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IGCSE Chemistry > chemical energetics and reactions > Flashcards

chemical energetics and reactions Flashcards

1
Q

what happens in a chemical reaction

A

bonds break and new bonds form

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2
Q

what do collisions do in a chemical reaction

A

they provide the energy for chemical reactions

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3
Q

What is activation energy

A

the minimum amount of energy particles must collide with in order to break bonds

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4
Q

what is collision theory

A

You need a collision to have a reaction. ​
Collisions provide the energy required to break bonds.​
Most collisions are not successful.​
A successful collision is very important for any chemical reaction.

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5
Q

exothermic reaction equation

A

reactants -> products + energy

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6
Q

endothermic reaction equation

A

energy + reactants -> products

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7
Q

describe exothermic reactions

A
  • energy is released by reaction
  • energy level of reactants is higher than products
  • activation energy is achieved earlier (peaks earlier)
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8
Q

describe endothermic reactions

A
  • energy is absorbed by reaction
  • energy level of reactants is lower than products
  • activation energy is achieved later (peaks later)
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9
Q

how does concentration affect rate of reaction and why

A

Increase in the concentration of a solution, the rate of reaction will increase
This is because there will be more reactant particles in a given volume, allowing more frequent and successful collisions per second, increasing the rate of reaction

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10
Q

how does surface area affect rate of reaction and why

A

Increase in the surface area of the solid, the rate of reaction will increase
This is because more surface area particles will be exposed to the other reactant so there will be more frequent and successful collisions per second, increasing the rate of reaction

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11
Q

how does temperature affect rate of reaction and why

A

Increase in the temperature, the rate of reaction will increase
This is because the particles will have more kinetic energy than the required activation energy, therefore there will be more frequent and successful collisions per second, increasing the rate of reaction

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12
Q

how do catalysts affect rate of reaction and why

A

Catalysts reduce the activation energy as they create alternative pathways requiring lower activation energy, allowing more successful and frequent collisions
This shows that when a catalyst is used, the rate of reaction will increase

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13
Q

what is explosive combustion

A

Explosive combustion occurs when there are many fine particles in the air
Many industrial processes such as metal working, coal mining or flour milling produce very fine and tiny particles
These particles have a very large surface area and are combustible in air
Even a small spark may cause them to ignite and since the surface area is so large, the rate of reaction can be incredibly fast, hence they are explosive
Methane gas mixed with air in coal mines can also form an explosive mixture

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14
Q

investigation for surface area

A

Method:
Add dilute hydrochloric acid into a conical flask
Use a capillary tube to connect this flask to a measuring cylinder upside down in a bucket of water (downwards displacement)
Add calcium carbonate chips into the conical flask and close the bung
Measure the volume of gas produced in a fixed time using the measuring cylinder
Repeat with different sizes of calcium carbonate chips (solid, crushed and powdered)
Result:
Smaller sizes of chips causes an increase in the surface area of the solid, so the rate of reaction will increase
This is because more surface area of the particles will be exposed to the other reactant so there will be more frequent and successful collisions, increasing the rate of reaction

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15
Q

investigation for concentration

A

Method:
Measure 50 cm3 of Sodium Thiosulfate solution into a flask
Measure 5 cm3 of dilute Hydrochloric acid into a measuring cylinder
Draw a cross on a piece of paper and put it underneath the flask
Add the acid into the flask and immediately start the stopwatch
Look down at the cross from above and stop the stopwatch when the cross can no longer be seen
Repeat using different concentrations of Sodium Thiosulfate solution (mix different volumes of sodium thiosulfate solution with water to dilute it)
Result:
With an increase in the concentration of a solution, the rate of reaction will increase
This is because there will be more reactant particles in a given volume, allowing more frequent and successful collisions, increasing the rate of reaction

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16
Q

investigation for temperature

A

Method:
Dilute Hydrochloric acid is heated to a set temperature using a water bath
Add the dilute Hydrochloric acid into a conical flask
Add a strip of Magnesium and start the stopwatch
Stop the time when the Magnesium fully dissolves
Repeat at different temperatures and compare results
Result:
With an increase in the temperature, the rate of reaction will increase
This is because the particles will have more kinetic energy than the required activation energy, therefore more frequent and successful collisions will occur, increasing the rate of reaction

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17
Q

graph for higher concentration

A

Compared to a reaction with a reactant at a low concentration, the graph line for the same reaction but at a higher concentration has a steeper gradient at the start and becomes horizontal sooner
This shows that with increased concentration of a solution, the rate of reaction will increase

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18
Q

graph for particle size/surface area

A

Compared to a reaction with lumps of reactant, the graph line for the same reaction but with powdered reactant has a steeper gradient at the start and becomes horizontal sooner
This shows that with increased surface area of the solid, the rate of reaction will increase

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19
Q

graph for catalyst

A

The diagram shows that when a catalyst is used, the activation energy is reduced as it creates an alternative pathway requiring lower activation energy, allowing more successful and frequent collisions
This shows that when a catalyst is used, the rate of reaction will increase

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20
Q

graph for temperature

A

Compared to a reaction at a low temperature, the graph line for the same reaction but at a higher temperature has a steeper gradient at the start and becomes horizontal sooner
This shows that with increased temperature, the rate of reaction will increase

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21
Q

photochemical reactions + properties

A

Light wave energy (UV or Radiation = α,β,γ)acts as electromagnetic waves
considered as an energy ‘bullets’ called photons ​
have sufficient energy to overcome the activation energy to break chemical bonds.
greater the intensity of light , the more reactant molecules are likely to gain the required energy (activation energy) - reaction speed increases.​
not a catalyst - the photons of energy are used up in the chemical changes they induce, the photons cannot be recycled

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22
Q

photosynthesis characterisitc

A

Requires the input of sunlight energy and the green chlorophyll molecules absorb the photon energy packets of light and initiate the chemical changes.
> Intensity Of Light, > Rate Of Photosynthesis.
Factors affecting the rate - light, temperature & CO2 concentration

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23
Q

photography characteristic

A

Each silver halide (AgCl, AgBr, AgI) salt has a different sensitivity to light (photosensitive).​
All three are used in the production of various types of photographic film to detect visible light and beta and gamma radiation from radioactive materials.
AgI is the least sensitive and used in X-ray radiography, AgCl is the most sensitive and used in ‘fast’ film for cameras.

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24
Q

equation for photography

A

When radiation hits the film the silver ions in the salt are reduced by electron gain to silver​:
Ag+ + e- → Ag​
Halide ion is oxidized to the halogen molecule by electron loss​:
2X- → X2 + 2e- (X = halogen atom, Cl, Br or I)​
Balance Equation :
2AgX → 2Ag + X2

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25
Q

what are NPK fertilisers

A

Fertilisers contain nitrogen, potassium and phosphorus
Nitrogen promotes healthy leaves, potassium promotes growth and healthy fruit and flowers and phosphorus promotes healthy roots

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26
Q

what do fertiliser compounds contain

A

Fertiliser compounds contain the following water-soluble ions:
Ammonium ions, NH4+ and nitrate ions, NO3–, are sources of soluble nitrogen
Phosphate ions, PO43- are a source of soluble phosphorus
Most common potassium compounds dissolve in water to produce potassium ions, K+

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27
Q

what do fertiliser compounds contain

A

Fertiliser compounds contain the following water-soluble ions:
Ammonium ions, NH4+ and nitrate ions, NO3–, are sources of soluble nitrogen
Phosphate ions, PO43- are a source of soluble phosphorus
Most common potassium compounds dissolve in water to produce potassium ions, K+

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28
Q

what is displacement of ammonia

A

Ammonia can be displaced from its salts by the addition of an alkali substance

29
Q

use of ammonia displacement

A

Farmers regularly add basic substances such as calcium hydroxide to their soil to neutralise any excess soil acidity
If too much of the basic substance is added or if it has been added too soon after fertiliser has been added, then an ammonia displacement reaction may occur
This involves the loss of nitrogen from the fertiliser, nullifying its effectiveness as a fertiliser
For example, the salt ammonium chloride is used extensively in fertilisers and reacts with calcium hydroxide:
2NH4Cl + Ca(OH)2 → CaCl2 + 2NH3 + 2H2O

30
Q

what are the sources of sulfur

A
  • Sulfur is found in its elemental state underground in the USA, Mexico and Poland
  • It is also a by-product from the removal of sulfur from petroleum and natural gas
  • Sulfur can also be obtained from sulfide ores
31
Q

what are the uses of sulfur

A
  • The main use of sulfur is in making sulphuric acid which is a very important chemical used in many industries
  • It is also used extensively in making rubber tyres more flexible (vulcanising), where the rubber is heated with sulfur
32
Q

making of sulfur dioxide

A

Sulfur dioxide can be made by the direct combination of sulphur with oxygen
This is the method used in the first stage of the manufacture of sulfuric acid:
S + O2 → SO2

33
Q

how is sulfuric acid manufactured

A

Sulfuric acid is synthesised by the Contact process which use sulfur and oxygen from air and is done in three distinct stages

34
Q

stages of sulfuric acid

A

Stage 1: The first stage is the oxidation of sulfur:
S + O2 → SO2

Stage 2: The main stage is the oxidation of sulfur dioxide to sulfur trioxide using a V2O5 catalyst:
2SO2 + O2 ⇌2SO3
The conditions for the main stage of production of sulfur trioxide need to be considered:
Temperature: 450ºC
The reaction is exothermic, so increasing the temperature shifts the position of equilibrium to the left in the direction of the reactants
Therefore the higher the temperature, the lower the yield of sulfur trioxide
The optimum temperature is a compromise between a higher rate of reaction at a higher temperature and a lower equilibrium yield at a higher temperature
Pressure: 2 atm
An increase in pressure shifts the position of equilibrium to the right in the direction of a smaller number of gaseous molecules
However the position of equilibrium lies far to the right (the equilibrium mixture contains about 96% sulfur trioxide)
So the reaction is carried out at just above atmospheric pressure because:
a) it is not worth spending the extra energy or money required to produce high pressures
b) a higher pressure would increase the problems of dealing with the corrosive mixture of gases
Stage 3: Once stage 2 is completed, the sulfur trioxide is absorbed into a solution of 98% sulphuric acid to produce a thick liquid called oleum:
SO3 + H2SO4 → H2S2O7
It is not absorbed into water because a fine mist of sulfuric acid would be produced and this would be difficult to condense and is also highly dangerous
Oleum is added to water to form concentrated sulfuric acid:
H2S2O7 + H2O → 2H2SO4

35
Q

properties of sulfuric acid

A

Sulfuric acid is a strong dibasic acid as two of its hydrogen atoms can be replaced by a metal
It reacts in a similar way to other acids with metal carbonates, oxides, hydroxides (and ammonia) and metals, e.g:
ZnO + H2SO4 → ZnSO4 + H2O
Mg + H2SO4 → MgSO4 + H2
Na2CO3 + H2SO4 → Na2SO4 + CO2 + H2O
Concentrated sulphuric acid is corrosive and a powerful oxidising agent
Concentrated sulphuric acid is also a very powerful dehydrating agent and is very good at removing water from other substances
For example, if mixed with sugar (C6H12O6), concentrated H2SO4 will remove water molecules and leave behind carbon in a spectacular looking reaction that produces a tower of pure carbon

36
Q

what are reversible reactions

A

In reversible reactions, the product molecules can themselves react with each other or decompose and form the reactant molecules again
It is said that the reaction can occur in both directions: the forward reaction (which forms the products) and the reverse direction (which forms the reactants)

37
Q

what are hydrated salts

A

Hydrated salts are salts that contain water of crystallisation which affects their molecular shape and colour
Water of crystallisation is the water that is stoichiometrically included in the structure of some salts during the crystallisation process

38
Q

what are anhydrated salt

A

Anhydrous salts are those that have lost their water of crystallisation, usually by heating, in which the salt becomes dehydrated

39
Q

Dehydration of Hydrated Copper (II) Sulfate:

A

Hydrated Copper (II) Sulfate ⇌ Anhydrous Copper (II) Sulfate + Water
When anhydrous copper (II) sulfate crystals are added to water they turn blue and heat is given off (exothermic); this reaction is reversible
When Copper (II) Sulfate crystals are heated in a test tube, the blue crystals turn into a white powder and a clear, colourless liquid (water) collects at the top of the test tube
The form of Copper (II) Sulfate in the crystals is known as Hydrated Copper (II) Sulfate because it contains water of crystallisation
When Hydrated Copper (II) Sulfate is heated, it loses its water of crystallisation and turns into anhydrous Copper (II) Sulfate:
CuSO4.5H2O (s) ⇌ CuSO4 (s) + 5H2O (l)

40
Q

Dehydration of Hydrated Cobalt (II) Chloride:

A

Hydrated Cobalt (II) Chloride ⇌ Anhydrous Cobalt (II) Chloride + Water
When anhydrous blue cobalt(II) chloride crystals are added to water they turn pink and the reaction is reversible
When the cobalt(II) chloride crystals are heated in a test tube, the pink crystals turn back to the blue colour again as the water of crystallisation is lost
The form of cobalt(II) chloride in the crystals that are pink is known as hydrated cobalt (II) chloride because it contains water of crystallisation
When hydrated cobalt(II) chloride is heated, it loses its water of crystallisation and turns into anhydrous cobalt(II) chloride:
CoCl2.6H2O (s) ⇌ CoCl2 (s) + 6H2O (l)

41
Q

what is a state of equilibrium

A

When during the course of reaction, the rate of the forward reaction equals the rate of the reverse reaction, then the overall reaction is said to be in a state of equilibrium

42
Q

characteristics of equilibrium

A

It is dynamic eg the molecules on the left and right of the equation are changing into each other by chemical reactions constantly and at the same rate
The concentration of reactants and products remains constant (given there is no other change to the system such as temperature and pressure)
It only occurs in a closed system so that none of the participating chemical species are able to leave the reaction vessel

43
Q

The reaction between H2 and N2 in the Haber process

A

When only nitrogen and hydrogen are present at the beginning of the reaction, the rate of the forward reaction is at its highest, since the concentrations of hydrogen and nitrogen are at their highest
As the reaction proceeds, the concentrations of hydrogen and nitrogen gradually decrease, so the rate of the forward reaction will decrease
However, the concentration of ammonia is gradually increasing and so the rate of the backward reaction will increase (ammonia will decompose to reform hydrogen and nitrogen)
Since the two reactions are interlinked and none of the gas can escape, the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached:
3H2 (g) + N2 (g) ⇌ 2NH3 (g)

44
Q

what is the position of equilibrium

A
  • Equilibrium position refers to the relationship between the concentration of reactants and products at the equilibrium state
  • When the position of equilibrium shifts to the left, it means the concentration of reactant increases
  • When the position of equilibrium shifts to right, this means the concentration of product increases
45
Q

what is the effect of catalysts on equilibrium positions

A

The presence of a catalyst does not affect the position of equilibrium but it does increase the rate at which equilibrium is reached
This is because the catalyst increases the rate of both the forward and backward reactions by the same amount (by providing an alternative pathway requiring lower activation energy)
As a result, the concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst

46
Q

what is le chatelier’s principle

A

Le Chatelier’s Principle states that when a change is made to the conditions of a system at equilibrium, the system automatically moves to oppose the change.
The principle is used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration.

47
Q

effect of temperature in equilibrium

A

increase in temperature: equilibrium moves in the endothermic direction to reverse the change

decrease in temperature: equilibrium moves in the exothermic direction to reverse the change

48
Q

effect of pressure in equilibrium

A

increase in pressure: equilibrium shifts in the direction that produces the smaller number of molecules of gas to decrease the pressure again

decrease in pressure: equilibrium shifts in the direction that produces the larger number of molecules of gas to increase the pressure again

49
Q

effects of concentration in equilibrium

A

increase in concentration: equilibrium shifts to the right to reduce the effect of increase in the concentration of a reactant

decrease in concentration: equilibrium shifts to the left to reduce the effects of a decrease in reactant (or an increase in the concentration of a product)

50
Q

what is a fuel

A

A fuel is a substance which releases energy when burned

51
Q

combustion reaction with hydrocarbon fuels what r the products

A

When the fuel is a hydrocarbon then water and carbon dioxide are produced in combustion reactions

52
Q

what is fuel efficiency

A

The efficiency of a fuel refers to how much energy is released per unit amount
We can measure the efficiency of fuels by calorimetry

53
Q

how to find a fuels heat of combustion

A

A known mass of the fuel is combusted and used to heat up a known mass of water to calculate its heat of combustion
Different fuels heat the water by different amounts and they can be analysed and compared in this way

54
Q

calorimetry experiment - combustion

A

Using a measuring cylinder, place 100 cm3 of water into a copper can
Measure and record the initial temperature of the water
Fill the spirit burner with test substance and measure and record its mass
Place the burner under the copper can and light the wick
Stir the water constantly with the thermometer and continue heating until the spirit burner burns out
Measure and record the highest temperature of the water
Calculation:

Temperature change of water = final temperature – initial temperature

Number of moles burned = change in mass ÷ molecular mass

Amount of energy = change in temperature x mass of water x specific heat capacity

Amount of energy per mole (J mol-1) = total amount of energy ÷ moles burned

55
Q

hydrogen fuel advantages disadvantages

A
  • Hydrogen is used in rocket engines and in fuel cells to power some cars

Advantages:

  • It releases more energy per kilogram than any other fuel (except for nuclear fuels)
  • It does not pollute as it only produces water on combustion, no other product is formed

Disadvantages:

  • Expensive to produce and requires energy for the production process
  • Difficult and dangerous to store and move around (usually stored as liquid hydrogen in highly pressurized containers)
56
Q

radioactive isotopes as fuels

A

Uranium-235 undergoes decay and gives off heat energy which nuclear power stations harness
The heat it produces is used to heat water to steam, which in turn is used to power turbines to generate electricity
Nuclear fuel energy is clean as it does not produce pollutants such as CO2 or oxides of nitrogen or sulfur
But nuclear power plants are expensive to build and maintain as well as being potentially dangerous in the event of an accident as radioactive materials may be released

57
Q

what is a fuel cell

A

A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode
These cells are becoming more common in the automotive industry to replace petrol or diesel engines

58
Q

hydrogen fuel cells reactions

A

H2 and O2 are pumped through two porous electrodes where the half-reactions occur
The following reaction occurs at the anode:
2H2 → 4H+ + 4e-
At the cathode the following reaction takes place:
4H+ + O2 + 4e- → 2H2O
The overall reaction is:
2H2 + O2 → 2H2O
The electrons move around the external circuit from the cathode to the anode
This movement of electrons is used to drive an electric motor

59
Q

advantages of fuel cells

A

Advantages
They do not produce any pollution
They produce more energy per kilogram than either petrol or diesel
No power is lost on transmission as there are far fewer moving parts than in an internal combustion engine

60
Q

disadvantages of fuel cell

A

Materials used in producing fuel cells are expensive
High-pressure tanks are needed to store the oxygen and hydrogen in sufficient amounts
Fuel cells are affected by low temperatures, becoming less efficient
Hydrogen is expensive to produce and store

61
Q

what are physical changes

A 
Physical changes (such as melting or evaporating) do not produce any new chemical substances
These changes are often easy to reverse and mixtures produced are usually relatively easy to separate
Chemical change
62
Q

what are chemical changes

A

In chemical reactions, new chemical products are formed that have very different properties to the reactants
Most chemical reactions are impossible to reverse
Energy changes also accompany chemical changes and energy can be given out (exothermic) or taken in (endothermic)
The majority of chemical reactions are exothermic with only a small number being endothermic

63
Q

what are redox reactions

A

Oxidation and reduction take place together at the same time in the same reaction
These are called redox reactions

64
Q

what are oxidation reactions

A

Oxygen is added to an element or a compound
An element, ion or compound loses electrons
The oxidation state of an element is increased

65
Q

what are reductions

A

Oxygen is removed from an element or a compound
An element, ion or compound gains electrons
The oxidation state of an element is decreased

66
Q

what is an oxidising agent

A

A substance that oxidises another substance, in so doing becoming itself reduced
Common examples include hydrogen peroxide, fluorine and chlorine

67
Q

what is a reducing agent

A

A substance that reduces another substance, in so doing becoming itself oxidised
Common examples include carbon and hydrogen
The process of reduction is very important in the chemical industry as a means of extracting metals from their ores

68
Q

how to identify redox reactions

A

Redox reactions can be identified by the changes in the oxidation states when a reactant goes to a product

69
Q

identify redox reactions by color change

A

The tests for redox reactions involve the observation of a colour change in the solution being analyse
Two common examples are acidified potassium manganate(VII), and potassium iodide
Potassium manganate (VII), KMnO4, is an oxidising agent which is often used to test for the presence of reducing agents
When acidified potassium manganate (VII) is added to a reducing agent its colour changes from pink-purple to colourless
Potassium iodide, KI, is a reducing agent which is often used to test for the presence of oxidising agents
When added to an acidified solution of an oxidising agent such as aqueous chlorine or hydrogen peroxide, the solution turns a brown colour due to the formation of iodine

IGCSE Chemistry (15 decks)

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