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Rates of reaction
Rate of reaction = amount of product formed
time
Core Practical: Investigate effects of changing conditions of reaction on rates of chemical reactions by:
Measuring production of gas
Hydrochloric acid + marble chips (calcium carbonate) in conical flask
To measure change in mass: scale
Mass lost = gas
To measure volume of gas: bung (air-tight) & gas syringe
Repeat with
different concentration
Or marble chips more crushed
Observing colour change
Sodium thiosulphate + hydrochloric acid → sodium chloride + water + sulphur
Both colourless – form yellow precipitate of sulfur
Measure amount of time taken for precipitate to form
Measure fixed amounts of solutions & use water bath to heat them both to desired temperature
Mix in conical flask & place it over black mark on piece of paper – watch it disappear
Repeat at different temperatures
practical methods for determining rate of given reaction
Precipitation – colour change / clouds solution
Mark on paper disappears
Change in mass
Number on scale decreases
Volume of gas
Gas syringe measures volume of gas
Collision theory
Collision frequency: more successful collisions = faster rate of reaction
Energy transferred during collision: particles need to collide with at least the activation energy in order to be successful
effects on rates of reaction:
changes in temperature
Increased temperature: particles move faster – more frequent collisions
Increased energy of collisions – enough for activation energy
Changes in concentration/pressure (reactions involving gases)
More particles of reactant in given volume – more frequent collisions
Increased pressure: particles more crowded – more frequent collisions
Changes in surface area : volume ratio of a solid
More exposed solid = more area to react with other particles – more frequent collisions
graphs of mass/volume/concentration of reactant/product against time
faster rate of reaction = steeper gradient
Draw tangent to find gradient of curve
catalyst: substance that speeds up rate of reaction
without chemically changing itself / products / mass at end of reaction
creates alternative reaction pathway that has lower activation energy so more particles have required energy to have
successful collisions
enzymes
Biological catalyst: Respiration / photosynthesis / protein synthesis
Fermentation: enzymes from yeast cells
Catalyse reaction from sugars to ethanol + carbon dioxide

Heat energy changes in chemical reactions
change in heat energy when:
Salts dissolving in water
Dissolve salt in water in polystyrene cup and measure temperature change
Neutralisation reactions acid + base
Most exothermic
Endothermic: ethanoic acid + sodium carbonate
displacement reactions
Exothermic: more reactive displaces less reactive – release of energy
precipitation reactions
Exothermic
Exothermic Neutralisation, respiration, combustion
negative energy change: heat energy given out –
products have less energy than reactants
Forming bonds releases energy
More energy released in products’ formation than
required to break reactants’ bonds
Endothermic photosynthesis, thermal decomposition
Positive energy change: heat energy taken in –
products have more energy than reactants
breaking bonds needs energy
More energy needed to break reactant bonds than
is released forming product bonds
Calculate energy change in a reaction given the energies of bonds (in
kJ/mol)
Overall energy change = energy required to break bonds – energy released
by forming bonds
activation energy
minimum amount of energy needed for bonds to break – need to be
broken for new ones to form
On graph: difference between reactants & highest point



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