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A-level Chemistry Paper 1- Periodic Table and Energy > Enthalpy > Flashcards

Enthalpy Flashcards

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1
Q

What is the definition of delta change?

A

The heat energy change measured at constant pressure

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2
Q

What can exothermic reactions be identified by?

A

Detecting a temperature increase

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3
Q

What can endothermic reactions be identified by?

A

Detecting a temperature decrease

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4
Q

What does breaking bonds do?

A

Absorbs energy

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5
Q

what does forming bonds do?

A

Releases energy

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6
Q

What type of reaction is intermolecular and covalent bond breaking?

A

Endothermic

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7
Q

What type of reaction is forming bonds?

A

Exothermic

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8
Q

When does an exothermic reaction occur?

A

When more energy is released from bond making than is absorbed during bond breaking

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9
Q

what happens when bonds are formed?

A

Energy is released (exothermic, delta H is negative)

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10
Q

What happens when bonds are broken?

A

Energy is absorbed (endothermic, delta H is positive)

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11
Q

what happened in most actual chemical reactions?

A

Both bond breaking and bond forming occur so which ever process is greater magnitude will result in the overall enthalpy change

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12
Q

What is the definition of exothermic reactions?

A

A reaction will be exothermic when more energy is released in bond forming than is absorbed in bond breaking.
This results in an overall negative delta

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13
Q

what is the definition of an endothermic reaction?

A

A reaction will be endothermic when more energy is absorbed in bond breaking than is released in bond forming.
This results in an overall positive delta H

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14
Q

what reactions are exothermic?

A

-All combustion (burning) reactions
-Test tube heating up when you add a metal such as magnesium to an acid such as HCl

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15
Q

what are examples of endothermic reactions?

A

-Photosynthesis
-The decomposition of carbonates
-Cool packs used on the football pitch
(less obvious in the real world)

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16
Q

What is activation energy (Ea) of a reaction?

A

the minimum energy required for a reaction to occur

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17
Q

What is the activation energy (Ea) oh of a reaction?

A

-Minimum energy required for a reaction to occur
-always positive

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18
Q

what type of reactions oxidation (combustion of CH4)?

A

Exothermic

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19
Q

What type of reaction is thermal decomposition?
(CaCo3 —> CaO + CO2)

A

Endothermic

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20
Q

What type of reaction is respiration?

A

Exothermic

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21
Q

what type of reaction is acids reacting with metals?
(Li + HCl —> LiCl + 1/2 H2)

A

exothermic

22
Q

what type of reaction is photosynthesis?

A

endothermic

23
Q

What does the size of the enthalpy change of a particular chemical reaction depend upon?

A

temperature; pressure; physical state (s,l,g); amount of reactants

24
Q

what are the standard conditions that all energy changes must be measured under?

A

-temperature= 298K
-Pressure= 100kPa
-Solution concentration= 1 moldm-3
All substances should be in their standard states at 298K and 100 kPa

25
Q

what are any enthalpy changes measured on standard conditions termed?

A

A standard enthalpy change

26
Q

what is the definition of enthalpy change of formation?

A

The enthalpy change for the formation of 1 mole of a compound from its elements.

27
Q

what is an example of the enthalpy change of formation?

A

C (s) + 2H2 (g) —> CH4 (g)

28
Q

what is the definition of the enthalpy change of combustion?

A

The enthalpy change for the complete combustion of 1 mole of a substance.

29
Q

what is an example of the enthalpy change of combustion?

A

CH4 (g) + 2O2 (g) → CO2 (g) + 2Н2O (l)

30
Q

what is the enthalpy change of neutralisation?

A

The enthalpy change for the formation of 1 mol of water from neutralisation.

31
Q

What is an example of the enthalpy change of neutralisation?

A

HCl (aq) + NaOH (aq) —> NaCl (aq) + H2O (l)

32
Q

what is definition of the standard enthalpy of reaction?

A

The enthalpy change associated with the stated equation

33
Q

what is an example of the standard enthalpy reaction?

A

N2 (g) + 3H2 (g) —> 2NH3 (g)

34
Q

what is the average bond enthalpy?

A

The enthalpy change for the breaking of one mole of bonds in gaseous molecules

35
Q

what is an example of average bond enthalpy?

A

1/3 NH3 (g) —> 1/3 N(g) + H (g)

36
Q

what type of reaction is boiling water? (going from liquid to gas)

A

Endothermic

37
Q

how many moles are burned in combustion?

A

1 mole

38
Q

Why is the bond enthalpy of C-H bond in methane different as that of a C-H bond in methanol?

A

Methanol is a bigger molecule than methane

39
Q

how is energy (q) calculated?

A

Q (energy) J = MCT delta T

40
Q

what does Q stand for?

A

Heat energy lost or gained (for the moles in the experiment), in J

41
Q

what does M stand for?

A

Mass of substance heated or cooled (where is thermometer is in the experiment)

42
Q

What does C stand for?

A

specific heat (for water 4.18 g-1 k-1)

43
Q

What is the unit for delta change temp?

A

K

44
Q

what is specific capacity defined as?

A

The quantity of energy required to heat 1 g of substance by 1°K

45
Q

What are the steps to an experimental calculation of Delta H?

A
  1. Calculate energy (q)
  2. Convert to KJ
  3. Calculate moles of something
  4. calculate Delta H.
  5. Sign, significant figures and unit.
46
Q

How is uncertainty calculated?

A

N.O of x error/ amount measured

47
Q

What is Hess’s law?

A

-(consequence of the first law of thermodynamics- energy cannot be created or destroyed)- if a reaction can take place by two routes, and the starting finishing conditions are the same, the total enthalpy is the same for each route

48
Q

what are the steps of finding an enthalpy change of combustion?

A

1.Write the equation for the enthalpy we need to calculate. Make the top arrow long.
2.At the bottom, write the combustion products of carbon dioxide and water which are common to both sides of the equation and draw in the arrows to make a triangle.
3. Add the values
4. Applying Hess’ Law to calculate the missing value on the top equation.

49
Q

what are the steps of finding an enthalpy change of formation?

A

1.Write the reaction for the enthalpy we need to calculate
2. Add the elements for formation which are common to both sides of the - equation
3. Complete the triangle by adding the values and arrows
4. Applying Hess Law will calculate the value as required

50
Q

what is the definition of average bond enthalpy?

A

The energy required to break one mole of a specific type of bond in a gaseous molecule

51
Q

what is an average bond enthalpy calculated from?

A

the actual bond enthalpy e.g. C-H bond enthalpy in methane is slightly different to the C-H bond enthalpy in ethane

A-level Chemistry Paper 1- Periodic Table and Energy (7 decks)

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