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Chemistry OCR A AS > Module 3.2.1 - Enthalpy changes > Flashcards

Module 3.2.1 - Enthalpy changes Flashcards

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1
Q

What is enthalpy change?

A

The amount of heat released (or absorbed) by a chemical reaction, carried out at constant pressure. Units = kJ mol^-1

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2
Q

What is an exothermic reaction?

A

A reaction where heat energy is released to the surroundings. Enthalpy change is negative

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3
Q

What is an endothermic reaction?

A

A reaction where heat energy is absorbed from the surroundings.
Enthalpy change is positive

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4
Q

What is average bond enthalpy?

A

The average enthalpy change for the breaking of 1 mole of bonds in gaseous molecules (by homolytic fission)

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5
Q

What are the standard conditions?

A
  • Temperature = 25°C (298K)
  • Pressure = 1atm (100kPa)
  • Solutions must have a concentration of 1.00 moldm^-3
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6
Q

What is standard enthalpy change of reaction?

A

The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states

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7
Q

What is standard enthalpy of neutralisation?

A

The enthalpy change that accompanies the formation of one mole of H2O(l) from neutralisation, under standard conditions

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8
Q

What is 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

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9
Q

What is standard enthalpy change of combustion?

A

The enthalpy change for complete combustion of one mole of a substance under standard conditions, all reactants and products being in their standard states

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10
Q

What does Hess’ Law state?

A

The enthalpy change of a reaction depends only on the initial and final states and is independent of the route taken

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11
Q

What is activation energy?

A

The minimum energy required to start a reaction by the breaking of bonds.

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12
Q

What does the first law of thermodynamics state?

A

Energy can be converted from one form to another and cannot be created or destroyed

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13
Q

How do you measure enthalpy?

A

The enthalpy change is measured when heat is either added to, or released from a system. **It is not possible to directly measure enthalpy **

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14
Q

When reading the temperature using a thermometer, what temperature is being measured?

A

The temperature of the surroundings

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15
Q

What is the enthalpy change of a reaction equal to?

A

The enthalpy of products - enthalpy of reactants

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16
Q

How is an exothermic reaction shown on a enthalpy profile diagram?

A
  • The enthalpy of products lie below the enthalpy of reactants
  • Enthalpy change is negative and points downwards
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17
Q

What are examples of exothermic processes?

A
  • Combustion
  • Respiration
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18
Q

How are endothermic processes shown on an enthalpy profile diagram?

A
  • Enthalpy of products lies above the enthalpy of reactants
  • Enthalpy change is positive, and the arrow points upwards
19
Q

What are examples of endothermic processes?

A
  • Photosynthesis
  • Thermal decomposition
20
Q

What is bond fission?

A

The breaking of a chemical bond

21
Q

What is homolytic fission?

A

Where a covalent bond breaks equally, hence one electron goes to each atom shown by half arrows
- forms two radicals (a species with an unpaired electron)

22
Q

What is heterolytic fission?

A

Where a covalent bond breaks unequally, hence both electrons go to one atom shown by a full arrow
- forms a positive and a negative ion

23
Q

What does a molecular chemical reaction involve?

A

The breaking of covalent bonds in reactant molecules and forming new covalent bonds in the product molecules

24
Q

What process are breaking bonds?

A

Endothermic ; require energy to be absorbed from surroundings to the system to break bonds

25
Q

What process are forming bonds?

A

Exothermic ; Energy is released from forming bonds from the system to the surroundings

26
Q

Why can the strength of bonds vary?

A

It depends on the environment in which it is found

27
Q

How is the enthalpy change of reaction calculated?

A

Enthalpy change of breaking bonds - enthalpy change of forming bonds

28
Q

What do you do when dealing with compound ions in relation to standard enthalpy change of formation?

A

Compound ions are split into their elements (into their standard states and conditions)

29
Q

What does a system mean in terms of a chemical reaction?

A

The atoms and bonds involved in the chemical reaction

30
Q

On a enthalpy profile diagram, which way does the arrow point for activation energy?

A

Upwards

31
Q

What is an enthalpy change measured under standard conditions referred to as?

A

Standard enthalpy change

32
Q

What does in standard state mean?

A

The state an element (or compound) exists at in standard conditions

33
Q

When are enthalpy cycles used?

A

When enthalpy changes cannot be measured directly from experiment

34
Q

What two types of enthalpy cycles are there?

A
  1. Using standard enthalpy of formation data
  2. Using standard enthalpy of combustion data
35
Q

How do you determine which enthalpy cycle to draw out?

A

Use the data given in the question, not what is being asked in the question

36
Q

How do you draw a hess cycle to calculate enthalpy change of formation?

A
  1. Re-write out the equation leaving space underneath
  2. Under the equation, write out the component elements in their standard states and conditions
  3. Connect lower elements to the overall equation with 1 arrow pointing towards the reactants and 1 arrow pointing towards the products (both arrows pointing up)
  4. Clockwise = anti-clockwise // Use FliPeR (formation = products - reactants)
37
Q

How do you draw a hess cycle for standard enthalpy change of combustion?

A
  1. Re-write the equation provided in the exam question (with state symbols)
  2. Underneath the overall equation, write out the products of complete combustion i.e. CO2 (g) + H2O (l)
  3. Balance the elements you can (C & H atoms)
  4. Using CRaP (combustion = reactants - products), arrows pointing down to the products of combustion.
38
Q

What is a calorimeter?

A

insulators e.g. polystyrene cups

39
Q

How can you calculate enthalpy change from experimental data?

A

equation = Q = mc (delta)T
m = mass of the substance being heated (density for liquids)
c = specific heat capacity of substance (water = 4.18)

40
Q

How do you calculate the experimental enthalpy change of solution?

A
  1. Calculate the total heat energy (Q=mc(delta)T) of substance being heated, convert to kJ by / 1000
  2. Calculate moles of substance added to substance heated (usually solid)
  3. Calculate enthalpy change for reaction, step 1 (in kJ) / step 2 (mols), add sign that corresponds to if the temperature has increased(-)/decreased(+)
41
Q

How do you calculate experimental enthalpy change of combustion?

A
  1. Calculate Q= mc (delta)T of substance being heated, divide by 1000 to get kJ
  2. Calculate the moles of the substance combustion (e.g. alcohol)
  3. Calculate enthalpy change, step 1 (kJ) / step 2. (mol), combustion = sign is negative
42
Q

How do you calculate the experimental enthalpy change for neutralisation?

A
  1. Calculate Q=mc (delta)T for substance(s) being heated (add together if more than one), /1000 for kJ
  2. Calculate the moles of both acid & alkali to find limiting reactant and use molar ratios to calculate mol of H2O (using limiting reactant)
  3. Calculate enthalpy change, step 1 (kJ)/ step 2. (mol of H2O), use corresponding sign if either exo/endo -thermic
43
Q

Why might experimental methods for enthalpy determination not be accurate? (3)

A
  • Heat lost to the surroundings
  • Not in standard conditions
  • Reaction may not be complete

Chemistry OCR A AS (27 decks)

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