Equilibria - 5.5
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What determines whether a reversible reaction goes forwards or backwards at any point in time?
depends on the amounts of reactants and products and the conditions (such as temperature and pressure)
What does the enthalpy change for a reversible reaction always refer to?
always refers to the forwards reaction for the backwards reaction, enthalpy change is the same number but with the opposite sign
How do you make the dehydration of copper sulphate crystals reaction go forwards?
heat the hydrated copper sulphate crystals the crystals turn from blue to white as they lose all their water
How do you make the dehydration of copper sulphate crystals reaction go backwards?
add water to the anhydrous copper sulphate white powder absorbs the water and turns blue in the process
How do you make the decomposition of ammonium chloride reaction go forwards?
heat the NH4Cl(s) nothing appears to change because NH3 and HCl are both colourless gases
How do you make the decomposition of ammonium chloride reaction go backwards?
allow NH3 and HCl to mix in the absence of heat white NH4Cl forms immediately wherever the cold gases have met
How can the decomposition of ammonium chloride be demonstrated?
can be demonstrated all-at-once by gently heating NH4Cl in a test tube: * The NH4Cl at the bottom of the tube decomposes to form invisible NH3 and HCl. * The invisible NH3 and HCl cool as they move up the tube, recombining to form NH4Cl
What is dynamic equilibrium?
When a reversible reaction is left in a sealed container, the forwards and backwards reactions will eventually ‘balance out’. From that point onwards, the mixture of reactants and products in the container is referred to as being in dynamic equilibrium
What is meant by the term dynamic?
the forwards and backwards reactions are still happening, just at the same rate
What is meant by the term equilibrium?
the concentration of the reactants and products remain unchanged
What is the equilibrium position?
the resulting balance of products and reactants in an equilibrium mixture is called the equilibrium position
What is meant by the terms ‘on the right’ and ‘on the left’?
‘on the right’ - more products than reactants in the mixture and the products are on the right-hand-side of the equation ‘on the left’ - products are on the left-hand-side of the equation
What is it called when there are more reactants than products in the mixture?
equilibrium is ‘on the left’ the greater the imbalance in reactants and products, the further an equilibrium is to the left or right
What is meant by the term equilibrium shifts?
The position of an equilibrium can be shifted left or right by changing any of these variables: amounts of reactants or products gas pressure temperature
What does Le Chatelier’s principle state?
The equilibrium shifts to counteract any change imposed in it.
Change: adding a substance
Shift direction: in whichever direction uses it up
Change: removing a substance
Shift direction: in whichever direction creates it
Change: increasing gas pressure
Shift direction: to whichever side has fewest moles of gas
Change: decreasing gas pressure
Shift direction: to whichever side has most moles of gas
Change: increasing temperature
Shift direction: to whichever direction is endothermic
Change: decreasing temperature
Shift direction: in whichever direction is exothermic
Concentration changes
If you add something, the equilibrium will shift to remove the extra you just added. If you remove something, the equilibrium will shift to create more of it.
Gas pressure changes
If you increase gas pressure, the equilibrium will shift to lower it back down. If you decrease gas pressure, the equilibrium will shift to bring it back up The equilibrium can affect the gas pressure by changing the moles of gas.
Temperature changes
If you increase temperature, the equilibrium will shift to cool it back down. If you decrease temperature, the equilibrium will shift to heat it back up