Chemistry module C3
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What are reactions that release energy called?
- exothermic -gives out energy in form of heat
- combustion / burning examples of exothermic
What are reactions that take in energy called?
- less common
- usually causes temperature to drop
- e.g. thermal decomposition where heat needs to be applied
acid added to alkali - what sort of reaction?
Give an example of endothermic reaction?
dissolving ammonium nitrate in water - causes a drop in the temperature of the water
what effect do any changes in bonds have on what type of reaction is taking place?
- in a chemical reaction old bonds are broken, new ones are formed.
- energy must be supplied to break bonds (endothermic process)
- energy is released when new bonds are formed (exothermic process)
- If more energy released the reaction is exothermic.
- If more energy is required the process is endothermic
How are ice packs for injuries made?
- cold pack has an outer bag of ammonium nitrate and an inner bag of water.
- Pressure causes inner bag to break allowing them to mix, ammonium nitrate dissolves in the water
- this is an endothermic reaction
- it draws heat from the injury
What is specific heat capacity?
amount of energy required to raise the temperature of a 1 gram of a material by 1 degree C.
What is the specific heat capacity of water?
4.2 J / g / °
What is the calorimetric experiment?
- heat water by burning a liquid fuel
- measure how much fuel burnt and temperature change of the water
- shield water from draughts
What are the calculations used in the calorimetric experiment
- find mass of fuel used by taking away the final mass of the burner and fuel from the initial mass of fuel and burner
- energy used (in Joules) = (mass of water ) x (specific heat capacity of water
formula for calorimetric energy?
- energy transferred = mass of water (grams) x specific heat capacity of water (4.2 J/g °C) x temp change (°C)
- energy = m x c x temp change ( symbol is triangle then "T" in book)
- for energy per gram of fuel divide by mass of fuel burnt
How can you make sure energy of fuels test results are fair?
- Repeat experiment several times
- keep everything except fuel the same
- ignore any anomalous (Exceptional) results
What is the rate of a chemical reaction?
- how fast reactants are changed into products
- reaction is over when one reactant is completely used up
Examples of slow chemical reactions?
- rusting of iron
- chemical weathering like acid rain damage to limestone buildings
Example of moderate chemical reaction?
a metal, e.g. magnesium, reacting with dilute acid to produce a steady stream of bubbles
Example of a fast chemical reaction?
- explosion is really fast - over in fraction of a second
How can you do an experiment to follow a reaction?
- rate of reaction that a gas is produced;
- measure how quickly gas is produced
- measure mass falling a gas released, or
- measure volume of gas given off
- plot results against time
What is the collision theory?
- rate of reaction depends on;
- collision frequency of reacting particles (how often they collide)
- more collisions the faster the reaction
- energy is transferred during collision
- particles must collide with enough energy for the collision to be successful.
what does the amount of product you get from a reaction depend on? (the yield)
- depends on the amount of reactant you start with
- more reactant = more particles = more reactions = more product
- amount of product is directly proportional to the limiting reactant (half particles of limiting reactant you get half the yield)
- limiting reactant is the reactant that is all used up.
- any of other reactant left is in excess
Collision theory - name four factors that increase rate of reaction?
- 1 increasing the temperature
- 2 increasing concentration or pressure
- 3 smaller solid particles / more surface area
- 4 a catalysts
What is the effect of temperature on reactions?
- temp increased = particles move more quickly
- quicker particles = more collisions
- temp increased = more energy in collisions
- reactions only if collisions with sufficient energy.
- Therefore increase of temp - increases rate of reaction.
What effect does concentration or pressure on rate of reaction
- if solution more concentrated more particles of reactant in same volume
- collisions more likely
- in gas, increase pressure means molecules are more crowded so frequency of collision increases
- both increase rate of reaction
effect of smaller solid particles / more surface area on rate of reaction?
- if reactant solid, breaking it up into smaller bits increases surface area
- particles around it have more area to work with
- frequency of collisions increases
- soluble pain killer dissolve quicker if broken up
- fine combustible powders in air burn very very fast - with spark can explode, factories making custard powder and flour and powdered sulfur have to be very careful
effect of catalyst on rate of reaction?
- catalyst = substance which speeds up reaction without being chemically changed or used up
- only need tiny bit to catalyse large reactions
- catalysts fussy as which reactions they work for
- works by giving reacting particles a surface to stick where they bump into each other
- reduces energy required
- increases successful number of collisions not the overall number of collisions
In periodic table, elements have two numbers
what is the bigger one, at the top?
In periodic table, elements have two numbers
what is smaller number at the bottom?
atomic mass (also known as proton number)
What is relative formula mass?
- all relative atomic masses added together
How do brackets effect the relative formula mass of compounds?
- If there is a 2 after brackets, e.g Ca(OH)2 , then there is two times everything within the brackets.
- e.g. Ca =40, O=16, H=1.
- therefore =40 + (16 +1) x 2
- therefore = 40 + 34 = 74
What happens to atoms in a chemical reaction?
- No atoms are destroyed
- No atoms are created
What happens to mass in chemical reactions?
- as no atoms created or destroyed there are same number and type of atoms on each side of a reaction equation
- mass is conserved
What is atom economy of a reaction?
- how much of the mass of the reactants is wasted when manufacturing a chemical.
- atom economy = atom Mr of desired product / total Mr of all products x 100
- 100 atom economy means all atoms in reactants turned into useful desired products.
- higher % greener
- useful by-products increase to increase
How is percentage yield worked out?
actual yield / predicted yield x 100
Give 4 reasons why yields are less than 100% for chemical reactions?
- 1 evaporation
- 2 not all reactant react to make product, e.g. Haber process - reversible reaction so reverse reaction taking place
- 3 filtration - will always lose some solid or liquid
- 4 transferring liquids - some always left behind
What are the benefits of batch production and when is it used?
- pharmaceutical drugs - complicated to make with fairly low demand.
- batch production cost effective as low start up cost - cheaper multi purpose machine
- (however labour intensive - set up, cleaning etc, plus hard to keep same quality)
When is continuous production used and what are the pro's and con's?
- large scale industrial manufacture of popular chemicals E.g Haber process for ammonia
- production never stops therefore not emptying reactor etc
- runs automatically
- quality is very consistent
- start up costs huge and needs to be run at full capacity.
Why are pharmaceutical drugs expensive?
- research and development
- trialling to get legal approval
- manufacture - multi step, labour intensive
- takes about 12 years & £900m to develop new drug and get on market
How do you extract a substance from a plant?
- 1) crush
- 2) boil and dissolved in suitable solvent
- 3) extract with chromatography
What happens to a pure substances during chromatography?
- pure substances are not separated during chromatography.
- It will move in one blog
What is the effect on boiling and melting points if a substance is not pure?
- Each pure substance will have specific melting and boiling points.
- If the substance is not pure these will change.
- e.g. water 0°C, 100°C - if add salt may melt at -2°C.
What are allotropes?
- different structural forms..
- .. of same element..
- .. of same physical state...
Name two allotropes of carbon
Four facts about diamond?
- 1) diamonds are lustrous (sparkly) and colourless (ideal for jewellery)
- 2) each carbon atom form 4 covalent bonds. in a very rigid covalent structure. (ideal cutting tools).
- 3) all those strong covalent bonds take a lot of energy to break -very high melting point.4) doesn't conduct electricity because it has no free electrons or ions.
Five facts about graphite?
- 1) graphite is black and opaque but still kind of shiny.
- 2) each carbon atom only forms three covalent bonds creating sheets of carbon atoms which are free to slide over each other.
- 3)layers are held weakly so they are slippery and can be rubbed off onto paper (pencil) (also ideal lubricating material).
- 4) graphite has high melting point - the covalent bonds need loads of energy to break.
- 5) since only three of each carbon's four outer electrons are used in bonds - there are lots of delocalised (spare) electrons that can move. This means graphite conducts electricity.
Diamond and graphite are G.... M........ S.........?
Giant Molecule Structures
Why can carbon form structures like diamond and graphite?
- Carbon can form lots of covalent bonds with itself.
- This means can form giant molecular structures.
- Due to all covalent bonds, giant molecular structures are strong, high melting points, don't dissolve in water.
- Don't usually conduct electricity - as no free electrons or ions (note graphite is an exception).
What are fullerenes?
- molecules of carbon
- shaped like closed tubes, or hollow balls or ellipsoid
How can fullerenes be used?
- can be used to 'cage' other molecules
- another atom or molecule can be inside
- this can be used to deliver a drug into the body e.g. slow release
- fullerenes can be joined to form nanotubes - tiny hollow carbon tubes
- nanotubes have huge surface area
- nanotubes could help make great industrial catalysts
- individual catalysts could be attached to the nanotubes
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