c: unit 2, topic 2 & 3 - aqueous solutions and molarity + rates of chemical reactions

what is the structure of water

UB 2HO ED H

• unsymmetrical

• bent polar molecule

• 2 polar O-H bonds

  • hydrogens: electron deficient

  • oxygen: electron rich

• electronegativities: electron pairs are shared unequally due to large difference in electronegativities

• dipole: can have dipole-dipole interactions with itself or other polar compounds

• hydrogen: main bonds are hydrogen

are water particles more or less dense when solid compared to liquid? explain why

less dense

• crystal: ice forms a tetrahedral crystalline structure that is less dense than the liquid, so it floats

• 9%: expands approximately 9% by volume

are all water molecules hydrogen bonded equally? explain why

no

the below bullet points create surface tension

AAS

• attractions: in liquid water, each molecule has attractions in all directions and hydrogen bonds, except for the water molecules at the surface

• air: surface molecules are exposed to the air above and are only attracted to other water molecules on the sides and below

  • skin: this imbalance causes surface molecules to be pulled inward and closer together, minimising the surface area and creating a skin-like effect at the surface

• this is responsible for the meniscus formed in a glass of water

surface tension definition

the ability of the surface of a liquid to resist an external force (because of the strong intermolecular interactions)

why is water’s boiling temperature so high

because of the intermolecular hydrogen bonds holding water molecules together

definition of solute, solvent & solution

solute: the substance that is dissolved in a solvent

solvent: the substance that dissolves a solute

solution: formed when a solute dissolves in a solvent

• the solvent and solute particles are evenly spread throughout the solution and can’t be distinguished from one another

why is water the best solvent

• easily dissolves polar covalent compounds

• ionic compounds

why do most ionic compounds easily dissolve in water?

dissociation process

1. water molecules orientate themselves so that they interact with the oppositely charged ion of the solute

2. water molecules ‘pull’ the ions, separating them form the solute, forming ion-dipole interactions

definition of an ion-dipole interaction (section: why water is the best solvent)

electrostatic attraction between an ion and a molecule that has a dipole

what are the 3 steps that need to occur for any substances to dissolve in a solvent

1. solvent-solvent forces broken

2. solute-solute forces broken

3. solute-solvent forces form

what is concentration (in chem)

• the amount of solute in a given volume of solvent, typically expressed in mol L^-1 or M or square brackets []

concentrated solution: large ratio of solute compared to solvent

dilute solution: large ratio of solvent compared to solute

define dissociate (in terms of chem solutions)

the process by which substances separate into smaller particles in solutions

difference between acid strength and acid concentration

acid strength: the degree of dissociation in water

acid concentration: the amount of solute in a given volume

saturated, unsaturated and supersaturated (solutions)

saturated: can’t dissolve any more solute as it contains the maximum amount of solute

unsaturated: can dissolve more solute as it contains less than the maximum amount of solute

supersaturated: containing more than the maximum amount of solute that can be dissolved

• achieved through temperature increase, and then very slow cooling

formulas for calculating concentration

definition of anion & cation

anion: a negatively charged ion

cation: a positively charged ion

what is pH

A1 HH >7 LHL

• acidity: used for determination of the alkalinity or acidity of a substance

• 1-<7: acidic

  • higher concentration of hydrogen ions

  • hydrogen: soluble substances that dissociate in water to form hydrogen/hydronium ions

• >7-14: basic (alkalis)

  • lower concentration of hydrogen ions

  • hydroxide: soluble bases that dissociate in water to form hydroxide ions

• log: is a log scale. each value is a whole log bigger than the last

indicator definition (in relation to pH)

a substance that changes colour to indicate a pH range

what is the arrhenius model

describes the acid-base theory

what determines the strength or weakness of an acid/base

• the degree of dissociation to form H+/OH- ions respectively

• different to concentration, which refers to the amount of solute in a given volume of a solution

is water an acid or a base?

BS

• both: it can act as both a weak acid + base (amphiprotic)

• self: because it can self-ionise

what is a neutralisation reaction

• a reaction between an acid and alkali/base that forms a salt and awter (and carbon dioxide when a metal carbonate is a reactant)

what are the 4 types of neutralisation reactions

1. acid + metal hydroxide - salt + water

2. acid + metal carbonate - salt + water +CO2

3. acid + metal hydrogen carbonate - salt + water +CO2

4. acid + metal oxide - salt + water

what is the difference between an alkali and a base

alkali: typ eof base that is soluble in water

base: broader term for any substance that reacts with an acid

what 3 things must occur for a chemical reaction to occur

the reactants:

CAO

1. collide: physically collide with each other

2. activation: have sufficient activation energy

3. orientation: collide with a specific orientation

how can rates of reaction be increased

increasing the

FP

• frequency of collisions

• proportion of successful collisions

how does temperature affect the rate of reaction and why

temp increases, rate of reaction also increases

why?

KCC TP

• kinetic: increases particles average kinetic energy,

  • causes:

    • collisions: more frequent collisions

    • threshold: the particles to exceed the activation energy threshold

      • proportion: increasing the proportion of successful collisions

how does surface area affect the rate of reaction and why

only has an impact if the reactant is a solid

sa increases, ror increases

• increases the amount of exposed space that a reactant can collide with

  • increases frequency of collisions

how does pressure affect the rate of reaction and why

changing pressure only applies if the reactant is ag as

pressure increases, ror increases

• by increasing pressure of gaseous molecules, there are more molecules in a smaller volume

  • increases the frequency of successful collisions and the ror

how does pressure affect the rate of reaction and why

only applies if a solution

concentration increases, ror increases

• increases number of particles available for collision

  • increases total number of successful collisions

what is activation energy

• the minimum amount of energy required for a reaction to proceed

  • required to break the chemical bonds in the reactants

• not the only condition that must be met for a successful collision

how do catalysts affect the rate of reaction and why

provide an alternate reaction pathway

they

RA

• reactants: bind to the reactants

• activation: lower activation energy

• proportion: increase proportion of successful collisions

what are the 5 things that affect the overall rate of a chemical reaction

TSP CC

1. temp

2. surface area (if solid)

3. pressure (if gaseous)

4. concentration (if solutions)

5. catalysts

Why do stronger bonds require more activation energy to be broken

because they are more stable, thus requiring more energy to be broken

what is the Maxwell-Boltzmann distribution curve and what can it be used to predict

describes the kinetic energy of particles of an ideal gas at a fixed temp

can be used to predict the:

PAP TC

• probable: number of particles with the most probable kinetic energy

• average kinetic energy of particles

• proportion of particles with energy greater than the activation energy

• temp: effect of changing temperature

• catalyst: effect of adding a catalyst

what are energy profile diagrams and what can they be used to predict

similar to enthalpy change level diagrams

used to predict:

EA BE CI

• exothermic: whether a reaction is exothermic or endothermic

• activation: the activation energy for the forward and backward reaction

• break: energy required to break or make bonds

• enthalpy: overall enthalpy change

• catalyst: effect of adding a catalyst

• intermediate state

rate of reaction formula

how can a rate of reaction be measured

by observing changes in:

CVM

• concentration

• volume

• mass