Topic 7 - Rates of reaction and Energy Changes Flashcards
What is the formula to calculate the rate of reaction
amount of reactant used or amount of product formed / time
How we measure rate of reaction with a precipitation reaction
-mix any two clear solutions to form a cloudy precipitate solution ontop of paper with a pencil cross on
-measure how long it takes for the cross to become invisible/ obscured
-faster = faster reaction
(is subjective however as people might not agree on when it disappears)
How can we measure the rate of reaction with change in mass (where gas is given off)
- reaction carried out on mass balance (where gas is produced)
- carry out in conical flask on mass balance with cotton wool to only let gas out and no liquids or anything flying out in reaction
- you know its finished when mass doesnt change
- plot change in mass against time graph
(gas may be harmful so do in fume cupboard)
How can we measure rate of reaction with volume of gas given off?
- use a gas syringe to measure volume of gas given off
- more gas given off in less time means faster rate
- you know its finished when no more gas is produced
- plot gas volume against time
-make sure you use the right size syringe as if the reaction is too vigorous then it might blow the plunger off the end.
describe an experiment to see how surface area effects the rate
- using different sized marble chips in HCl
- measure the volume of gas poduced into gas syringe at regular time intervals and plot time against volume graph
- repeat with diiferent sized chips
what would having finer particles have an effect on the rate of reaction?
The smaller the particles the faster the rate as theres a larger surface area for the reaction to take place on
Describe an experiment to measure the rate of reaction with varying temperatures
- measure out fixed volumes of sodium thiosulfate and hydrochloric acid using a measuring cylinder
- use hot water bath to gently heat solutions to desired temperatures before mixing
- mix together and time how longer you can no longer see the cross on paper beneath flask
- repeat for several tempertures
If the gradient on a graph is steeper what does it show
a faster rate of reaction
What must particles do in order to react?
collide with each other
What does the rate of reaction depend on
How often they collide (frequency)
The energy transferred in a collision (minimum energy is activation energy)
The more collisions the …… the rate of reaction
higher
What does increasing the temperature do to the rate of reaction? and why
It increases the rate of reaction because the particles have more energy and move faster and therefore have more successful collisions
What does increasing the concentration (or pressure in gases) do to the rate of reaction? and why
If it is more concentrated then there are more particles in the same volume so it makes collisions more likely
In a gas the particles are more crowded in a smaller area so frequency of collisions will increase –so rate increases
What does increasing the surface area do to the rate of reaction? and why
Smaller solid particles means a larger surface area as there is more space for the reaction to take place on so frequency of collisions increases
rate of reaction is faster for solids with a larger surface area to volume ratio
What does a catalyst do to the rate of a reaction
- (wont change products)
- work by decreasing the activation energy needed for reaction by providing ALTERNATIVE REACTION PATHWAY so minimum amount of energy is needed for reaction
- catalysts are very particular though about which reactions they catalyse
What do enzymes do
control living cell reactions
- they are biological catalysts
- used in respiration, photosynthesis, and protein synthesis
- yeast cell enzymes used in fermentation
- enzymes have a high specificity for the reaction they catalyse
Define an exothermic reaction
a reaction which gives out energy to surroundings usually from heat and is shown by a rise in temperature of the surroundings
Define an Endothermic reaction
a reaction which takes in heat from the surroundings in the form of heat and is shown by a fall in temperature of surroundings
What is the activation energy
The minimum amount energy needed to start a reaction to make BONDS BREAK
describe a reaction profile for an exothermic reaction
draw
- reactant line higher
- which then increases a bit before decreasing and making products
- product line is lower
- energy after is lower because energy is given out
describe a reaction profile for an endothermic reaction
draw
- reaction line lower
- which then increases lots and then decreases a bit before making the products line
- product line is higher in terms of energy as energy taken in
When bonds are broken its an …..reaction
And why?
endothermic
because greater energy is used to break bonds than energy released by forming them
when bonds are formed its an….. reaction
And why?
exothermic
because energy released by forming bonds is greater than energy to break them
what is the formula to work out bond energy calculations
Overall energy change = energy required to break bonds - energy required to make bonds
