Chemical reactivity

What are acids and bases

Acids are proton (H+ ion) donors and bases are proton acceptors. An acid-base reaction involves the transfer of protons (H+ ions).

Weak acids when added to water

Poor proton donors, only partially ionise in water to produce H3O+ ions. Only a few of the potential H+ ions are donated to H2O and the reaction reaches equilibrium.

Weak acids examples

R-COOH, NH4, H2CO3, HF

Strong acids when added to water

Good proton donors and completely ionise in water to produce H30+ ions. All the potential H+ ions are donated to H2O and the reaction goes to completion.

Strong acids examples

HCl, HBr, H2SO4, HNO3

Strong bases when added to water

Strong bases are ionic compounds and dissolve in water rather than react with it.

Completely dissolve to produce OH- ions, which are good proton acceptors. Since all potential OH- ions dissolve into solution the reaction goes to completion.

Strong bases examples

NaOH, KOH

Weak bases when added to water

Poor proton acceptors, only partially ionise in H2O to produce OH- ions. Reaction reaches equilibrium.

Weak bases examples

NH3, R-NH2, R-COO, CO3, HCO3

What is a conjugate acid base pair

A conjugate acid base pair is two substances that differ by only one proton (H⁺ ion). The acid always has an extra H+ ion than its conjugate base.

When writing acid is always written first e.g CH3COOH/CH3COO-

What are amphiprotic substances

Amphiprotic substances can donate and accept protons (H+ ions) e.g water can behave as a weak acid or weak base, and can dissociate into H3O+ and OH- ions. It is an equilibrium reaction.

What does the value of Kw indicate about the dissociation of water?​

Since Kw is extremely small the reaction is very reactant favoured and very few products form. The [H3O+] and [OH-] is very small.​

What does the value of Kw remaining constant mean in terms of changes to ions

If [H3O+] is increased then [OH-] is decreased in order to keep Kw constant.​

If [OH-] is increases then [H3O+] is decreased in order to keep Kw constant.​

State collision theory

A reaction only occurs when effective collisions between the particles happen. This requires particles colliding with enough energy and the correct orientation to break existing bonds so that new ones can form.

The minimum required energy to break these bonds is equivalent to the activation energy.

How does SA affect rate of reaction?

Greater surface area results in more available particles to react, therefore there are more effective collisions per second, and therefore a higher rate of reaction.

How does temperature affect rate of reaction?

At high temperatures, particles have more kinetic energy, meaning they move faster. This increases rate of reaction as particles have more energy therefore more of them will have the sufficient energy for a successful collision. The particles move faster, so there are more frequent collisions, and an increased chance of effective collisions.

How does concentration affect rate of reaction?

Higher concentration = more H+ particles per L of solution. More particles in the same space leads to more collisions occurring, and an increased chance of effective collisions, speeding up the rate of reaction.

How do catalysts affect rate of reaction?

A catalyst/enzyme makes a reaction go faster by providing an alternate pathway by lowering the reaction’s activation energy. It is not used up by the reaction and is unchanged at the end of it.

The catalyst/enzyme lowers the activation energy required to break existing bonds, and more particles have this lowered activation energy, sp there is an increased chance of effective collisions and faster ROR.

When calculating using Kw, what should you count for the mole ratio?

How many H’s are in the acid/base, that is the ratio e.g H2SO4 —> H30+ HSO4, ratio 1:2 as one mole of H2SO4 has 2 H’s and so there must be two moles of H3O+ to be even

pH definition

pH of a solution is a measure of its acidity and is related to the concentration of H3O+ ions present.

Concentration of H3O+ and OH- ions in terms of pH

Higher [H3O+] = lower pH = stronger acid

Higher [OH-] = higher pH = stronger base

What is required for a solution to conduct electricity

A solution containing freely moving ions will conduct electricity, the higher the concentration of ions, the greater the conductivity of the solution

Strengths of acids and bases and ion concentration in terms of electrical conductivity

Strong acids bases and salts have high conductivity because they completely dissociate in water to produce a high concentration of ioms.

Weak acids and bases have low conductivity because they only partially dissociate in water to produce a low concentration of ions.

What is equilibrium

When a reversible (can go in both directions) reaction happens in a closed system (neither products nor reactants can escape), chemical equilibrium can occur.

At equilibrium, the rate of the forward reaction = the rate of the backward reaction.

• the reactants are not all used up

• the reaction does not go to completion

• the container contains both reactants and products

• the amount (conc) of products is not necessarily the same as the reactants

• the reaction appears to be static (e.g colour doesn’t change) even though reactants are continually being made into into products and products are continually being broken down to reform reactants

What is equilibrium constant

K = [C]^c x [D]^d / [A]^a x [B]^b

a,b,c,d = the amounts of mol in each species

Only species that are (g) or (aq) are included in K expression

Define large and v large Kc values

If Kc is large (>1) then at equilibrium there will be more products than reactants. The reaction is said to be product favoured.

If Kc is very large (>10^ 20) then the reaction will have gone to completion.

Define small and v small Kc values

If Kc is small (<1) then at equilibrium only a small amount of products have formed. The reaction is said to be reactant favoured.

If Kc is very small (<10^-20) then the reaction will not occur.

What does K tell us?

The size of K indicates how far a reaction has gone to completion.

K = products/reactants

More products than reactants = `K is large = product favoured reaction = gone a long way towards completion

More reactants than products = `K is small = reactant favoured reaction = made little progress towards completion

What changes the value of K

Temperature the reaction is done at - The change depends on whether the forward reaction is endothermic/exothermic

What is le chatlier’s principle?

Any change made to a system in equilibrium results in a shift of the equilibrium in the direction that minimises the change.

The effect on equilibrium when temperature is changed steps!

1. recognise if it’s endothermic or exothermic

2. Identify the change - increase/decrease in temp

3. Work out shift in response to change: mixture heated = equilibrium shifts to RHS/forward reaction, mixture cooled = equilibrium shifts to LHS/backward reaction

4. Consider the impact of the shift

• reactant and product ratios higher/lower concentrations: heated = product conc increase, reactant conc decrease or cooled = product conc decrease, reactant conc increase

• the value of Kc increase/decrease: heated = higher Kc value (increase), cooled = lower Kc value (decrease)

The effect on equilibrium when concentration is changed steps!

1. Identify change e.g increase of reactant [A]

2. Shift equilibrium to use up excess

• increase in [A/B] decrease in [C/D] equilibrium moves to the right towards products

• decrease in. [A/B] increase in [C/D] equilibrium moves to the left towards reactants

3. Impact of shift e.g observation colour intensity

Increase in reactant conc = shift towards products = turns colour of products

Increase in product conc = shifts towards reactants = turns colour of reactants

The effect on equilibrium when pressure is changed steps!

1. State Le Chatelier's principle

2. Count the gas moles: Count the total number of moles of gas (g) particles on the reactant and product side

3. State the direction of the shift:

   • An increase in pressure favours a move to the side with fewer gaseous molecules / moles of gas.

     A decrease in pressure favours a move to the side with more gaseous molecules / moles of gas.

4. Name the resulting change: State whether the concentration/amount of the reactants or products increases or decreases as a result.

The effect on equilibrium when a catalyst/enzyme is added

These do not affect the position of equilibrium but the rate at which equilibrium is reached.