energetics

Question 1

what do most chemical reactions give out or take in

Answer 1

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

Question 2

why is the amount of energy involved in a reaction important

Answer 2

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

Question 3

what is the form of the energy involved

Answer 3

the energy involved may be in different forms

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

Question 4

what is thermochemistry

Answer 4

Thermochemistry is the study of heat changes during chemical reactions

Question 5

what happens when a chemical reaction takes place

Answer 5

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

Question 6

how do bonds break

Answer 6

energy must be put in to break bonds

Question 7

what must happen in order for bonds to be made

Answer 7

energy is given out when bonds are formed

Question 8

how is a chemical reaction endothermic

Answer 8

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

Question 9

how is a chemical reaction exothermic

Answer 9

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

Question 10

what is the overall energy change of the reaction mean

Answer 10

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

Question 11

what is an exothermic reaction

Answer 11

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

Question 12

what is an endothermic reaction

Answer 12

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

Question 13

what is another example of an endothermic reaction

Answer 13

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

Question 14

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

Answer 14

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

Question 15

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

Answer 15

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

Question 16

what happens when fuels are burnt

Answer 16

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

Question 17

what do physiotherapists do to treat sports injuries

Answer 17

physiotherapists often treat sport injuries with cold packs

These produce “coldness” by an endothermic reaction

Question 18

what changes the amount of heat given out by reactions

Answer 18

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

Question 19

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

Answer 19

it is called an enthalpy change

Question 20

what is the symbol for enthalpy

Answer 20

H

Question 21

what is the symbol for enthalpy change

Answer 21

ΔH

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

Question 22

what is the standard conditions for measuring enthalpy changes

Answer 22

pressure of 100kPa (approximately normal atmospheric pressure)

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

Question 23

what is the standard state of an element

Answer 23

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

Question 24

what happens when you measure hat changes at constant temperatures

Answer 24

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

Question 25

what happens to an endothermic reason under standard conditions

Answer 25

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

Question 26

how does pressure affect the amount of heat given out

Answer 26

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

Question 27

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

Answer 27

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

Question 28

what are enthalpy levels diagrams

Answer 28

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

Question 29

what is the general name for the enthalpy change

Answer 29

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-)

Question 30

what must you do before you measure an enthalpy change

Answer 30

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

Question 31

what are the standard enthalpies

Answer 31

commonly used reactions are given names | e.g. the enthalpy change of formation ΔfH° and the enthalpy change of combustion ΔcH°

Question 32

what is the formal definition of the enthalpy change of formation

Answer 32

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

Question 33

what is the formal definition of the enthalpy change of combustion

Answer 33

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

Question 34

what is temperature

Answer 34

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

Question 35

is the amount of particles important for temperature

Answer 35

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

Question 36

what is temperature measured with

Answer 36

temperature is measured with a thermometer

Question 37

what is heat

Answer 37

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

Question 38

what is the flow of heat

Answer 38

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

Question 39

how do we measure the enthalpy change of an reaction

Answer 39

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

Question 40

how do you find out the temperature change

Answer 40

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

Question 41

how can we accurately calculate the temperature change

Answer 41

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

Question 42

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

Answer 42

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)

Question 43

what is a better version of the simple calorimeter

Answer 43

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

Question 44

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

Answer 44

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

Question 45

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

Answer 45

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

Question 46

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

Answer 46

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

Question 47

what is Hess's Law

Answer 47

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

Question 48

how do we use Hess's Law

Answer 48

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

Question 49

what does Hess's law mean

Answer 49

ΔH1 = ΔH2+ΔH3

Question 50

what does this method of calculating enthalpy change depends on

Answer 50

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

Question 51

how else can you work out an enthalpy change | enthalpy change of formation

Answer 51

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)