chemistry y4

exothermic

delta H <0

bond breaking < bond forming

energy released

temperature of surrounding rise

eg: combustion, neutralisation, respiration, metal displacement, condensation / freezing

endothermic

delta H >0

bond breaking > bond forming

energy absorbed

temperature of surrounding decrease

eg: electrolysis of water, thermal decomposition, photosynthesis, melting / boiling, dissolution

what is activation energy

activation energy, Ea, is the minimum amount fo energy the reactant particle must possess in order for a reaction to occur

how does concentration affect rate of reaction

when concentration increase, there is an increase in the number of reacting particles per unit volume. the frequency of collisions and frequency of effective collisions increase, hence rate of reaction increases

how does particle size affect rate of reaction

smaller particles increases the surface area to volume ration, hence the frequency of collisions and frequency of effective collisions increase and the rate of reaction also increases.

how does pressure of reactant affect the rate of reaction

at higher pressure, there is an increase in the number of gaseous particles per unit volume. frequency of colisions and frequency of effective collisions increase, hence rate of reaction also increases.

how does temperature affect rate of reaction

at higher temperatures, the particles have more kinetic energy and move faster. this increases the frequency of collisions and frequency of effective collisions increases. the particles also have energy equal to or more than the activation energy. hence, rate of reaction increases

definition of catalysts

catalusts are substances that increases the speed of a chemical reaction by providing an alternative pathway with lower activation energy. catalysts also remain chemically unchanged

why is the volume of H+ produced in cathode twice of oxygen produced at anode

for every 4 moles of electron transferred, 2 moles of hydrogen gas is produced at the cathode and 1 mole of oxygen gas is produced at anode. By avogadro’s law, mol = volume divided by 24. hence mole ratio is equal to volume ratio.

carbon monoxide

from incomplete combustion in insufficient oxygen.
it binds irreversibly with haemoglobin which lowers the ability of blood to transport oxygen, leading to loss of consciousness and even death

sulfur dioxide and nitrogen oxides

reacts with oxygen in the air to form acidic compounds which dissolve in rain water to form acid rain.

acid rain

corrodes buildings made of limestone marble and metals.

lowers the pH of water, killing plants and aquatic life

acid rain leaches essential nutrients like magnesium from the soil which hampers the health and growth of plants

unburnt hydrocarbons

reacts with NO2 in the prescence of sunlight and form smog which irritates the eye and lead to breathing difficulties

catalytic converters

used for car engines. carbon monoxide, and unburnt hydrocarbons are oxidised to form carbon dioxide. nitrogen oxides are reduced to for nitrogen

honeycomb structure maximises surface area for reaction to take place

flue gas desulfurisation

remove sulfur dioxide from flue gases (waste product from combustion)

ozone layer depletion

caused due to CFCs
CFCs break down under ultraviolet radiation into chlorine atoms. chlorine atoms react with the ozone molecules to form chlorine oxide. chlorine oxide then further reacts with another ozone molecule to form. chlorine and oxygen.
chlorine atoms are catalysts

global warming

melting of polar icecaps causing sea levels to rise → low lying land will be flooded

more occurence of unusual weather conditions such as droughts and floods → vegetation is affected and crops yield will be reduced

rapid evaporation of seawater, leading to more carbon dioxide being released into the atmosphere → earth’s average temperature will rise further

explain the separation of hydrocarbons from fractional distillation

petroleum is heated in a furnace and the vapour is passed up the fractionating column. as hot vapour rises, it loses heat and condenses.
lighter fraction, with lower boiling points are collected near the top of the fractionating column where it is cooler. it is collected as gas at the top of the column.
heavier fraction, with higher boiling poitn is collected near the bottom of the fractionating column as it has stronger intermolecular forces of attraction. is the condensed and collected as liquid