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Higher Chemistry > Chemical Energy > Flashcards

Chemical Energy Flashcards

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1
Q

are combustion reactions exo- or endothermic

A

exothermic - fuels release energy

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2
Q

problems with exothermic reactions

A

may incur costs in supplying heat to keep the reaction rate

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3
Q

problems with endothermic reactions

A

may require heat to be removed to prevent the temperature from rising. could cause an explosion

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4
Q

4 main ways to calculate chemical energy

A
  1. proportion
  2. Eh=cm delta T
  3. bond breaking/ bond making
  4. Hess’s law
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5
Q

what is enthalpy

A

a measure of chemical energy in a substance

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6
Q

what is the enthalpy of combustion

A

the energy released when one mole of substance burns completely in oxygen

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7
Q

complete combustion

A

plentiful supply of oxygen

products - carbon dioxide and water (unless only carbon or hydrogen is being burned)

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8
Q

incomplete combustion

A

not enough O2
when there is a poor supply of oxygen, the hydrocarbon cannot burn completely and so the products are carbon monoxide, carbon and water.

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9
Q

steps for calculating the enthalpy of combustion from proportion

A
  1. rewrite the information in the question as a proportion
  2. now input the mass of one mole (gfm)
  3. multiply diagonal numbers and divide by number in corner
    * negative sign to show energy is released*
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10
Q

set up used to calculate the energy released by a fuel

A

a thermometer in glass beaker on top of alcohol in the spirit burner

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11
Q

measurements taken in energy released by fuel experiment

A
  1. mass of burner before + after burning

2. temperature of water before and after (temperature rise)

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12
Q

specific heat capacity equation

A

Eh=cm delta T

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13
Q

specific heat capacity definition

A

the energy required to raise the temperature of 1kg of a substance by 1-degree Celcius

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14
Q

c of water

A

4.18 kJ per kg per degree

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15
Q

general rule of thumb for decimal places

A

100s/1000s - no dp
10s - 1 dp
units - 2 dp

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16
Q

calculating enthalpy of combustion

A
  1. work out energy released (no need for negative sign)

2. use proportion to work out enthalpy of combustion

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17
Q

why are experimental and theoretical values different

A

heat lost to the surroundings

18
Q

ways to improve enthalpy of combustion experiment

A
  1. use a heat shield to prevent heat escaping

2. use a copper can around the glass beaker as copper is a good conductor of heat

19
Q

other sources of error in enthalpy of combustion experiment

A
  • not enough oxygen for complete combustion
  • some of the fuel evaporates (Mass decreases but fuel not burned to release energy)
  • no lid on container
  • if water has not been stirred
  • some heat energy absorbed by beaker/ other apparatus
20
Q

the bomb calorimeter

A

can be used to improve experiment

  • heat no longer lost to surroundings
  • complete combustion occurs
  • diagram in notes and camera roll*
21
Q

why is bond breaking positive

A

bonds form to allow atoms to achieve stability so to break chemical bonds, energy needs to be put in to destroy stability (endothermic process)

22
Q

unstable atoms

A

high in energy - reactive

23
Q

stable atoms

A

low in energy - less reactive

24
Q

why is bond making negative

A

when chemical bonds are formed, the new product is more stable and therefore lower in energy than the reactants
excess energy is released to surroundings as heat
(exothermic process)

25
Q

bond breaking…

A

energy required
endothermic
enthalpy change = positive

26
Q

bond making…

A

energy released
exothermic
enthalpy change = negative

27
Q

molar bond enthalpy

A

the energy required to break one mole of chemical bonds in the gaseous state and from two moles of gaseous atoms

28
Q

bond enthalpy table in data booklet

A

gives us bond enthalpies of common diatomic molecules

these bonds only exist in these diatomic molecules so values are very accurate

29
Q

why are values for triple bonds higher than single/double

A

it takes more energy to break a triple bond than a single or double bond as it is stronger

30
Q

mean bond enthalpy table in data booklet

A

these bonds can be found in more than one substance
eg: C-H found in alkanes, alkenes, alcohols, aldehydes…
the mean (average) value of the C-H bond in all compounds is calculated and displayed in the table

31
Q

bond breaking, bond making equation

A

enthalpy change = sum of all bonds broken + sum of all bonds made
(break up AND make up)
bonds made will have a negative value

32
Q

steps when using bond breaking, bond making

A
  1. draw full structural formula to show all bonds
  2. make a list of all bonds which are broken in reactants
  3. fill in values using page 10 of db
  4. repeat process for bond making
  5. calculate total energy in and total energy given out.
  6. calculate enthalpy change
33
Q

what is enthalpy of formation?

A

the energy change one mole of a compound from its elements in their standard states.

34
Q

what is Hess’s law?

A

Hess’s law states that the enthalpy change for a chemical reaction is independent of the route taken

35
Q

steps for basic Hess’s law question

A
  1. find starting point
  2. follow alternative route and change signs if arrows are in wrong direction
  3. enthalpy change is the sum of all steps
36
Q

applications of Hess’s law

A

Hess’s law can be used to calculate enthalpy changes, which are difficult to determine by experiment.
Each of the substances shown in the equation can be burned and their enthalpies of combustion found by experiment.

37
Q

what is enthalpy of formation

A

quantity of energy taken in or given out when 1 mole of a substance is formed

38
Q

alternative method when it is difficult to determine by the direct route (enthalpy of formation)

A

use enthalpy of combustion to help work out the energies of the different steps (pg10)

39
Q

Hess’s law calculations steps

A
  1. write out target equation
  2. write out balanced equations for the enthalpy of combustion for each substance
  3. compare and rearrange the known equations to get to target equation
  4. cancel out and add equations and enthalpy change values to get the target.
40
Q

enthalpy of combustion equations for C and H

A

C(s)+02–>CO2

H2(g)+1/2O2(g)–> H20(g)

Higher Chemistry (16 decks)

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