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A Level / GCSE Chemisty > 🟤 TOPIC 8 - Energetics I > Flashcards

🟤 TOPIC 8 - Energetics I Flashcards

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1
Q

Define enthalpy (H)

A

The heat energy stored within a chemical system (in bonds/intermolecular forces) measured at a constant pressure

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2
Q

Define exothermic

A

Energy released to surroundings, temp. goes up (of surroundings), 🔺H-
(More energy released from making bonds)
Reactants above products

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3
Q

Give examples of exothermic reactions

A
  • metals + acids
  • neutralisation
  • water + CaO (hand warmers)
  • combustion
  • oxidation
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4
Q

Define endothermic

A

Energy taken in from surroundings, temp. goes down (of surroundings), 🔺H+
(More energy taken in to break bonds)
Reactants below products

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5
Q

Give some examples of endothermic reactions

A
  • thermal decomposition
  • citric acid + baking soda
  • sports injury packs
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6
Q

Define energy level diagram

A

Shows relative energy of the reactants and products only

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7
Q

Define enthalpy profile diagram

A

Shows the relative energy of the reactants and products, and the activation energy [think ‘profile’]

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8
Q

Define activation energy

A

Energy required to break all the bonds in the reactants.
Minimum energy required by colliding particles for a reaction to occur

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9
Q

What are the standard conditions?

A

Pressure: 100kPa = 1atm
Temperature: 298K = 25°C
Concentration: 1moldm-3

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10
Q

What is a standard state?

A

The physical state (g/s/l) of a substance at standard conditions

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11
Q

Define standard enthalpy change of formation

A

(🔺Hf ) enthalpy change when one mole of a compound is formed from its constituent elements in their standard states under standard conditions
[may have to use fractions to get only 1 mole]

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12
Q

What is the standard enthalpy change of formation for all elements in their standard state?

A

0kJmol-1 (eg. S(s) -> S(s), O2(g) -> O2(g))

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13
Q

Define standard enthalpy change of combustion

A

(🔺Hc) Enthalpy change when one mole of a substance reacts completely with (burned in excess) oxygen under standard conditions (all reactants and products being in their standard states)

(Although it is burned, it is still standard conditions (so 298K) so the water made as a product is a liquid NOT a gas)

(0kJmol-1 for O2)

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14
Q

Define standard enthalpy change of reaction

A

(🔺Hr) Enthalpy change which accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions (all reactants and products being in their standard states)

(‘per mole’ refers to per mole of equation - use no. moles of limiting reactant)

🔺H is equal + opposite for reversible reactions

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15
Q

Define standard enthalpy change of neutralisation

A

(🔺Hn) Enthalpy change when one mole of water is produced by the reaction of an acid with a base under standard conditions (all reactants and products being in their standard standard states)

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16
Q

Define enthalpy change (🔺H)

A

The heat energy exchange with the surroundings in a reaction at S.T.P
(A circle with a line through it represents “standard”)

17
Q

Define calorimetry

A

A process that allows us to calculate the energy change (Q) of a reaction by measuring the temperature change of a known volume of water/solution

18
Q

What can be used as a calorimeter?

A

Either,
A polystyrene cup (when reaction is taking place in solution)
Or a copper (metal) can (when you are burning a fuel underneath to heat water)

19
Q

How would you calculate the energy change of a reaction?

A

Q = -mc🔺T
Q: energy change (J)
m: mass of WATER/SOLUTION (g) [1cm3 = 1g - but may need to do density calculation if different substance] - remember to account for what’s been added in
c: specific heat capacity (4.18J/g/K)
🔺T: change in temperature (°C/K) (Remember negatives if temp. goes down)

20
Q

How do you calculate enthalpy change?

A

Enthalpy change (🔺H) (kJmol-1) = Q (converted to kJ) / n
[where n = no. of moles of the limiting reactant / of the specific substance (eg. for 🔺Hn or 🔺Hc)

21
Q

What is the specific heat capacity?

A

The amount of energy needed to raise a temperature of 1g of a substance by 1 degree/1 kelvin (4.18Jg-1K-1)

22
Q

What does a catalyst do?

A

A catalyst increases the rate of a reaction by providing an alternative reaction pathway with a lower activation energy. The catalyst is chemically unchanged after a reaction.

23
Q

Why will a result in a practical to determine enthalpy change be different to the actual value?

A
  1. Not at STP
  2. Heat loss
24
Q

Define Hess’s Law

A
  • Takes the same amount of energy for a reaction to take place no matter the route taken
  • The enthalpy change for a reaction (🔺Hr) is independent of the route taken
25
Q

Which way do the arrows point for 🔺Hf in a Hess cycle?

A

Upwards (‘forming’/building up)

26
Q

Which way do arrows point for 🔺Hc in a Hess cycle?

A

Down (‘burning down’)

27
Q

What goes at the bottom for 🔺Hf in a Hess cycle?

A

The constituent elements

28
Q

What goes at the bottom for 🔺Hc in a Hess cycle?

A

The combustion products of the reactants (or products) - eg. When reacted with O2

29
Q

What do you always need to remember for enthalpy change?

A

Signs (+ / -)

30
Q

Define mean bond enthalpy

A

The average energy required/released when 1 mole of covalent bonds are broken/formed (broken via homolytic fission)
Bond enthalpies are equal and opposite (require same amount of energy to break as they release when formed)
Bond enthalpy tells you the strength of the bond

31
Q

Why is 🔺H not as accurate when using mean bond enthalpies as when using Hess’s law?

A

They are only average bond enthalpies, which actually differ slightly between molecules

32
Q

How do you do calculations using mean bond enthalpies? (ie. Calculating all the bond enthalpies in a molecule using values given in a table)

A

BREAKING TAKE MAKING

33
Q

If when using calorimetry the water’s initial temperature is quite high, how will this effect the value calculated for the enthalpy change of combustion, and why?

A

The enthalpy change of combustion would be less exothermic than if the water was at room temperature.
This is because some energy will be used to boil the water.

34
Q

If when using calorimetry the water is heated for longer, how will this effect the value calculated for the enthalpy change of combustion, and why?

A

The enthalpy change of combustion will be less exothermic as there will be a greater heat loss to the surroundings due to the higher temperature
OR
The enthalpy change of combustion will be the same as both the energy change (/temperature change) and mass of the fuel burned will be higher

A Level / GCSE Chemisty (26 decks)

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