topic 7 - rates of reaction and energy changes Flashcards
rate of reaction formula
time
what are three ways of measuring reaction rate
change in mass
volume of gas given off
precipitation
describe how rate of reaction is measured in terms of change in mass (when a gas is produced)
using a mass balance
as the gas is released, the lost mass is easily measured on the balance (the quicker the reading is dropping, the higher the reaction rate)
plot graph of change in mass against time elapsed
what should you do if the gas being released is dangerous
use a fume cupboard
describe how rate of reaction is measured in terms of volume of gas given off
using a gas syringe
the more gas given off in set time interval, the faster the reaction
once no more gas is produced, the reaction is finished
plot results on graph of gas volume against time elapsed
describe how rate of reaction is measured in terms of a precipitation
two reactant solutions are mixed in a flask which is placed on a piece of paper with a mark on it
observe the mark through the mixture and time how long it takes for it to be obscured (the faster the mark disappears, the faster the reaction)
why is measuring rate of reaction in terms of a precipitation subjective
people may have different opinions on when the mark disappears
what three factors affect reaction rate
surface area
concentration/pressure
temperature
the steeper the slope on a rate of reaction graph…
the faster the rate of reaction
to find the gradient on a curve
draw a tangent and calculate the gradient of that straight line
that gradient is the rate at that point
investigating the effect of surface area on rate of reaction: uses what reactants
hydrochloric acid
marble chips
investigating the effect of surface area on rate of reaction: describe the method
measure volume of gas with a gas syringe (taking readings at regular intervals and recording them in a table)
plot results on a graph (volume against time)
repeat with the same VOLUME and CONCENTRATION of acid and same MASS of chips but with the chips MORE crushed up, to increase surface area
REPEAT
investigating the effect of surface area on rate of reaction: results
the finer the particles, the greater the rate of reaction
investigating the effect of concentration on rate of reaction: uses what reactants
hydrochloric acid
marble chips
investigating the effect of concentration on rate of reaction: describe the method (2 possible)
OR
–
measure the change in mass during the reaction
place a flask with a set concentration of HCl on a mass balance
add marble chips and immediately note the mass shown on the balance
record the mass at regular intervals
plot mass lost against time on a graph
repeat with different concentrations of acid
investigating the effect of temperature on rate of reaction: reactants and reaction
observed colour change from precipitation reaction between sodium thiosulfate and hydrochloric acid
investigating the effect of temperature on rate of reaction: describe the method (6/7)
1) (sodium thiosulfate solution and hydrochloric acid react to form a yellow precipitate of sulfur)
2) measure out fixed volumes of the two solutions using a measuring cylinder
3) gently heat both solutions to the desired temperature using a water bath before mixing them
4) mix solutions in a conical flask and place flask over a black mark
5) time how long it take for the mark to disappear as the precipitate forms
6) repeat at different temperatures
7) plot results on graph of time taken for mark to disappear against temperature of reacting solutions
investigating the effect of temperature on rate of reaction: results
increasing temperature, increases rate of reaction
activation energy
the minimum energy that particles need in order to react when they collide
AND
the minimum amount of energy for bonds to break
why does increasing temperature, increase rate of reaction
when temperature increases, particle have more kinetic energy so move more quickly
when they move more quickly, there are more frequent collisions
therefore at higher temperatures, THERE WILL BE MORE SUCCESSFUL COLLISIONS, increasing rate of reaction
why does increasing concentration/pressure, increase rate of reaction
if a solution is more concentrated, there are more reactant particles in the same volume/are more crowded so collisions are more likely and the rate of reaction increases
why does increasing surface area, increase rate of reaction
increasing the surface area of an object gives it a greater surface area to volume ratio
the particles surrounding it have a larger area to work on so the frequency of collisions will increase
this increases rate of reaction
catalyst
a substance that increases the rate of reaction without being chemically change or being used up in the reaction itself
how do catalysts increase rate of reaction
catalysts lower the activation energy needed for a reaction to occur by providing an ALTERNATIIVE REACTION PATHWAY (which has a lower activation energy)
therefore, more particles have the minimum amount of energy needed for a reaction when particles collide so more successful collisions occur
what are enzymes
biological catalysts
what do biological catalysts do (examples of reactions they are involved in)
speed up chemical reactions within cells such as respiration, photosynthesis and protein synthesis
what do enzymes in yeast cells do
they are involved in fermentation, which is used to make alcoholic drinks
they catalyse the reaction converting sugars to ethanol and CO2
exothermic reaction
a reaction which gives out energy to the surroundings, usually in the form of heat and is shown by a rise in the temperature of the surroundings
endothermic reaction
a reaction that takes in energy from the surroundings, usually in the form of heat and is show by a fall in the temperature of the surroundings
describe the curve on the reaction profile of an exothermic reaction
curves downwards (ends lower than it started)
describe the curve on the reaction profile of an endothermic reaction
curves upwards (ends higher than it started)
what is the activation energy on a reaction profile
the distance between the reactants and the highest point on the curve
how to measure temperature change in surroundings
- place a polystyrene cup in a large beaker of cotton wool
- add known volume of first reagent to cup
- measure initial temperature
- add a measured mass/volume of second reagent to cup and stir
- put lid on cup (to reduce energy loss by evaporation)
- record maximum or minimum tempature
- calculate temperature change
precipitation reaction
when two solutions are mixed together to form an insoluble solid
are precipitation reactions endothermic or exothermic
exothermic
displacement reaction
a more reactive element displaces a less reactive element from a compound
are displacement reactions endothermic or exothermic
exothemic
neutralisation reaction
acid and base react together to form a salt and water
are most neutralisation reactions endothermic or exothermic
most neutralisation reactions are exothermic
give an example of an endothermic neutralisation reaction
ethanoic acid + calcium carbonate
dissolving salts in water is exothermic or endothermic
can be both
bond breaking
energy must be supplied to break existing bonds
bond breaking is an ENDOTHERMIC process
bond forming
energy is released when new bonds are formed
bond formation is an EXOTHERMIC process
energy for bonds broken/formed in endothermic reactions
more energy is used to break bonds than is released by forming them
energy for bonds broken/formed in exothermic reactions
more energy is released by forming bonds than by breaking them
how to calculate overall energy change
energy need to break bonds - energy released by forming bonds
what does it mean if the overall energy change is POSITIVE
endothermic reaction
what does it mean if the overall energy change is NEGATIVE
exothermic reaction
