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Wjec Chemistry Unit 2 > 2.1 > Flashcards

2.1 Flashcards

1
Q

What is enthalpy, H

A

Heat content of a system at constant pressure

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2
Q

What is enthalpy change

A

Head added to a system at constant pressure

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3
Q

Equation to measure enthalpy change

A

Enthalpy of products - enthalpy of reactants

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4
Q

Exothermic reaction

A
  • releases energy to surroundings
  • temperature rise
  • enthalpy change is negative
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5
Q

Endothermic reactions

A
  • absorb energy from surroundings
  • temperature drop
  • enthalpy change is positive
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6
Q

What is the principle of conservation of energy

A

energy cannot be created or destroyed only changed from one form to another.

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7
Q

Conditions for standard enthalpy change

A
  • all substances in their standard states
  • 298 K (25 c)
  • 1 atm (101000 Pa)
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8
Q

What is the standard enthalpy change of formation

A

the enthalpy change when one mole of a substance is formed from its constituent elements in their standard states under standard conditions.

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9
Q

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

A

0 kJ mol-1

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10
Q

Standard enthalpy change of formation of water

A

H2 (g) + 1/2 O2 (g) —–> H2O (l) /\fH = -286 kJ mol-1

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11
Q

What is the standard enthalpy change of combustion

A

enthalpy change when one mole of a substance is completely combusted in oxygen under standard conditions.

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12
Q

What is the standard enthalpy change of combustion of methane

A

CH4 (g) + 2O2 (g) —-> CO2 (g) + 2H2O (l) /\cH = -891 kJ mol-1

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13
Q

Equation for the standard enthalpy of reaction

A

/\rH = sum/\fH(products) - sum/\fH(reactants)

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14
Q

What is Hess’s law

A

the total enthalpy change for a reaction is independent of the route taken from the reactants to the products

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15
Q

What is bond enthalpy

A

the enthalpy required to break a covalent X-Y bond into X atoms and Y atoms, all in the gas phase

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16
Q

What is average bond enthalpy

A

Average value of the enthalpy required to break a given type of covalent bond in the molecules of a gaseous species

17
Q

How to calculate bond enthalpy change

A

BURP

Sum of reactants - sum of products

18
Q

What can be done to measure enthalpy due to not being able to measure it directly

A

Measure the heat transferred to its surroundings

19
Q

What is a calorimeter

A

Insulated container that helps calculate the chemical change of a reaction
• change in temperature inside the calorimeter caused by the enthalpy change is measured with a thermometer

20
Q

What information is needed to calculate the enthalpy change when doing it physically

A
  • temperature change
  • mass
  • specific heat capacity of contents of calorimeter
21
Q

Equation that represents the relationship between the temperature change and the amount of heat transferred

A

q= mc/\t

c- specific heat capacity
m- mass of solution in cup (volume)

22
Q

What is done to obtain the maximum temperature change

A

allowances are made for heat lost to the surroundings
• temperatures of the solution are taken for a short period before mixing and some time after mixing
• graph of temperature against time is plotted and maximum temperature is obtained by extrapolating the graph back to the mixing time

23
Q

Look

A
24
Q

Equation to calculate the enthalpy change of reaction per mole

A

/\H = -q/n

reactant that is not in excess has to be accurately measured - mass of solid or concentration of solution must be known

25
Q

What is done when determining an enthalpy change of combustion

A

Known mass of fuel is burnt in air to heat a known mass of water and the temperature change in the water is recorded

26
Q

Main points to note when determining an enthalpy change of combustion

A
  • suitable gap between the base of the metal container and the top of the spirit burner
  • accurately measured the amount of water added
  • use an accurate thermometer to measure initial temperature and record the steady value obtained
  • weigh the spirit burner with fuel and record initial mass
  • adjust the gap between the metal container and the spirit burner if necessary after lighting the wick
  • allow the fuel to heat the water to a suitable temperature (about 20 c)
  • extinguish the flame and record final maximum temperature
  • allow spirit burner to cool thoroughly before re-weighing and recording final mass
27
Q

Why is the value of enthalpy change of combustion much lower than the book value?

A
  • some energy transferred from burning fuel is lost in heating the apparatus and the surroundings
  • fuel isn’t completely combusted
28
Q

Simplest type of calorimeter

A

Coffee cup calorimeter

29
Q

Examples of indirect determination of enthalpy change

A
  • magnesium oxide with carbon dioxide to magnesium carbonate (/\H of reaction between Mg and acid and MgCO3 and acid are separately measured)
  • Formation of MgO (adding Mg and MgO separately to HCl)
  • Formation of MgCO3 (adding Mg and MgCO3 separately to HCl)
  • Decomposition of sodium hydrogencarbonate (adding sodium hydrogencarbonate and sodium carbonate separately to HCl)
30
Q

Main points to note in the practical work of indirect determination of an enthalpy change

A

• measure an excess volume of acid using a burette or pipette and place in a polystyrene cup
• use accurate thermometer to measure initial temperature of acid, record the temperature when a steady value has been obtained
• accurately weigh the solid in powder (fast as possible) form in a suitable
container
• add all solid to the cup and stir the mixture well and start a stopwatch
• keep stirring with the thermometer and record the temperature regularly (every 30s)
• stop recording temperature when is has fallen for about 5 minutes
• re-weigh the weighing container to ensure correct mass of solid added is recorded
• plot a graph of temperature against time to calculate maximum temperature of the mixture might have reached
• calculate heat transferred
• calculate enthalpy change
• repeat with other solid
• use Hess’s law to calculate the required enthalpy change

31
Q

Direction of arrows in an enthalpy cycle of formation

A

From the common elements to the reactants and products

32
Q

Direction of arrows in enthalpy cycle of combustion

A

From reactants and products to the common combustion products

33
Q

Why are average bond enthalpies used - why aren’t they all the same for a specific bond?

A

Actual value of the bond enthalpy for a particular bond depends on the structure of the rest of the molecule.

The bond enthalpy of a given bond is similar in a wide range of compounds but the average is calculated using the values from many different compounds

Wjec Chemistry Unit 2 (8 decks)

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