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energetics Flashcards

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1
Q

what do most chemical reactions give out or take in

A

most chemical reactions give out or take in energy as they proceed

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2
Q

why is the amount of energy involved in a reaction important

A
  1. you can measure the energy values of fuels
  2. you can calculate the energy requirements for industrial processes
  3. it helps to predict whether or not a reaction will take place
  4. you can work our the theoretical amount of energy to break bonds and the amount of energy released when bond are made
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3
Q

what is the form of the energy involved

A

the energy involved may be in different forms

e.g. light, electrical or most usually heat energy

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4
Q

what is thermochemistry

A

Thermochemistry is the study of heat changes during chemical reactions

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5
Q

what happens when a chemical reaction takes place

A

when a chemical reaction takes place, chemical bonds break and new ones are formed

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6
Q

how do bonds break

A

energy must be put in to break bonds

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7
Q

what must happen in order for bonds to be made

A

energy is given out when bonds are formed

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8
Q

how is a chemical reaction endothermic

A

at the end of the reacion, if energy has been taken in, the reaction is endothermic

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9
Q

how is a chemical reaction exothermic

A

at the end of the reaction, if energy has been taken out, the reaction is exothermic

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10
Q

what is the overall energy change of the reaction mean

A

the overall energy change may result in energy being given out or taken in

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11
Q

what is an exothermic reaction

A

some reactions give out heat as they proceed.

These are called exothermic reactions
Neutralising an acid with an acid with an alkali is an example of an exothermic reaction

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12
Q

what is an endothermic reaction

A

some reactions take in heat from their surroundings to keep the reaction going

These are called endothermic reactions. The breakdown of limestone Calcium carbonate to lime (calcium oxide) and carbon dioxide is an example of an endothermic because the reaction needs heat to proceed

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13
Q

what is another example of an endothermic reaction

A

Another example of an endothermic reaction is heating copper sulfate.

Blue copper sulfate crystals have the formula CuSO4. 5H2O.
The water molecules are bonded to the copper sulfate.
In order to break these bonds and make white, anhydrous copper sulfate, heat energy must be supplied

This reaction takes in heat so it is an endothermic reaction:

CuSO4. 5H2O - CuSO4 +H2O

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14
Q

what is the reverse reaction of the blue copper sulfate to white anhydrous copper sulfate reaction

A

when you add water to anhydrous copper sulfate, the reactions gives out heat

CuSO4 +5H2O - CuSO4.5H2O

In this direction the reaction is exothermic

It is usually the case that a reaction s endothermic in one direction is exothermic in the reverse direction

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15
Q

what does the amount of heat given out/ taken in by a reaction depend on

A

it depends on the quantity of reactants

This energy that is given out is usually measured in Kilojoules per mol

To avoid any confusions of quantities you need to balance and give an equation

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16
Q

what happens when fuels are burnt

A

when fuels are burnt, there is a large heat output

These are therefore very exothermic reactions

e.g.
coal is mostly carbon
Carbon gives out 393kJ when one mole, 12g is burnt completely so that the most highly oxidised products is formed
This is carbon dioxide and not carbon monoxide
Carbon is the only product formed

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17
Q

what do physiotherapists do to treat sports injuries

A

physiotherapists often treat sport injuries with cold packs

These produce “coldness” by an endothermic reaction

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18
Q

what changes the amount of heat given out by reactions

A

the amount of heat given out or taken in by a reaction varies with the conditions:

  • temperature
  • pressure
  • concentration of solutions and so on

This means that you must state the conditions under which measurements are made
e.g. you normally measure heat changes at constant atmospheric pressure

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19
Q

what is it called when you measure a heat change at constant pressure

A

it is called an enthalpy change

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20
Q

what is the symbol for enthalpy

A

H

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21
Q

what is the symbol for enthalpy change

A

ΔH

the Greek letter for change is Δ (delta) is used to indicate a change in any quantity

22
Q

what is the standard conditions for measuring enthalpy changes

A

pressure of 100kPa (approximately normal atmospheric pressure)

temperature of 298k (around normal room temp, 25C)

23
Q

what is the standard state of an element

A

the standard state of an element is the state in which it exists at 298K and 100kPa

when an enthalpy change is measured under standard conditions it is written as ΔH° and pronounced as delta H standard

24
Q

what happens when you measure hat changes at constant temperatures

A

it seems strange to measure heat changes at constant temperate because heat normally causes temperature changes

TO EXPLAIN THIS:
imagine the reactants at 298k

mix the reactants and heat is given out the surroundings (heat is produced)

A reaction is not thought of as being over until the products have cooled back to 298k

The heat given out to the surroundings while the reaction mixture cools is the enthalpy change for the reaction ΔH°

In an exothermic reaction, the products end up with less heat energy than the starting materials because they have lost heat energy when heating up their surroundings

Some endothermic reactions that take place in aqueous solutions absorb heat from the water and cool it down, e.g., dissolving ammonium nitrate in water.

Again you don’t think the reactions to be over until the products have warmed up to the temp at which they started

25
Q

what happens to an endothermic reason under standard conditions

A

Some endothermic reactions that take place in aqueous solutions absorb heat from the water and cools it down, e.g., dissolving ammonium nitrate in water.

Again you don’t think the reactions to be over until the products have warmed up to the temp at which they started

In this case the solution has to take in heat from the surroundings to do this

In an endothermic reaction the product ends up with more energy than the starting materials, so the ΔH is a positive sign

26
Q

how does pressure affect the amount of heat given out

A

pressure affects the amount of heat energy given out by reactions that involves gases.

If a gas is given out, some energy is requires to push away the atmosphere
The greater the atmosphere pressure, the more the energy is used for this

This means that less energy remains to be given out as heat by the reaction

This is why it is important to have standard pressure for measuring energy changes

27
Q

why are the physical states of reactants and products also affects the enthalpy change of reaction

A

the physical states (gas, liquids or solid) of reactants and products also affect the enthalpy change of a reaction

e.g. heat must be put in to change a liquid to a gas and is given out when a gas is changed to a liquid

This means that you must always include state symbols in your equations

e.g. hydrogen burns in oxygen to form water but there are two possibilities:

  1. forms liquid:
    water the enthalpy change is -285.8 kJ mol-1
  2. forms steam:
    the enthalpy change is -241.8kJ mol-1
28
Q

what are enthalpy levels diagrams

A

enthalpy level diagrams, sometimes called energy diagrams are used to represent enthalpy changes

in an exothermic reaction, the reactants have more energy than the products so it is higher than the products (in an endothermic reaction, it is the opposite)

They show the relative enthalpy levels of the reactants (starting materials) and the products
The vertical axis represents enthalpy and the horizontal axis, the extent of the reaction

29
Q

what is the general name for the enthalpy change

A

the general name for the enthalpy change for any reaction is THE STANDARD MOLAR ENTHALPY CHANGE OF REACTION ΔH°.

It is measured in kilojoules per mol (kJmol-)

30
Q

what must you do before you measure an enthalpy change

A

you must write a balanced symbol for the reaction and then find the heat change for the quantities in moles given by the equation

essentially, you need to work out the quantities
e.g.
ΔH for 2NaOH +H2S04 - NA2SO4 +2H2O

is the enthalpy change when 2 moles of NaOH react with 1 mole of H2SO4

31
Q

what are the standard enthalpies

A

commonly used reactions are given names

e.g. the enthalpy change of formation ΔfH° and the enthalpy change of combustion ΔcH°

32
Q

what is the formal definition of the enthalpy change of formation

A

The standard molar enthalpy of formation, ΔfH°, is the enthalpy change when one mole of substance is formed from its constituent elements under standard conditions, all reactants and products being in their standard states

33
Q

what is the formal definition of the enthalpy change of combustion

A

the standard molar enthalpy of combustion, ΔcH°, is the enthalpy change when 1 mole of substance is completely burnt in oxygen under standard conditions, all reactants and products and products being in their standard states

34
Q

what is temperature

A

temperature is related to the average kinetic energy of the particles in a system

As the particles move faster, their average kinetic energy increases and the temperature goes up

35
Q

is the amount of particles important for temperature

A

it does not matter how many particles there are, temperatures is independent of the number present

36
Q

what is temperature measured with

A

temperature is measured with a thermometer

37
Q

what is heat

A

heat is a measure of the total energy of all the particles present in a given amount of substance

It does depend on how much of the substance present

The energy of every particle is included so a bath of lukewarm water has much more heat than a red hot nail because there are so many more particles

38
Q

what is the flow of heat

A

heat always flows from high to low temperature, so heat will flow from the nail ( example from previous card) into the bath water even though the water has much more heat than the nail

39
Q

how do we measure the enthalpy change of an reaction

A

enthalpy change of an reaction is the heat given out or taken in as the reaction proceeds

There is no instrument that measures heat directly however, to measure an enthalpy change you arrange for the heat to be transferred into a particular mass of a substance, often water

Then, you need to know three things:

  1. mass of the substance that is being heated up/ cooled down
  2. temperature change
  3. specific heat capacity
40
Q

how do you find out the temperature change

A

for reactions that happen in a solution, you just put the reactants in a container and use a thermometer to measure the temperature of the mixture at regular intervals

It’s best to use a polystyrene beaker, to reduce the amount of heat lost or gained through the sides

41
Q

how can we accurately calculate the temperature change

A

the most obvious way of finding the temp change in a calorimetry experiment is to subtract the starting temp from the highest temp you recorded

That won’t give you an accurate value, because of the heat lost from the calorimeter to the surroundings
Instead, you can use a graph of your results to find a more accurate value

HERE’S WHAT YOU DO:

  1. during the experiment, record the temp at regular intervals, beginning a couple minuted before you start the reaction
  2. Plot a graph of your results
  3. draw two lines of best fit:
    One going through the points from BEFORE the reaction started and one going through the points from AFTER it started
  4. extend both lines so that they both pass the time when the reaction started
  5. the distance between the two lines at the time the reaction started (before any heat was lost) us the accurate temperature change for the reaction
42
Q

how do you work out the heat change of a combustion reaction

A

you can use a simple calorimeter to work out the heat change

you can use the apparatus to find the approximate enthalpy change when a fuel burn:

  1. a thermometer
  2. 200g of water
  3. spirit burner
  4. ethanol

You burn the fuel to heat a known mass of water and then measure the temp change of the water

You assume that all the heat from the fuel goes into the water
The apparatus used is called a calorimeter ( from the Latin meaning heat)

43
Q

what is a better version of the simple calorimeter

A

we can use a flame calorimeter set up instead of a simple calorimeter set up

It has the following features to reduce heat loss even further:

  1. spiral chimney is made of copper
  2. the flame is enclosed
  3. the fuel burns in pure oxygen rather air
  4. there is a draught screen
44
Q

what is the equation we use to work out an enthlpy change

A

heat change = mass of substance x specific heat capacity x temperature change

heat change/ mole = enthalpy change

we might also need to work out the mass of a substance from the volume and density

we use this density to work this out
= density x volume = mass

45
Q

what do chemists do to measure an enthalpy change (Hess’s law)

A

the enthalpy change for reactions cannot be measured directly

To find these you use an indirect approach

Chemists use enthalpy changes that they can can measure to work out enthalpy changes that they cannot

e.g. in particular, it is often easy to measure enthalpies of combustion

46
Q

how do Chemists use enthalpy changes that they can can measure to work out enthalpy changes that they cannot

A

To do this, chemists use Hess’s law, first stated by German Hess, a swiss - born Russian chemist, born 1802

47
Q

what is Hess’s Law

A

Hess’s law states that the enthalpy change for a chemist reaction is the same whatever route is taken from reactants to products

This is at the consequence of a more general scientific law:
THE LAW OF CONSERVATION OF MASS
where energy can’t be created or destroyed
Therefore, provided that the starting and ending points of a reaction are the same, the energy change must be the same

  • if not, energy has been created or destroyed
48
Q

how do we use Hess’s Law

A

ethyne, C2H2 is converted to C2H6, by two different routes

ROUTE 1:
the reaction takes place directly - ethyne reacts with two moles of hydrogen to give ethane:
C2H2(g)+2H2(g) - C2H6(g)
but how do we work out the enthalpy change

ROUTE 2:
the reaction takes place in two stages

a) ethyne, C2H2, reacts with one mole of hydrogen to give ethene C2H4

C2H2 +H2 - C2H4(g) ΔH2 = -176kJ mol-1

b) ethene then reacts with a 2nd mole of hydrogen to give ethene , C2H6

C2H2+H2 - C2H4(g) ΔH3 = -137kJ mol-1

Hess law tells us that the total energy change is the same whichever route you take
(you can show this using a thermochemical cycle)

knowing the equation for Hess’s law, ΔH1= (-176)+(-137) = -313 kJ mol-1

49
Q

what does Hess’s law mean

A

ΔH1 = ΔH2+ΔH3

50
Q

what does this method of calculating enthalpy change depends on

A

This method of calculating ΔH1 is fine if you know the enthalpy changes for the other tow reactions

There are certain enthalpy changes that can be looked up for a large rang for compounds that can have looked up for range of compounds

These include the enthalpy change of formation and combustion - in practice, many ΔfH are calculated from ΔcH via Hess’s law cycle

51
Q

how else can you work out an enthalpy change

enthalpy change of formation

A

e.g.

another theoretical way to convert ethyne to ethane could be via the elements carbon and hydrogen

  • ethyne is first converted to its elements carbon and hydrogen
    This is the reverse of formation and the enthalpy change is the negative of the enthalpy of formation

therefore:

This is a genral rule. The reverse of a reaction has the negative of its ΔH value - it is in fact a consequence of Hess’s law

Then the carbon and hydrogen react to form ethane - this is the enthalpy of formation for ethane

(reference notes for this due to diagrams)

chemistry a level (24 decks)

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