AS CHEMISTRY OCR SALTER B - REACTION RATES

NOTES ON REACTION RATES & DYNAMIC EQUILIBRIA & LE CHATELIERS PRINCIPLE - INCLUDES EXAMPLES.

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AS CHEMISTRY OCR SALTERS B NOTES
UNIT 2 TOPIC 2 - THE ATMOSHPERE: REVERSIBLE REACTIONS
REVERSIBLE REACTIONS CAN REACH DYNAMIC EQUILIBRIUM
- the symbol used to represent reversible reactions (this isn't the exact symbol,
just the best I could do)
Reactants get used up ­ forward reaction slows down.
More product formed ­ reverse reaction speeds up.
Eventually both reactions will go at the same rate ­ product and reactant amounts
won't change, so it looks like nothings happening.
THIS IS CALLED DYNAMIC EQUILIBRIUM
(this only happens in a closed system!)
LE CHATELIERS PRINCIPLE ­ PREDICTS WHAT WILL HAPPEN IF CONDITIONS CHANGE
"IF THERE IS A CHANGE IN PRESSURE, TEMP. OR CONCENTRATION THE EQUILIBRIUM WILL
SHIFT TO TRY AND COUNTERACT THAT CHANGE."
Changing pressure, temp. or concentration alters the position of the equilibrium ­ so
you'll get different amounts of products and reactants at equilibrium.
EQUILIBRIUM MOVES TO THE LEFT ­ MORE REACTANTS
MOVES TO THE RIGHT ­ MORE PRODUCTS
Raised temperatures ­ equilibrium shifts to cool it down
Raised concentration & pressure ­ equilibrium shifts to reduce it
CATALYSTS DO NOT AFFECT THE POSITION OF EQUILIBRIUM
EQUILIBRIUM IS JUST REACHED FASTER.
RULES FOR LE CHATELIERS PRINCIPLE:
EXAMPLE EQUATION: 2SO(g) + O(g) 2SO(g)
CONCENTRATION:
1. INCREASED CONCENTRATION REACTANTS ­ equilibrium will try to remove extra
reactant by making more product (SO) ­ SHIFTS TO THE RIGHT.
2. INCREASED CONCENTRATION PRODUCT ­ tries to remove extra product, so reverse
reaction goes faster ­ SHIFTS TO THE LEFT.
3. DECREASING CONCENTRATION ­ opposite effect of increasing.
PRESSURE: (ONLY AFFECTS EQUILIBRIA WITH GASES)
1. INCREASING PRESSURE ­ equilibrium shifted to side w/ least gas molecules ­ reduces
pressure.
2. DECREASING PRESSURE ­ equilibrium shifted to side w/ most gas molecules ­ raises
pressure.
IN THE EXAMPLE EQUATION THERE ARE 3 MOLES ON THE LEFT & 2 ON THE RIGHT. AN
INCREASE IN PRESSURE SHIFTS EQUILIBRIUM TO THE RIGHT.
TEMPERATURE: (INCREASE ­ ADD HEAT, DECREASE ­ REMOVE HEAT)
1. INCREASING TEMP. ­ equilibrium shifts to endothermic end of reaction to absorb the
heat.
2. DECREASING TEMP. ­ equilibrium shifts to exothermic end of reaction to replace the
heat.
EG ­ FORWARD REACTION IS EXOTHERMIC & REVERSE IS ENDOTHERMIC. INCREASE
IN TEMPERATURE = EQUILIBRIUM SHIFTS TO LEFT TO ABSORB EXTRA HEAT.

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