gcse science chemistry unit C5 cards

• 6.023x10^23
• one mole of atoms/ molecules of a sustance will have a mass in grams equal to Ar/Mr 
• e.g Carbon Ar 12       +      carbon dioxide Mr 44

• formula for finding no. of moles: 

relative masses are masses compared to carbon-12:

• relative atomic mass is compared to 1/12th the mass of an atom of carbon-12 (isotope of carbon)

use ratios or moles to calculate masses in reacting 

• 1)write out balanced euqation+ calculate no. of moles in the desired mass of element 
• 2)look at the ratio + calculate the mass from the moles 

calculating % composition by mass of compounds: 

empirical formulas- simplest ratio of atoms in compound 

finding it from given masses+elements 

• 1)list all elements in compound 
• 2)underneath write masses being used in experiment 
• 3)divide each mass by Ar for particular element 
• 4)turn numbers into simple ratio by multiplying/dividing 
• 5)get ratio in simplest from- giving you empirical fromula 

• measure of how crowded things are (moles per dm^3)
• the more solute dissolved the more crowded solute molecules+the more concen.
• 1dm^3=1 litre 

calculating concen.

e.g. whats the concen. of a solution with 2 moles of salt in 500cm^3?

converting moles per dm^3 to grams per dm^3

e.g. you have solution of sulphuric acid of 0.04mol/dm^3 what is the concen. in grams per dm^3?

given concentrated solution+ asked to dilute it to weaken solution

e.g. produce 500cm^3 of 0.1 mol/dm^3 solution of KOH if you're given 1.0 mol/dm^3 solution of KOH + water? 

• 1) work out ratios of two concen. - divide to get no. less than 1 
• 2) multiply ratio by vol. of solution you want to end up with- tells you how much of original acid tou need to dilute 
• 3) work out vol. of water you will need 

GDA's are rought estimates

• amounts given in 100g + might eat more or less of product 
• may add other things- milk to cereals 

using sodium content to estimate salt 

e.g. slice of bread contains 0.2g sodium- how much salt does it contain?

• 1) find ratio of sodium chlorides Mr to sodiums Ar 
• 2) multiply this by amount of sodium 
• wont all come from sodium as may be other compounds- sodium nitrate (preservative)

• used to find out concen.+exactly how much acid needed to neutralise alkali (vice versa)
• use pipette+pipette filler to add alkali+indicator, fill burette with acid, at eye level add drops of acid swirling after every few drops-when end-point reached(indicator changes colour)
• first experiment is rough idea of how much needed- then repeat to spot anomalies 

use single indicators- universal indicators shows range of colour+only rought end point 

• phenolphthalein- clear (alkalis) to pink (acid)
• litmus- blue (alkalis) red (acids)

pH curves show pH against vol.- sudden change in pH gives S-shape

calculate concen.-

e.g. start with 25cm^3 of NaOH +concen. is 0.1 moles/dm^3 takes 49cm^3 to neutralise what is concen of the acid? 

collection methods depends on gas:

• 1) gas syringe- any gas to nearest cm^3 (quite accurate) but delicate 
• 2) upturned m. cyclinder/burette- bubble gas into water, no good for hydrogen chloride or ammonia as they dissolve, burette measures to 0.1 m^3 (accurate)
• 3)mass of gas produced- gas released mass dispering easily recorded- releases gas straight into room but is most accurate 

one mole of gas occupies 24 dm^3 at room temp (25d.c)+ pressure (1 atmosphere)

reactions stop when one reactant is used up:

• used up reactant= limiting reactant   + other reactants are in excess 
• amount of product is directly proportional to amount of limiting reactant 

faster rates of reaction shown by steeper curves

• when lines reach same amount of product it means they have same amount of limiting reactant 
• more limiting reactant makes reaction faster+more product 

reversible reaction- where products of reaction can react themselves to produce original reactants

reversible reactions reach equilibrium:

• start- forward reaction falls as reactants turn to products then backward reaction inc. as products turn back into reactants 
• eventually forward+backward goes at same rate- reached equilibrium 
• only takes place in closed system+concen. of reactant+products are equal +wont change 

position of equilibrium can be right or left:

• lie to right- lots of products not much reactants (concen. of product greater than reactant) 
• lie to left- lots of reactants not much products 

exact postion depends on conditions 

• temp
• pressure- only involving gases
• concen. 
• *catalyst-- doesnt change e postion- speeds up forwar+backward reaction same amount - makes quicker but still end up with same amount of product 

equilibrium tries to minimise changes made:

• temp- all reactions exo. in one direction + endo. in the other 
• dec. temp - e moves in exothermic direction to produce more heat (vice versa) lies with what is causing exothermic reaction
• pressure- only with gases- inc pressure move in direction where fewer moles of gase (vice versa)
• concen.- inc. concen. by adding more reactants- e tries to reduce shifting to right to produce more products (vice versa)

used to make sulphuric acid:

• 1) sulphur(s) + oxygen(g) = sulphur dioxide(g)
• 2)sulphur dioxide(g) + oxygen = sulphur trioxide(g) (reversible reaction)
• 3)sulhur trioxide(g) + water = sulphuric acid 

conditions in stage 2 carefully chose as reversible 

• temp-exothermic- think that reducing temp will shift e to right inc. heat+products but instead slows reaction down- temp is compromised at 450d.c for quite high yield 
• pressure- two moles product three moles reactant- inc. pressure e shifts to right t oreduce pressure+inc. product- expensive + e already on right so only 1 atmospheres used 
• catalyst- increase rate (vanadium pentoxide)- doesnt affect position of e 

good yield of around 99% of SO3  

acids produce protons in water- ionise+ produce hydrogen ions 

strong acids(sulphuric,hydrochloric,nitric)-

• ionise completely in water- every H2 atoms releases H+ 
• HCL=H+ + CL- 

weak acids(ethanoic,citric,carbonic)-

• dont fully ionise- only some H2 atoms release H+             
• CH3COOH=H+ + CH3COO- (reversible reaction)- equilibrium lies well to left 
• lower concen. of ions means less charge can be carried 

strong acids= what proportion of acid molecules ionise in water

concentrated acids=how many mole/dm^3 of acid(how watered down acid is)-total no. of dissolved

strong+weak acids react with carbonates in the same way:

• both react with magnesium to give hydrogen 
• both react with calcium carbonate to give carbon dioxide 

diference between reactions is the rate

• weak acid slower due to equilibrium- releases few H+ ions so add Mg or CaCO+ collision frequency is low, when H+ react concen. dec. so e shifts to left so more H+ ions releases + so on 
• stong acid all acid molecules ionised+ H+ ions fully disassociate- so high collision frequency

vol. of gas produced depends on amount of acid:

• rate of reaction is diff. but amount of product is the same- starting with same amount+concen.
• as concen. the same no. of molecules in litre will be the same + each can only let go of 1 H+ therefore total vol. of H2 gas will be he same 

• involve ions- to react ions need to move + collide 
• so therefore ionic substance has to be molten or in solution- really quick reaction as high collison frequency

ionic equations show the useful bits:

• precip. reaction- start with 2 solutions (aq) + end up with a solid (s)
• spectator ions= ions that dont change in the reaction- dissolved before + still afterwards 
• ionic equations ignore the spectator ions

testing for sulphate ions 

• add dilute HCL then barium chloride 
• white precip. of barium chloride means original compound was sulphate 

testing for halide ions 

• add dilute nitric acid then lead nitrate 
• chloride gives white precip. of lead chloride 
• bromide gives cream precip. of lead bromide 
• iodide gives yellow precip. of lead iodide 

picking right reactants 

• e.g. making lead iodide need lead ions + iodide ions + need to be in solution so cant move out
• nitrates soluble so used solution of lead nitrate + can get iodide ions from potassium iodide 
• mix ingredients togeth gives a salt (precip reaction)

then... precipitate, filter, rinse, dry... (use distilled water as contains no other ions)

• precipitate(mix) solutions in small beaker with distilled water so ions dissolve- salt precips out
• fliter- using filter paper+funnel stick in concical flask+ pour beaker contents into it+rinse  beaker with distilled water + do it again making sure all product is gone 
• rinse content with distilled water making sure all soluble salts washed away 
• 
  
• scrape onto fresh filter paper+leave to dry