Chemical Reactivity Level 2 NCEA

Collision Theory

For a reaction to occur, the particles must collide, they must collide with the appropriate orientation, and they must collide with sufficient energy.

Concentration

Collision Theory: An increase in concentration means the particles are closer together so there is an increased chance of colliding, therefore the frequency of collisions increases.

Surface Area

Collision Theory: Increasing the ______ _____ means exposing a greater part of one reactant to the other reactant e.g using a powder instead of a solid. As a result there is an increased chance of colliding, therefore the frequency of collisions increases and the rate of reaction

Temperature

Collision theory: Is a measure of the kinetic energy of particles. At higher ______ the collisions are more energetic and the particles will have more energy than the activation energy making the reaction faster. As a result there is an increased chance of colliding, therefore the frequency of collisions increases and the rate of reaction

Catalyst

Collision Theory: A _____ speeds up the reaction by lowering the activation energy for the reaction, this means that collisions are more effective as more particles have sufficient energy to react.

Le Chateliers Principle

If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts in the direction that minimises change

Equilibrium Constant

A measure of how far forwards or backwards a reaction goes. Kc

Concentration

Effect: If the ________ is changed, the equilibrium system responds to counteract this change:

Fe 3+ (aq) + SCN - (aq) = [FeSCN]2+

if more Fe 3+ is added, the equilibrium shifts to reduce the effect of this change by moving in the forward direction, a darker red colour change would be observed. No change in Kc

Pressure

Effect: can be altered by changing the amount of gas or changing the volume of gas in a closed system. It will only have effect if gases are present

Pressure Change

Equilibrium: _____ ____ will be counteracted by decreasing the number of gas particles in the equilibrium mixture. Hence, the equilibrium shifts to the side with the least number of moles of gas. (always include the number of moles of gas in the answer, eg 4 mols to 2 mols). No change in Kc

Temperature Change

Equilibrium: increasing the temperature will favour the endothermic reaction, this absorbs the increase heat energy supplied to the system. A decrease in temperature will favour the exothermic reaction, this releases heat energy to replace energy removed by lowering the temperature. Kc changes in both circumstances.

Concentration Change

Equilibrium: If extra reactant or product is added to a reaction mixture, the response is to consume the extra material added. If reactant or product is removed the equilibrium will shift to favour the production of the removed species. No change in Kc

Catalyst

Definition: Speed up the reaction rate by lowering the activation energy. They are not used up in reactions, it provides an alternative pathway for the reaction.

Catalyst Effect

Equilibrium: Catalysts speed up both the forwards and backwards reaction causing equilibrium to be reached more quickly. There is no overall change in the amount of products or reactants present and there is no change in Kc.

- ∆Hr

When this is negative it means the reaction is Exothermic - produces heat

+ ∆Hr

When this sign is positive it means the reaction is Endothermic - absorbs heat

Endothermic

Absorption of heat in a chemical reaction

Exothermic

Production or release of heat in a chemical reaction

∆H

Change in enthalpy ____ H, heat change involved during a chemical reaction

Acid

Molecules such as HCl, H2SO4, and Ethanoic acids CH3COOH act as _____ in aqueous solutions by donating protons. The protons are transferred to water molecules to form Hydronium ions H3O+ (aq)

Bases

are substances that accept protons

Acid - base reaction

in which a proton is transferred from an acid to a base.

Strong Acid

Acids which completely dissociate in water, for example HCl, HNO3 and H2SO4

Weak Acid

acids that dissociate only to a limited extent in solution CH3COOH

Strong Base

A base that dissociates entirely into metal ions and hydroxide ions in aqueous solutions. They readily accept protons. Sodium and Potassium Hydroxide are strong bases

Weak Base

React to only a small extent with water, and an equilibrium is established. They accept protons only to a limited extent. Ammonia solution is a weak base

Amphiprotic

Substances like water can either accept or donate protons.

Neutral

Substances that neither accept nor donate protons are ______

Conjugate Base

When an acid loses a proton it forms a conjugate base

Conjugate Acid

When a base gains a proton, it forms a conjugate acid

Acid/base Pairs

CH3COOH / CH3COO -

NH4+ / NH3

H2O / OH-

H3O+ / H2O

(species that differ by 1 proton)

pH

An acidic solution has a pH of less than 7

A neutral solution has a pH of 7

A basic solution has a pH of more than 7

Ionic Product

The product of the concentrations is called the Dissociation constant or ionic product of Kw. Kw is 1 x 10^-14

Strong Acids

Properties: donate protons readily in an aqueous solution to become completely ionised or dissociated. Strong acid has more ions and is a better conductor of electricity, greater concentration of H ions so has a lower pH, it will react faster with metals and carbonates because of the higher concentration of H3O+

Weak Acids

Properties: donate protons only to a limited extent in aqueous solution, so are only partly ionised or dissociated.