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Chemistry Module 3 > Enthalpy Changes > Flashcards

Enthalpy Changes Flashcards

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1
Q

What is enthalpy?

A

The heat stored within a chemical system which cannot be measured directly

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2
Q

What is enthalpy change?

What is its symbol?

A

The heat exchange with the surroundings during a reaction.

🔺H

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3
Q

If heat is:
1. Lost
2. Gained
In a system what does it mean?

A
  1. Heat gained to surroundings and temperature increase

2. Heat lost from surroundings and a temperature decrease

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4
Q

How do we work out enthalpy change?

A

= H products- H reactants

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5
Q

What happens in an exothermic reaction?

A

Heat is lost from the system to the surroundings, so the temperature increases.

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6
Q

What type of reaction is making chemical bonds?

A

Exothermic

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7
Q

Draw an enthalpy diagram for an exothermic reaction and label delta H:

A

Reactants higher than products
Arrow down is labelled as delta H
-Ve

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8
Q

What is an example of an exothermic reaction?

A

Combustion of fuels

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9
Q

What happened during an endothermic reaction?

A

Heat is gained to the system from the surroundings and the temperature decreases

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10
Q

What type of reaction is breaking chemical bonds?

A

Endothermic

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11
Q

Draw an enthalpy profile diagram for an endothermic reaction and label the delta H:

A

Reactants lower than products
Arrow up is delta H
+ Ve

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12
Q

What is an example of an endothermic reaction?

A

Thermal decomposition

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13
Q

What is the activation energy and its symbol?

A

The minimum energy required by reactants in order to form products
Ea

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14
Q

Draw an enthalpy profile diagram including the activation energy for an exothermic reaction:

A

Activation energy line goes up from reactants down to products

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15
Q

Draw an enthalpy profile diagram including the activation energy for an endothermic reaction:

A

Activation energy goes up and above products from reactants then down back to products

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16
Q

What are the standard conditions? And for a standard enthalpy change to be measured what also must we make sure?

A

100kPa
298K 25 degrees Celsius
1 mol dm3

Substances are in their standard states

17
Q

For an enthalpy change of reaction what must we need to specify?

A

An equation showing the amounts in moles of reactants and products

The states of every species

Enthalpy change in KJ mol-1

18
Q

What is the enthalpy of reaction?

A

The heat change when the number of moles of reactants specified in a given reaction equation react

19
Q

What is enthalpy of combustion?

A

The heat change due to the complete combustion of one mole of a substance

20
Q

What is the enthalpy of formation?

A

The heat change when one mole of a compound is formed from its elements in their natural states

21
Q

What is the enthalpy of formation of an element?

A

0

22
Q

What is the enthalpy of neutralisation?

A

The heat change which accompanies the formation of 1 mole of water when an acid and base/alkali react under the conditions stated.

23
Q

What is the average bond enthalpy? (E)

A

The breaking of one mole of covalent bonds in a gaseous molecule.
(Endothermic)

24
Q

What is calorimetry a measure of?

A

Enthalpy change

25
Q

What is the equation to work out heat energy exchanged the untis?

A

Q= mc(delta)T

Mass (g)
Specific heat capacity
Temp change (degrees Celsius)
Q- heat exchanged J

26
Q

What is the equation that works out delta H? And units:

A

Delta H= Q/ n

Delta H- kJmol-1
Q- kJ
n- mol

27
Q

Which mol do we use?

A

Use the mol of the substance that IS NOT in excess in Q/n

28
Q

What steps do we use to work out enthalpy change of reaction!

A

Q= mc(delta) T
Divide Q by 1000 to get kJ
Work out moles
Delta H= Q/n

29
Q

What must we make sure we do when writing out a combustion equation? How do we do this?

A

The mole of reactant is 1!

Balance C, H then the O

30
Q

How do we work out enthalpy change of combustion?

A

Q= mc(delta) T
Convert Q in kJ (divide by 1000)
Work out mol n= m/M
Delta H= Q/n

31
Q

What are the experimental errors in a combustion reaction?

A
  1. Heat loss- inefficient transfer of heat from flame to water
  2. Incomplete combustion
  3. Evaporation of substance combusted
  4. Non-standard conditions
32
Q

What must we do when writing out the equation for a neutralisation reaction?

A

Make sure there is 1 mole of water!

33
Q

How do we work out enthalpy change for a beaut realisation reaction?

A 
Q= mc(delta) T
Divide Q by 1000
Work out mole of water-
By working out mole of reactant which IS NOT IN EXCESS and working out ratio of this to water
Then Q/ n (of water)
34
Q

What are the steps to working out the delta H using bond enthalpies!

A
  1. Write balanced equation
  2. Draw out the molecules
  3. Add up bond enthalpies (E values) for bonds broken
  4. Add up bond enthalpies (E values) for bonds made
  5. Delta H= total E(bonds broken) - total E (bonds made)
35
Q

What does Hess’ law state?

A

The total enthalpy change from reactants to products is independent of the route taken.

36
Q

Draw a hess’ cycle for a combustion reaction:

A

Arrows point down to combustion products

37
Q

What is the equation we use when applying Hess’ law in a combustion reaction and how do we re- arrange it?

A

Reaction H+ combustion H products= combustion H Reactants

Reaction H= combustion H reactants- combustion H products

38
Q

Draw Hess’ cycle for a formation reaction:

A

Arrows point up from formation elements

39
Q

What is the equation we use when applying Hess’ law in a formation reaction and how do we re- arrange it?

A

Reaction H+ formation H reactants = formation H products

Reaction H= formation H products- formation H reactants

Chemistry Module 3 (6 decks)

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