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A-Level Chemistry > 9 Enthalpy > Flashcards

9 Enthalpy Flashcards

1
Q

What is enthalpy?

A
  • Molar heat energy change
  • The measure of the heat energy in a chemical system
  • Energy stored within bonds
  • Enthalpy cannot be measured but change can
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2
Q

What is a chemical system?

A

The atoms, molecules, or ions making up the chemicals

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3
Q

What is enthalpy change?

A

Difference in the enthalpies of the reactants and products
^H = H(products) - H(reactants)

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4
Q

What is the law of conservation of energy?

A

Energy cannot be created or destroyed

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5
Q

What happens during a chemical reactions involving an enthalpy change?

A

Heat energy is transferred between the system and the surroundings:
- System is the reactants and products
- Surroundings is what envelopes the system e.g. apparatus, lab
- Universe is everything

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6
Q

What is an exothermic change?

A
  • System releases heat energy to the surroundings
  • Energy loss by system = energy gain by the surrounding
  • Enthalpy change is negative
  • The temperature of the surroundings increases as they gain energy
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7
Q

What is an endothermic change?

A
  • Energy transfer from the surroundings to the system
  • Chemical system takes in heat energy from the surroundings
  • Energy gain by system = energy loss by surroundings
  • The temperature of the surroundings decreases as they lose energy
  • Enthalpy change is positive
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8
Q

What is activation energy?

A

Definition: The minimum amount of energy for a successful collision to take place resulting in a reaction
- The energy input required to break bonds

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9
Q

What is the standard pressure?

A

100 kPa

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10
Q

What is the standard temperature?

A

298K (25C)

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11
Q

What is the standard concentration?

A

1 moldm-3

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12
Q

What is the standard state?

A

The physical state of a substance under standard conditions

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13
Q

What is the standard enthalpy change of reaction?

A

The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states
- Always refers to a stated equation

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14
Q

What happens if the balancing numbers are doubled in a reaction?

A

The standard enthalpy change is doubled

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15
Q

What is the standard enthalpy change of formation?

A

The enthalpy change when:
- one mole of a compound
- is formed from its elements
- under standard conditions
(with all reactants and products in their standard states)

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16
Q

What is the standard enthalpy change of combustion?

A

The enthalpy change when:
- one mole of a substance
- reacts completely with oxygen
- under standard conditions
(with all reactants and products in their standard states)

17
Q

What are the products of combustion?

A

The oxides of the elements in the substance
- Carbon dioxide and water

18
Q

What is the standard enthalpy change of neutralisation?

A

The enthalpy change when:
- one mole of water
- is formed when an acid and a base react
- under standard conditions
(with all reactants and products in their standard states)

19
Q

How do you convert from Kelvin to celsius?

A

C = K - 273

20
Q

What is the equation for an energy change?

A

q = mc^T

21
Q

What is the ‘m’ in the energy change equation?

A

The mass of the material changing temperature

22
Q

What is the ‘c’ in the energy change equation?

A

The specific heat capacity - the energy required to raise the temperature of 1g of a substance by 1K

4.18Jg-1K-1

23
Q

Describe a method to find the enthalpy change of the combustion of methanol

A
  1. Measuring cylinder to measure 150cm3 of water into the beaker. Record the initial temperature
  2. Weight the spirit burner containing methanol with the lid on
  3. Place the spirit burner under the beaker. Light the burner and burn the methanol whilst stirring the water with the thermometer
  4. Heat until 30C rise and record the highest temperature reached
  5. Reweigh the spirit burner
24
Q

What are the calculation to find the enthalpy change in a methanol combustion reaction?

A
  1. Energy change (q =mc^T) of water
  2. Mol of methanol burnt
  3. Enthalpy change
25
Q

Give experimental errors of a combustion reaction which would affect the enthalpy change value

A
  • Heat loss to the surroundings
  • Incomplete combustion of methanol so CO or C produced instead of CO2
  • Evaporation of methanol from wick
  • Non-standard conditions
26
Q

What are the 2 calorimetry equations?

A

q=mc^T

^H = -q/1000n

27
Q

How can heat loss be minimised?

A

Insulating cup
No draughts
Small distance between reaction and water

28
Q

What are the key points of a cooling curve?

A
  • Monitor temperature before initiating the reaction
  • Measure the temperature regularly even after the reaction has finished
  • Identify the section where the solution is cooling due to heat loss and extrapolate back to the start of the reaction
29
Q

What is average bond enthalpy?

A

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

30
Q

Key points about average bond enthalpy

A
  • Energy is always required to break bonds
  • Bond enthalpies are always endothermic
  • Bond enthalpies always have a positive enthalpy value
31
Q

What type of energy required to break bonds?

A

Bond breaking is endothermic
^H is positive

32
Q

What is the energy required to make bonds?

A

Bond making is exothermic
^H is negative

33
Q

How do you find the enthalpy change of reaction?

A

Calculating the bond enthalpies of the bonds in the reactants and the products
= sum of bond enthalpies in reactants - sum of bond enthalpies in products

34
Q

What is Hess’ law?

A

If a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route

35
Q

How would you find the enthalpy change of formation/combustion/neutralisation?

A
  1. Think about the definition!
  2. Construct the enthalpy cycle between the reactants, products and elements
  3. Add ^H and calculate unknown enthalpy change by following direction of the reaction

A-Level Chemistry (15 decks)

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