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A Level Chem - OCR > Topic 3 > Flashcards

Topic 3 Flashcards

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1
Q

what is activation energy

A

minimum amount of energy required for a reaction to take place

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2
Q

what is average bond enthalpy

A

the energy required to break one mole of gaseous bonds. Actual bond enthalpies may differ from the average as the average bond enthalpy considers a particular bond in a range of molecules.

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3
Q

what does endothermic mean

A

a reaction that takes in energy
ΔH is ALWAYS positive
more energy is required to break the bonds than is released forming them

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4
Q

What is Enthalpy

A

value that represents heat content in a system

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5
Q

what is enthalpy change of combustion

A

when 1 mol of a substances completely combusts with oxygen under standard conditions

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6
Q

what is enthalpy change of formation

A

enthalpy change when 1 mol of a substance is formed from its elements under standard conditions

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7
Q

what is enthalpy change of neutralisation

A

The enthalpy change that takes place when
one mole of water is formed from a neutralisation reaction

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8
Q

what is enthalpy change (ΔH)

A

the change in the heat content of a system during a reaction. This can be determined from experimental results

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9
Q

What is the equation for enthalpy change and what does each symbol stand for

A

q = MCΔT
q = heat change of surroundings
m = mass
c = specific heat capacity
ΔT = change in temperature

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10
Q

what does exothermic mean

A

a reaction that gives out energy
ΔH is ALWAYS NEGATIVE
more energy is released by bonds forming than is when bonds are broke

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11
Q

What is enthalpy change of reaction

A

the enthalpy change that is associated with a
particular chemical equation.

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12
Q

What is Hess’s Law

A

the enthalpy change of a reaction is independent of the route it takes

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13
Q

what are standard conditions?

A

1atm and 298K

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14
Q

what is standard state

A

physical state of a substance under standard conditions

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15
Q

how do you calculate percentage error

A

% error = resolution of equipment/measured value x 100

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16
Q

when given ΔHf, what way do the arrows face on your enthalpy profile diagram

A

upwards

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17
Q

when given ΔHc what way do the arrows face on your enthalpy profile diagram

A

downwards

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18
Q

how do do you find ΔHr for ΔHf

A

-ΣΔHf (reactants) + ΣΔHf(products)

19
Q

how do do you find ΔHr for ΔHc

A

ΣΔHc (reactants) - ΣΔHc (products)
flip the sign on the products as you go against the arrow

20
Q

what is a system in a chemical reaction

A

the atoms and bonds involved in the reaction

21
Q

which way does the arrow for activation energy point on a reaction profile

A

always points up

22
Q

what are the advantages of a bomb calorimeter

A

minimises heat loss
pure oxygen is used which ensures complete combustion

23
Q

what are the reasons that experimental methods for enthalpy determination might not be accurate

A

heat loss to the surroundings
not in standard conditions
may not have a complete reaction

24
Q

why is using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation

A

bond enthalpies are a mean average for the same bond across different molecules whereas standard enthalpy apply to just that molecule therefore they are more accurate

25
Q

what is the equation to calculate rate

A

rate = change in concentration/time

26
Q

what is the unit for rate of reaction

A

mol dm ^-3 s ^-1

27
Q

what must particles do to react

A

collide with enough energy (activation energy) and correct orientation

28
Q

how many collisions result in a reaction

A

not many

29
Q

what are factors affecting rate of reaction

A
  • temperature
  • pressure
  • concentration
  • surface area
  • catalyst
30
Q

how does an increase in temperature increase rate

A

leads to an increase in rate
higher proportion of particles have more energy than activation energy
more successful collisions –> faster rate

31
Q

what effect does an increase in pressure/concentration have on rate

A

leads to an increase in rate as there are more particles in a given volume -> more frequent successful collisions = faster rate reaction rate

32
Q

how do you calculate rate from a concentration time graph

A

draw tangent
work out gradient of tangent using
change in y/change in x

32
Q

what variables can be monitored to calculate rate

A

conc of reactant/product
gas volume of product
mass of substances formed

33
Q

definition of a catalyst

A

substance that increases rate of reaction by lowering activation energy but is not used up in the reaction

34
Q

how does a catalyst work

A

provide another reaction pathway that has a lower activation energy
so particles have more energy - collide more often successfully - increased reaction rate

35
Q

what is dynamic equilibrium

A

occurs when rate of forwards reaction equals rate of backwards reaction and concentration of both remain constant in a closed system

36
Q

what factors affect position of equilibrium

A

temperature
pressure
concentration

37
Q

what is Le Chatelier’s principle

A

if a system is disturbed, the position of equilibrium shifts to oppose/reduce the change

38
Q

if a forwards reaction is endothermic
what effect would increasing temperature have on equilibrium

A

equilibrium would shift right - endothermic side - to lower the temperature

39
Q

why is high pressure good for production of some chemicals

A

increase in pressure means more particles in a given volume - more successful collisions - faster rate

40
Q

does a catalyst ahve an effect on position of equilibrium

A

no - effects both sides equally

41
Q

what is a heterogenous catalyst

A

a catalyst that is a different state to reactants

42
Q

what is a homogenous catalyst

A

a catalyst that is the same state as reactants

43
Q
A

A Level Chem - OCR (2 decks)

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