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chemistry a - level > Topic 8 - Energetics I > Flashcards

Topic 8 - Energetics I Flashcards

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1
Q

What are the standard conditions for a reaction?

A
  • 298K
  • 100kPa
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2
Q

What is the definition of the Standard Enthalpy change of Formation?

A

The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions of 100kPa and a specified temperature (normally 298K)

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3
Q

What is the definition of the Standard Enthalpy change of Combustion?

A

The enthalpy change when one mole of a compound is completely burned in excess oxygen, converted to standard conditions of 100kPa pressure and a specified temperature

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4
Q

What is the definition of the Standard Enthalpy change of Reaction?

A

The enthalpy change when the chemical equation molar quantities react under standard conditions and in their standard states

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5
Q

What is the definition of the Standard Enthalpy change of Neutralisation?

A

The enthalpy change when the acid and alkali, in the equation for the reaction, neutralise each other to form one mole of water

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6
Q

What is the definition of specific heat capacity?

A

The energy required to raise the temperature of one gram of a substance by 1K without a change in state

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7
Q

What is the equation for the energy change of a substance in a Calorimetry experiment?

A

Q = mc(delta)T

m - mass of water/solution (g)
c - specific heat capacity of water
(delta)T - temperature increase or decrease caused by reaction (K)

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8
Q

What must we do when using the Q = mc(delta)T equation in Calorimetry calculation questions?

A

Ensure the units are all in the correct form

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9
Q

What is the equation used to calculate enthalpy change?

A

(delta)H = Q/n

(delta)H - enthalpy change (Kjmol^-1)
Q - energy change (Kj)
n - number of moles of the limiting reagent

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10
Q

What units is Q in when you use the equation Q = mc(delta)T?

A

Joules, J

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11
Q

What does the n in the equation (delta)H = Q/n stand for?

A

Number of moles of the limiting reagent (the reagent NOT in excess)

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12
Q

What do you have to do between using the equation Q = mc(delta)T and (delta)H = Q/n?

A

Convert your value of Q from Joules, J, to Kilojoules, Kj

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13
Q

What must your value of enthalpy change always have?

A

a sign, + or -, to indicate whether the reaction is exothermic or endothermic

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14
Q

What sign does your enthalpy change value have when the reaction is exothermic?

A

-ve , energy is released to the enivronment so your products have less energy than your reactants

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15
Q

What sign does your enthalpy change value have when the reaction is endothermic?

A

+ve, energy is taken in from the environment so your products have more energy than your reactants

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16
Q

What is the definition of the limiting reagent?

A

The reagent which is not in excess and, therefore, limits how far the reaction can proceed

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17
Q

What are the three approximations that we make when doing calorimetry calculations?

A
  1. The mass = mass of solution (the value of m you use in the equation)
  2. The density of water = density of your solution, 1gdm^3, therefore, the volume of your solution will equal its mass
  3. The specific heat capacity of water = specific heat capacity of the solution
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18
Q

What does the value of m in the equation Q = mc(delta)T stand for?

A

The mass of your solution, it DOES NOT include the mass of any solid added

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19
Q

What is the specific heat capacity of water?

A

4.18Jg^-1K^-1

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20
Q

What is your value of m in the equation equation Q = mc(delta)T when you have two solutions?

A

The sum of the two solutions

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21
Q

What do you have to do when you are not given which solution/substance is in excess in a Calorimetry calculation question?

A

You need to figure out the reacting moles of each reactant:
- If the reactants have the same number of moles, you use that number
- If the reactants have different numbers of moles, then you use the lower value (as this is the limiting value)

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22
Q

What are the axis for an energy level diagram?

A

X-AXIS - Progress of Reaction
Y-AXIS - Energy

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23
Q

What are the axis for a reaction profile?

A

X-AXIS - Progress of Reaction
Y-AXIS - Energy

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24
Q

What is the difference between an energy level diagram and a reaction profile?

A

ENERGY LEVEL DIAGRAM
- Shows the energies of the products and the reactants
- Shows the enthalpy change

REACTION PROFILE
- Shows the energies of the products and reactants
- Shows the enthalpy change
- Shows the activation energy (by a curve)

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25
Q

What does an endothermic reaction profile/energy level diagram look like?

A

The reactants have a lower energy than the products

26
Q

What does an exothermic reaction profile/energy level diagram look like?

A

The reactants have a higher energy than the products

27
Q

What do the big numbers in front compounds/elements in a reaction show?

A

The reacting moles NOT the actual moles

28
Q

In terms of bonds what does it mean when a reaction is endothermic?

A

More bonds are broken than made

29
Q

In terms of bonds what does it mean when a reaction is exothermic?

A

More bonds are made than broken

30
Q

Bond making…

A

releases energy

31
Q

Bond breaking…

A

requires energy

32
Q

What is the definition of the mean bond enthalpy?

A

The enthalpy change required to break one mole of the bond into gaseous atoms averaged over different molecules

33
Q

What is the equation for the enthalpy change of a reaction using bond enthalpies?

A

Bond breaking - Bond making = enthalpy change

34
Q

Describe how to undertake a Calorimetry practical in order to determine the enthalpy change of the reaction?

A
  • Record the mass of your solid being used using the weigh by difference method - weigh the bottle + solid on a balance, pour your solid into your calorimeter then weigh the bottle once more. The difference in mass is the mass of solid you have added to your calorimeter.
  • Pour a known volume of solution into a beaker
  • Stir your solution and record its initial temperature
  • Add your solution to the calorimeter and stir
  • Record the highest temperature reached
35
Q

What is the definition of the bond dissociation enthalpy?

A

The energy required to break one mole of the bond to give its separated atoms in the gaseous state

36
Q

Describe how to draw a calorimeter?

A

Use a polystyrene cup with a thermometer placed in it in order to record the temperature change - could also use a lid and further insulating with cotton wool around the outside to prevent heat loss to the surroundings

37
Q

What do you have to do when determining the maximum temperature change in a calorimetry experiment?

A
  • Plot a graph of temperature change against time
  • Collect data values before you add the second reactant and after
  • Your graph should have a straight line of temperatures before adding the second reactant
  • At the time you add your second reactant your next data point should be much higher/lower
  • Following this, your data points should reach a peak and then slowly increase/decrease proportionally with time as heat is lost or gained from the surroundings
  • To calculate the maximum temperature change of your experiment you need to extrapolate your data values from after you added the second reactant. To do this you would draw a straight line through the data points, from when the relationship has become proportional, and draw the line back to the moment you added the second reactant
  • This temperature change between your extrapolated temperature and your original temperature is the maximum temperature change (the value you will use in your calculation)
38
Q

What type of reactions are exothermic?

A
  • Combustion
  • Respiration
  • Neutralisation
39
Q

What type of reactions are endothermic?

A
  • Decomposition
  • Photosynthesis
40
Q

Why can you not measure the enthalpy change of decomposition reactions directly?

A

The decomposition takes place at high temperatures which requires heating. Therefore, you cannot measure the enthalpy change directly as you cant measure how much energy you are putting into the reaction

41
Q

How can you minimise sources of error in a calorimetry experiment?

A
  • Add a lid to your calorimeter
  • Insulate the calorimeter further by surrounding it in cotton wool
42
Q

How do you calculate % (U)?

A

(uncertainty / reading value) x 100

43
Q

How do you determine the uncertainty of different apparatus?

A

Its resolution

44
Q

What do you have to do to your uncertainty if you have taken two readings?

A

Double it

45
Q

How can reduce the size of percentage uncertainties?

A
  • Do the experiment on a larger scale
  • Use apparatus with a smaller resolution
46
Q

What could the possible reasons be of a reaction being less exothermic than its accepted value?

A
  • No lid used
  • No insulation of cup
  • Irregular stirring
  • Particles on top could have had sufficient energy to boil + evaporate
47
Q

Describe the set up for determining the enthalpy of combustion of alcohols?

A
  • Use a spirit burner with an alcohol inside and place this on a heatproof mat
  • Place your calorimeter on a tripod above the spirit burner
48
Q

What are some control variables when conducting the experiment of determining the enthalpy of combustion of alcohols?

A
  • Distance between spirit burner and calorimeter
  • Type of calorimeter
  • Thermometer not touching the glass
49
Q

Why do you need to record the mass of the spirit burner with its cap on?

A
  • To stop the chemical reaction between the alcohol and the burner
  • To prevent fumes being produced, mass loss (as alcohols are very volatile and so would react with oxygen in the air)
50
Q

State Hess’s Law

A

if a reaction can take place by more than one route, and the initial and final conditions are all the same, then the total enthalpy change is the same for each route

51
Q

Why do we use Hess’s Law?

A

As sometimes the enthalpy change of a reaction cannot be measured directly

52
Q

Why are two enthalpy values for the same reaction likely to be different?

A

As the bond energies vary in different environments

53
Q

Why do we use extrapolation?

A

As the maximum temperature reached on the graph will be inaccurate, lower than its actual value due to cooling of the reaction mixture from the surroundings. Therefore, by extrapolating your graph you are finding the temperature change when no cooling has occurred, therefore, a more accurate result.

54
Q

How would using a copper calorimeter instead of a glass calorimeter, change the enthalpy change of the reaction?

A

The metal copper is a good conductor of heat, therefore, there will be more heat loss to the cup and the surroundings. The (delta)H(standard)c would therefore be more negative.

55
Q

How does carbon chain length affect the energy released during combustion?

A

Increasing carbon length -> Increases energy released from bonds when they are broken

56
Q

Why would the energy released during combustion be less for alcohols in comparison to alkanes?

A

The alcohols are already partially oxidised

57
Q

What do you have to account for when doing Hess Cycle calculations?

A

The reacting moles - you have to ensure your equation is balanced and from this need to multiply the enthalpy changes you are provided with by the reacting moles (as enthalpy values describe the energy released for one mole of a substance)

58
Q

What is a system?

A

The chemical reaction

59
Q

What are the surroundings?

A

Where the chemical reaction occurs

60
Q

Why are bond enthalpies less accurate values than enthalpies of formation/combustion etc.?

A

As bond enthalpies are a calculated average

61
Q

What is the symbol that shows the reaction is under standard conditions?

A

A circle with a line horizontally through it

chemistry a - level (26 decks)

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