Edexcel IGCSE Chemistry. Topic 1: Principles of chemistry. Chemical formulae, equations and calculations. Notes.

Transcription

1 Edexcel IGCSE Chemistry Tpic 1: Principles f chemistry Chemical frmulae, equatins and calculatins Ntes

2 1.25 write wrd equatins and balanced chemical equatins (including state symbls): fr reactins studied in this specificatin, fr unfamiliar reactins where suitable infrmatin is prvided (g) means gas, (s) means slid, (l) means liquid, (aq) means aqueus Example f wrd equatin: hydrchlric acid + sdium hydrxide -> sdium chlride + water Example f balanced chemical equatin: HCl + NaOH -> NaCl + H 2 O t balance an equatin: yu need t make sure there are the same number f each element n each side f the equatin and if there isn t use big numbers at the frnt f a cmpund t balance it e.g. 3H 2 O 1.26 calculate relative frmula masses (including relative mlecular masses) (Mr) frm relative atmic masses (Ar) Relative frmula mass (Mr) f a cmpund: sum f the relative atmic masses f the atms in the numbers shwn in the frmula In a balanced chemical equatin: sum f Mr f reactants in quantities shwn = sum f Mr f prducts in quantities shwn 1.27 knw that the mle (ml) is the unit fr the amunt f a substance Chemical amunts are measured in mles (therefre it is the amunt f substance). The symbl fr the unit mle is ml. The mass f ne mle f a substance in grams is numerically equal t its relative frmula mass. Fr example, the Ar f Irn is 56, s ne mle f irn weighs 56g. The Mr f nitrgen gas (N 2 ) is 28 (2x14), s ne mle is 28g. One mle f a substance cntains the same number f the stated particles, atms, mlecules r ins as ne mle f any ther substance 1.28 understand hw t carry ut calculatins invlving amunt f substance, relative atmic mass (Ar) and relative frmula mass (Mr) Yu can cnvert between mles and grams by using this triangle r the equatin: mles = mass relative atmic mass mass = mles x relative atmic mass E.g hw many mles are there in 42g f carbn? Mles = Mass / Mr = 42/12 = 3.5 mles

3 1.29 calculate reacting masses using experimental data and chemical equatins Chemical equatins can be interpreted in terms f mles E.g. Mg + 2HCl MgCl 2 + H 2 shws that 1 ml. Mg reacts with 2 ml. HCl t prduce 1 ml. MgCl 2 and 1 ml. H 2 Masses f reactants & prducts can be calculated frm balanced symbl equatins. If yu are given the reacting mass f ne reactant and asked t find the mass f ne prduct frmed: Find mles f that ne substance: mles = mass /mlar mass Use balancing numbers t find the mles f desired reactant r prduct (e.g. if yu had the equatin:2naoh + Mg Mg(OH) 2 + 2Na, if yu had 2 mles f Mg, yu wuld frm 2x2=4 mles f Na) Mass = mles x mlar mass(f the prduct) t find mass 1.30 calculate percentage yield Percentage yield = Amunt f prduct prduced x 100 Maximum amunt f prduct pssible It is nt always pssible t btain the calculated amunt f a prduct fr 3 reasns Reactin may nt g t cmpletin because it is reversible Sme f the prduct may be lst when it is separated frm the reactin mixture Sme f the reactants may react in ways different t the expected reactin Amunt f prduct btained is knwn as yield 1.31 understand hw the frmulae f simple cmpunds can be btained experimentally, including metal xides, water and salts cntaining water f crystallisatin example experiment t find frmula f magnesium xide: weigh sme pure magnesium Heat magnesium t burning in a crucible t frm magnesium xide, as the magnesium will react with the xygen in the air weigh the mass f the magnesium xide Knwn quantities: mass f magnesium used & mass f magnesium xide prduced Required calculatins: mass xygen = mass magnesium xide - mass magnesium mles magnesium = mass magnesium mlar mass magnesium mles xygen = mass xygen mlar mass xygen

4 calculate rati f mles f magnesium t mles f xygen use rati t frm empirical frmula 1.32 knw what is meant by the terms empirical frmula and mlecular frmula mlecular frmula- the number f atms f each element in a cmpund empirical frmula- the simplest whle number rati f atms f each element in a cmpund 1.33 calculate empirical and mlecular frmulae frm experimental data Empirical frmula frm the frmula f mlecule: if yu have a cmmn multiple e.g. Fe 2 O 4, the empirical frmula is the simplest whle number rati, which wuld be FeO 2 if there is n cmmn multiple, yu already have the empirical frmula Mlecular frmula frm empirical frmula and relative mlecular mass Find relative mlecular mass f the empirical frmula Divide relative mlecular mass f cmpund by that f the empirical frmula Multiply the number f each type f atm in the empirical frmula by this number e.g. if answer was 2 and the empirical frmula was Fe 2 O 3 then the mlecular frmula wuld be empirical frmula x 2 = Fe 4 O (chemistry nly) understand hw t carry ut calculatins invlving amunt f substance, vlume and cncentratin (in ml/dm3 ) f slutin Cncentratin f a slutin can be measured in mass per given vlume f slutin e.g. grams per dm 3 (g/dm 3 ) t calculate cncentratin f a slutin use the equatin cncentratin (g dm -3 ) = mass f slute (g) vlume (dm 3 ) T calculate mass f slute in a given vlume f a knwn cncentratin use the equatin: mass = cnc x vl i.e. g = g/dm 3 x dm 3 (think abut the units!)

5 1.35 (chemistry nly) understand hw t carry ut calculatins invlving gas vlumes and the mlar vlume f a gas (24dm 3 and cm 3 at rm temperature and pressure (rtp)) Equal amunts in ml. f gases ccupy the same vlume under the same cnditins f temperature and pressure (e.g. RTP) Vlume f 1 ml. f any gas at RTP (rm temperature and pressure: 20 degrees C and 1 atmsphere pressure) is 24 dm 3 This sets up the equatin: Vlume (dm 3 ) f gas at RTP = Ml. x 24 Use this equatin t calculate the vlumes f gaseus reactants and prducts at RTP e.g. 5 mles f H 2 wuld ccupy a vlume f 24 x 5 = 120 dm 3 at RTP 1.36 practical: knw hw t determine the frmula f a metal xide by cmbustin (e.g. magnesium xide) r by reductin (e.g. cpper(ii) xide) see 1.31