Rate and Extent of Chemical Change Flashcards
Rate of a chemical reaction
How fast reactants are turned into products
What a steeper line mean on graph of rate of reaction
Faster rate of reaction
What will happen to every line of rate of reaction graph
It will become less steep eventually as reactants are used up
What do flat lines show on rate of reaction graph
Reaction has finished
What do fastest reactions look like in rate of reaction graph
- steepest lines
- become flat in least time
Collision theory
- more collisions means faster rate of reaction
- particles must collide with enough energy for collision to be successful
Collision theory - what happens when particles collide
Energy is transferred
Activation energy
Minimum amount of energy particles need to break bonds in reactants to start reaction
Factors affecting rate of reaction
- temperature
- concentration of solution or pressure of gas
- surface area
- presence of catalyst
How does temperature affect rate of reaction
- increasing temperature increases rate of reaction
- temperature increases - particles move faster
- faster movement - more frequent collisions + more collisions with enough energy to make reaction happen
How does concentration of solution affect rate of reaction
- increasing concentration increases rate of reaction
- more particles in same volume of water/solvent
- more frequent collisions
How does gas pressure affect rate of reaction
- increasing pressure increases rate of reaction
- more particles in smaller space
- more frequent collisions
How does surface area affect rate of reaction
- increasing SA:V increases rate of reaction
- for same volume of solid, particles around it will have more area to work on
- solid reactants should be broken into smaller pieces
How does presence of catalyst affect rate of reaction
- using catalyst increases rate of reaction
- substance that speeds up rate of reaction without being used up in reaction itself (not part of reaction)
- decrease activation energy by providing alternative reaction pathway with lower activation energy
How to calculate mean rate of reaction
- quantity of reactant used / time taken
- quantity of product formed / time taken
units of rate of reaction
- cm³/s
- g/s
- mol/s
Ways of measuring rate of reaction
- colour change
- change in mass
- volume of gas given off
When can rate of reaction be measured with colour change
When initial solution is transparent and product is precipitate that clouds solution - turning it opaque
Measuring rate of reaction - colour change
- place laminated cross on surface
- place flask over cross
- observe + time how long it takes for cross to disappear
- faster disappearance = faster reaction rate
Problems with measuring rate of reaction with colour change
- subjective results - people may disagree on exact point mark ‘disappears’ when solution changes colour
- can’t plot graph of rate of reaction
How to measure rate of reaction with change in mass
- place flask on electric balance
- as gas released - mass on balance drops
- faster balance drops - faster reaction rate
When can rate of reaction be measured with change in mass
When reaction produces gas
Safety procaution for measuring rate of reaction with change in mass
Put cotton wool at top of flask to stop acid spitting out but allow gas to escape
Advantage of measuring reaction rate with change in mass
- most accurate of 3 methods
- can plot results on graph
Disadvantage of measuring rate of reaction with change in mass
Gas being released straight into room
How to measure reaction with volume of gas given off
- place bung + gas syringe in flask
- gas syringe measures volume of gas produced in reaction
- more mass given off in time interval - faster reaction rate
When can rate of reaction be measured with volume of gas given off
When reaction produces gas
Advantage of measuring rate of reaction with volume of gas given off
- gas syringes accurate - usually give volume to nearest cm³
- can plot results on graph
Disadvantage of measuring rate of reaction with volume of gas given off
Reaction too vigorous - can blow plunger out of end of syringe
How to find mean rate of reaction from graph
Gradient of that interval / whole reaction
How much of graph to find gradient of when calculating mean rate of whole reaction
From start to when graph becomes flat (reaction has already finished)
How to find rate of reaction at a particular point from graph
- draw tangent
- find gradient of tangent
Reversible reaction equation
A + B ⇌ C + D
Reversible reaction
Reaction where products can react to form reactants and will reach dynamic equilibrium
How does a reversible reaction reach equilibrium
- as reactants react, concentrations fall - forward reaction slows down
- as more products made, concentrations rise - backward reaction speeds up
- after while - forward + backward reaction happening at same rate (equilibrium)
What happens at equilibrium
Both reactions still happening but no overall effect, concentrations of reactants/products reached balance, won’t change
What must conditions be for equilibrium to be reached
Reaction must take place in closed system
Why can position of equilibrium lie to left/right
Amount of reactants/products not equal
What does it mean if equilibrium lies to right
Concentration of products greater than concentration of reactants
What does it mean if equilibrium lies to left
Concentration of reactants greater than concentration of products
What determines position of equilibrium
- temperature
- pressure if involving gases
- concentration of reactants products
If 1 direction of reaction is endothermic what must other direction be
Exothermic
If 1 direction of reaction is exothermic what must other direction be
Endothermic
Relationship between energy transferred in endo/exo in reversible reaction
Energy transferred from surroundings in endothermic equal to energy transferred to surroundings in exothermic
Le Chatelier’s Principle
If you try to change conditions of reversible reaction at equilibrium, system will try to counteract change
What happens if you decrease temperature of reversible reaction
- equilibrium will move in exothermic direction to produce more heat
- more products for exothermic reaction, fewer products for endothermic reaction
What happens if you increase temperature of reversible reaction
- equilibrium will move in endothermic direction to try decrease it
- more products for endothermic reaction, less products for exothermic reaction
What happens if you increase pressure of reversible reaction
Equilibrium tried to reduce it by moving in direction where there are fewer gas molecules
What happens if you decrease pressure of reversible reaction
Equilibrium tries to increase it by moving in direction where there are more gas molecules
What happens if you change concentration of reactants/products in reversible reaction
- system no longer at equilibrium
- will respond to bring it back
What happens if you increase concentration of reactants in reversible reaction
System makes more products to decrease it
What happens if you decrease concentration of products in reversible reaction
System less reactants to increase it
