# gcse science chemistry unit C5 cards

HideShow resource information
• Created by: charlie
• Created on: 08-06-13 17:18

## the mole

• 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)
1 of 14

## reacting masses + empirical formulas

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
2 of 14

## concentration

• 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?

3 of 14

## concentration

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)
4 of 14

## titrations

• 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?

5 of 14

## gas volumes

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)

6 of 14

## following reactions

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
7 of 14

## equilibrium

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
8 of 14

## changing equilibrium

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)
9 of 14

## the contact process

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

10 of 14

## strong + weak acids

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

11 of 14

## more on strong + weak acids

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
12 of 14

## precipitation reactions

• 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

• chloride gives white precip. of lead chloride
• bromide gives cream precip. of lead bromide
• iodide gives yellow precip. of lead iodide
13 of 14

## preparing insoluble salts

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
14 of 14