

1. Measuring a change in mass of reactant or product — change in mass recorded on a weighing balance in a given time; divide this change in mass by the time experimented to find the rate.
2. Measuring the formation of a produced gas — either use an inverted measuring cylinder via a displacement method, where gas displaces the water when delivered into the cylinder (delivery tube and measuring cylinder needed); or use a gas syringe, where the gas produced is directly delivered into the syringe for recording. Divide the recorded gas volume by the time measured to find the rate.
3. Measuring the time taken for reaction completion (often a visible change) — e.g. iodine clock; record the time taken for an image to be obscured, indicating reaction completion.

Reversible reactions and equilibrium

A reversible reaction is represented by the symbol: ⇌

Examples of reversible reactions include hydrated and anhydrous compounds:

• When you add water to anhydrous compounds they become hydrated
• When you heat hydrated compounds they become anhydrous

• Hydrated salt — chemically combined with H2O
• Anhydrous salt — does not contain H2O

Water of crystallisation — the water present in a hydrated compound.

A reaction in equilibrium — when the forward and the backward reaction are equal in rate, the concentration of reactants and products remain constant, and the reaction happens in a closed system.

Equilibrium shifts

How factors affect the yield of forward and backward reactions:

Yield and rate of reaction are NOT equivalent.

Processes studied

Haber process

• Creation of ammonia
• Reacting N2 and H2 to produce NH3
• An exothermic process
• Nitrogen from air and hydrogen from methane

N2 + 3H2 ⇌ 2NH3

Conditions:

1. 200 atm (20,000 kPa)
2. Iron catalyst
3. Temperature of 450 degrees celsius

Contact process

• Making of sulfuric acid
• Burn sulphur in air or roast sulphur ore to produce sulphur dioxide
• React sulphur dioxide with oxygen to produce sulphur trioxide (reversible part)
• Sulphur trioxide reacts with sulphuric acid to produce oleum (H2S2O7)
• Oleum reacts with water to produce sulphuric acid

Have to transfer into oleum because directly reacting SO3 with water is highly exothermic and the fumes are toxic.

S + O2 -> SO2

2SO2 + O2 ⇌ 2SO3

SO3 + H2SO4 -> H2S2O7 (oleum)

H2S2O7 + H2O -> 2H2SO4

Conditions:

• Vanadium(V) oxide catalyst
• Temperature of 450 degrees celsius
• 2 atm (200 kPa)

Why aren't lower temperatures used since the reactions are exothermic (exothermic reactions favour lower temperatures)?

• Below 450 degrees celsius, the rate of reaction is too slow
• Above 450 degrees celsius, the yield of the forward reaction decreases

Why aren't higher pressures used (since higher pressure favours the side with fewer moles)?

• Higher pressures are dangerous
• Higher pressures are more expensive to maintain

Redox

A reaction that undergoes both oxidation AND reduction.

What is oxidation and reduction?

Oxidation state rules:

1. Elements not combined (alone) have an oxidation state of 0, e.g. H2, Mg
2. Oxidation state = charge (it is represented as roman numerals), e.g. cobalt(II) has an oxidation state of +2
3. Sum of all oxidation states in a molecule is 0
4. In complex ions (e.g. SO4), the sum of oxidation numbers equals the charge

• Oxidising agent — the substance that causes another substance to be oxidised in a reaction. It itself is reduced.
• Reducing agent — the substance that causes another substance to be reduced in a reaction. It itself is oxidised.

Agent tests: