Topic 6 - Rates of reaction Flashcards
(26 cards)
1
Q
What is the rate of a chemical reaction?
A
how fast the reactants are changed into products
2
Q
How do graphs link with rates of reaction?
A
- steeper line on graph, faster rate of reaction
- over time, line becomes less steep as reactants are used up
- quickest reactions have steepest lines - become flat in least time
3
Q
What does the rate of chemical reactions depend on? Collision theory
A
- collision frequency of reacting particles - more collisions = faster reaction
- energy transferred during collision - particles have to collide with more energy for collision to be successful
- particles need activation energy to break bonds in the reactants + start reaction
4
Q
What 4 things does the rate of reaction depend on?
A
- temperature
- concentration of solution or pressure of gas
- surface area
- presence of catalyst
5
Q
How does increasing temperature increase the rate of reaction?
A
- particles have more kinetic energy - most faster
- more frequent collisions
- faster they move - more energy they have - more collisions have enough energy to make reaction happen (activation energy)
6
Q
How does increasing concentration or temperature increase rate of reaction?
A
- more concentrated solution = more particles present in same volume of water
- increased pressure - same number of particles occupy similar space
- makes collisions between particles more frequent
7
Q
How does increasing surface area increase rate of reaction?
A
- if one of the reactants is solid - breaking it up into smaller pieces increases surface area to volume ratio
- for same volume of solid, particles around it have more area to work on - more frequent collisions
8
Q
How does using a catalyst increase the rate of reaction?
A
- catalyst is substance that speeds up reaction without being used up in reactions itself - not part of overall reaction equation
- catalysts reduce activation energy needed for reaction to occur - do this by providing alternative reaction pathway with lower activation energy
9
Q
Rate of reaction formula?
A
Amount of reactant used or amount of product form / time
10
Q
Three ways to measure rate of reaction?
A
- precipitation and colour change
- change in mass (usually gas given off)
- volume of gas given off
11
Q
How can you use precipitation and colour change to measure rate of reaction?
A
- record visual change in reaciton if initial solution is transparent and product is precipitate that clouds the solution
- observe mark through the solution - measure how long it takens for it to disappear - faster mark disappears - quicker the reaction
- if reactants are coloured and products colourless (or vice versa) - time how long it takes for solution to lose/gain its colour
12
Q
Two issues with using colour change to measure rate of reaction?
A
- results are subjective - different people pay not agree on exact point mark disappears
- cant plot a graph from results
13
Q
How can change in mass be used to measure rate of reaction?
A
- measuring speed of reaction that produces a gas - carried out on mass balance
- as gas is released, mass disappearing is measured on the balance
- quicker reading on balance drops - faster rate of reaction
- disadvantage of gas being released straight into room - advantage - most accurate method
14
Q
How can volume of gas be used to measure rate of reaction?
A
- involves use of gas syringe to measure volume of gas given off
- more gas given off during given time interval = faster rate of reaction
- if reaction is too vigorous - plunger can be blown out of end of syringe
15
Q
Magnesium and HCL method?
A
- add set volume of HCL to conical flask - place carefully on mass balance
- add some magnesium ribbon to acid - quickly plug flask with cotton wool
- start stopwatch + record mass on balance - take readings of mass at regular time intervals
- plot results in table and work out mass lost for each reading
- repeat with more concentrated acid solutions
- control variables - amount of magnesium ribbon, volume of acid kept same each time
- plot results on a graph
16
Q
Sodium thiosulphate and HCL method?
A
- add set volume of dilute sodium thiosulphate to conical flask
- place flash on piece of paper with black cross drawn on it - add some dilute HCL to flask and start stopwatch
- time how long it takes for black cross to disappear through cloudy sulfur
- repeat with solutions of either reactant at different concentrations
- keep volume and temperature of reactants the same for control variables
17
Q
What happens with reversible reactions and equilibrium?
A
- as reactants react, their concentrations fall - forward reaction slows down - as more products are made and their cocentrations rise - backward reaction speeds up
- after a while - forward reaction going at same rate as backward reaction - system is at equilibrium
- at dynamic equilibrium - borh reactions still happening - but no overall effect - concentrations of reactants and products reached a ballance - they wont change
18
Q
In what system is equilibrium reached?
A
- only reached if reversible reaction happens in closed system - none of the reactants or products can escape - nothing else can get it
19
Q
How does the position of equilibrium alter?
A
- equillibrium can be on the right or left
- when reaction is at equilibrium - doesnt mean amount of reactants and products are equal
- if equilibrium lies to right - concentration of products is greater than that of reactantsa
- if equilibrium lies to left - concentration of reactants greater than than of products
20
Q
What is position of equilibrium affected by?
A
- temperature
- pressure (only affects equilibria involving gases)
- concentration of reacts and products
21
Q
Are reversible reactions endothermic or exothermic?
A
- they can be both
- in reversible reactions - if reaction is endothermic in one direction, it is exothermic in the other
- energy transferred from surroundings by endothermic reaction is equal to energy transferred to surroundings during exothermic reaction
22
Q
Example of reversible reactions being exothermic and endothermic?
A
- thermal decomposition of hydrated copper sulfate
- if you heat blue hydrated copper (II) solution - it drives the water off and leaves white anhydrous copper (II) sulfate powder - this is endothermic
- if you add a couple drops of water to white powder - you get blue crystals back again - exothermic
23
Q
What is Le Chatelier’s principle?
A
- idea that If a system at equilibrium is disturbed, the system will shift to counteract the change and restore equilibrium
- can be used to predicts effect of any changes you make to a reaction system
24
Q
Le Chatelier’s Principle - Temperature?
A
- all reactions are exothermic in one direction and endothermic in the other
- if you decrease temperature - equilibrium will move in the exothermic direction to produce more heat - more products for exothermic reaction + fewer products for endothermic reaction
- if you raise temperature - equilibrium will move to the endothermic direction to try and decrease it - more products at endothermic reaction than exothermic reaction
25
Le Chatelier's Principle - Pressure?
- only affects equilibrium involving gases
- if you increase pressure - equilibrium tries to reduce it - moves to direction where there are fewer molecules of gas
-decrease pressure - equilibrium tries to increase it - moves to direction where there are more molecules of gas
26
Le Chatelier's Principle - Concentration?
- if you change concentration of either reactants or products - system no longer at equilibrium
- esystem responds to bring itself back to equilibrium
- if you increase concentration of reactants - system tries to decrease it by making more products
- if you decrease concentration of products - system tries to increase it by making more reactants
