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RGS 4th Form Chemistry > 5 Energetics > Flashcards

5 Energetics Flashcards

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1
Q

Is bond making exothermic or endothermic?

A

exothermic

energy is released when bonds are formed

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2
Q

Is bond breaking endothermic or exothermic?

A

endothermic

Energy must be added to break bonds

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3
Q

What does the quantity symbol H stand for?

What is its unit?

A

molar enthalpy

kJ / mol

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4
Q

What does the quantity Q stand for in Q = mc(change in T)?

What is its unit?

A

Q stands for energy

Unit : Joules (J)

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5
Q

What is the relationship between energy, mass, specific heat capacity and change in temperature?

A

Q = mc (Change in T)

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6
Q

What does a negative change in enthalpy value mean?

A

The reaction is exothermic

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7
Q

What does a positive change in enthalpy value mean?

A

The reaction is endothermic

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8
Q

During the complete combustion of propanol, 100 cm3 of water is heated and the temperature of the water rose from 23ºC to 43ºC. The specific heat capacity of water is 4.2 J/gºC.

How is the energy released by the propanol be calculated?

A
  1. Find energy released using:

Q =mc(change in T)

Q = 100g x 4.2J/gºC x 20ºC

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9
Q

To determine the enthapy of reaction for the combustion of propanol, 100 cm3 of water is heated. The temperature started at 23ºC and rose to 46ºC.

What other measurement/s must be made to calculate the enthalpy of reaction in kJ/mol?

A

Mass of spirit burner before AND after.

  1. Find energy released using:

Q =mc(change in T)

2: Find moles of propanol reacted using:

mole of fuel burnt = mass / RFM of propanol

  1. Calculate enthapy change using

Enthalpy = Q / moles

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10
Q

To determine the enthapy of reaction for the complete combustion of propanol, 100 cm3 of water is heated. The temperature started at 23ºC and rose to 46ºC.

State three things which must be done to ensure accurate results?

A
  1. Water must be stirred
  2. Temperature must be read every minute after the spirt burner is blown out in order to record maximum temperature
  3. Thermometer must not be touching the bottom of the beaker
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11
Q

Using the energy profile diagram below, determine if the reaction is exothermic or endothermic

A

exothermic

change in enthalpy is negative

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12
Q

Using the energy profile diagram below, determine if the reaction is exothermic or endothermic?

A

Endothermic

Enthalpy change is positive

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13
Q

Below is a diagram showing that more energy is released making bond in this reaction than energy supplied breaking bonds.

Is this reaction exothermic or endothermic?

A

exothermic

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14
Q

Metal Displacement reactions of metals and metal salts are exothermic.

Mg + CuSO4 –> MgSO4 + Cu

How would the temperature change of the solution compare between magnesium in copper sulfate and zinc in copper sulfate?

A

The temperature rise would be greater for magnesium in copper sulfate than zinc in copper sulfate because magnesium and copper are futher apart in the reactivity series.

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15
Q

How does the addition of a catalyst alter the energy profile diagram?

A

A catalyst provides an alternative route with a lower activation energy.

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16
Q

Complete combustion of methane

CH4 + 2O2 –> CO2 + 2H2O

After calcuating the total energy required to break all the bonds in the reactants and the _total energy to make all the bond_s in all the products, how is the enthalpy change calculated?

How do you know if the reaction is exothermic or endothermic?

A

negative value = exothermic

positive value = endothermic

17
Q

Complete combustion of methane

CH4 + 2O2 –> CO2 + 2H2O

What is the total bond enthalpy released when the bond in carbon dioxide are made?

A

2x 730kJ/mol = 1460 kJ/mol

carbon dioxide has two C=O bonds!

18
Q

Complete combustion of methane

CH4 + 2O2 –> CO2 + 2H2O

What is the total bond enthalpy required to break both oxygen molecules?

A

2x 502kJ/mol = 1010 kJ/mol

oxygen has O=O bonds!

19
Q

Complete combustion of methane

CH4 + 2O2 –> CO2 + 2H2O

What is the total bond enthalpy released when both water molecules are made?

A

4x 464 kJ/mol = 1856 kJ/mol

H-O-H H-O-H

4 x O-H bonds are made!

20
Q

Below is the calculation for the theroetical value for change in enthapy of the complete combustion of methane

CH4 + 2O2 –> CO2 + 2H2O

How and why is the experimental value different?

A

The experimental value will be less than -810 kJ/mol.

Reasons:

  1. Not all the energy released by the fuel goes to the water- some lost to the surroundings
  2. Not all the full undergoes complete combustion - some reacts via incomplete combustion (flame was yellow not blue)
21
Q

Calculate the enthalpy of reaction for hydrogen reacting with chlorine to produce hydrogen chloride

A

Bonds broken

432 + 239 = 671 kJ/mol

Bonds made

2 x 427 = 854 kJ/mol

Enthalpy of reaction

671 - 854 = - 183 kJ/mol

EXOTHEMRMIC

22
Q

Define an exothermic reaction

A

heat energy given out - surrounding warm up

23
Q

Define endothermic reactions

A

heat is taken in - surrounding cool down

24
Q

What are the key marking points when creating an energy profile diagram?

A
  1. y-axis labelled with enthalpy (or energy)
  2. labelled reactants and products
  3. bump to represent reaction path
  4. activation energy arrow from reactants to top of bump
  5. enthalpy change labelled from reactants to products - arrow up (endo) arrow down (exo)
25
Q

Complete the steps to find the molar enthapy of a fuel?

  1. find the mass of the fuel and container before using a balance
  2. pour 100cm3 of water into the can using a measuring cylinder
  3. record the initial temperature of the water in the can
  4. Light the fuel and place it directly under the can
  5. blow out the flame when the temperature rises about 10 degrees.
A
  1. record the maximum temperature of the water
  2. find the new mass of the fuel and container
  3. Calculate the energy gained by the water

Q = 100g x 4.2J/gºC x(change in temperature)

  1. Calculate the moles of fuel burnt

moles = mass of fuel burnt / RFM of fuel

  1. Calculate the molar enthalpy by

Energy gained by water (in kJ) / mole of fuel burnt

26
Q

Complete the steps to find the molar enthapy of a metal displacement reaction?

  1. measure the mass of the metal using a balance
  2. pour 100 cm3 of metal salt (excess) into the styrofoam cup using a measuring cylinder
  3. record the initial temperature of the salt solution in the sytrofoam cup
  4. pour the metal into the cup & stir
A
  1. record the maximum temperature of the water
  2. Calculate the energy gained by the water

Q = 100g x 4.2J/gºC x(change in temperature)

  1. Calculate the moles of mass added

moles = mass of metal / RFM of metal

  1. Calculate the molar enthalpy by

Energy gained by solution (in kJ) / mole of metal

RGS 4th Form Chemistry (5 decks)

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