4-chemical changes Flashcards
explain how the ph scale works
it is a measure of how acidic or alkaline a solution is
the lower the ph of a solution the more acidic it is
the higher the ph of a solution, the more alkaline it is
a neutral substance has ph of 7(pure water)
give examples of things with ph 1
car battery acid
stomach acid
e.g. things with ph 3
vinegar
lemon juice
e.g. things with ph 4
acid rain
things with ph 5
normal rain
things with ph 9
washing up liquid
things with ph 10
pancreatic juice
ph 11 examples
soap powder
ph 12 examples
bleachh
ph 13/14 examples
causatic soda (drain cleaner)
what is an indicator
ways to measure pH
a dye that changes colour depending on whether its above or below a certain pH
Some indicators contain a mixture of dyes that mean they gradually change colour over a broad range of pH . =WIDE range of indicators, useful for estimating ph of a solution e.g. universal indicator gives colours
how to measure pH
pH probe
why are they more accurate than indicators
attached to a pH meter to measure pH electronically
probe is placed in the solution youre measuring and the pH is given on a digital display as a numerical value = more accurate than indicator
what is an acid
a substance that forms aqueous solutions with a ph less than 7
acids from H+ ions in water
what is an acid
a substance that forms aqueous solutions with a ph less than 7
acids from H+ ions in water
what is an alkali
a base is a substance with a ph greater than 7
an alkali is a base that dissolves in water to form a solution with a pH greater than 7
alkalis form OH- ions in water
what is the neutralisation equation
show in terms of words
show in terms of ions
acid + base => salt + water
H+(aq)+ OH- (aq)=> H2O(l)
explain what happens in neutralisation
when an acid neutralises base
products are neutral with ph of 7
indicator can be used to show that a neutralisation reaction is over
neutralisation reactions of strong acids and alkalis can be used to calculate the concentration of an acid or alkali by titration
acids ionise in aqueous solutions
give example equations
HCL => H+ + CL-
HNO3 => H+ + NO3-
explain how strong acids behave in aqueous solutions
eg sulfuric,hydrochlric,nitric
ionise completely in water
all acid particles disassociate to release H+ ions
explain how weak acids behave in aqueous solutions
ethanoic,citric, carbonic
dont fully ionise in solution
only a small proportion of acid particles disassociate to release H+ ions
explain ionisation of weak acids
reversible reaction
sets up an equilibrium between the undissociated and disassociated acid
since only a few particles release H+ ions, position of equilibruim lies well to the left
explain reactions of acids involving H+ ions
they react with other substances
if the concentration of H+ ions is higher, the rate of reaction will be faster so strong acids will be more reactive than the weak acids of the same concentration
what is the pH of an acid/alkaliis a measure of what
the concentration of H+ ions in the solution
for every decrease of 1 on the pH scale
the concentraition of H+ ions increases by a factor of 10
an acid which the ph of 4 has 10 x the conc of H+ ions of ann acid that has a pH of 5
for a decrease of 2 on the pH scale, the concentration of H+ ions increases by a factor of 100
rule=
factor H+ ion concentration changes by= 10^-x
x is the difference in pH, soif pH falls from 7 to 4, difference is -3
factor increased by is increased by 10-(-3) so 10^3
ph of a strong acid is less than ph of weaker acid if they have same concentration
what does strength of acid tell u
acid strength tels you what proportion of acid molecules ionise in water
what does the concentration of an acid tell u
it measures how much acid there is in a certain volume of water
concentration is basically how watered down your acid is
the larger the amount of acid there is in a certain volume of liquid, the more concentrated the acid is
you can have dilute but strong acid vice versa
ph will decrease with increasing acid conc regardless strong/weak acid
what are metal oxides and hydroxides and what they do in water
they are bases that dissolve in water
these compounds are alkalis
what happens to bases that dont dissolve in water
they take part in neutralisation reactions with acids
what do metal oxides and hydroxides form after reacting with acids s
salt and water
acid + metal oxide- salt + water
acid + metal hydroxide- salt+water
explain metal carbonates
reacts with acids to produe
acid + metal carbonate- salt + water+ carbon dioxide
explain metal carbonates
reacts with acids to produe
acid + metal carbonate- salt + water+ carbon dioxide
how can you make soluble salts from an insoluble base
gently warm dilute acid with bunsen burner then turn off
add insoluble base to acid at a time until no more reacts so base in in excess
all acid should be neutralised so excess solid sinks
filter out excess solid to get salt solution
gently heat solution using water bath/ electric heater to evaporate some water to make it more concentrated . leave to cool
crystals formed
filter out and dry
explain properties of reactivity series
reactivity determined by how easily metal leaves electron to form positive ions
the higher up the more reactive
metals react with water/acid to make positive ions
what is the order of the reacitivity series
potassium
sodium
lithium
calcium
magnesium
carbon
zinc
iron
hydrogen
copper
what does acid + metal give
salt + hydrogen
from a reaction between an acid and metal, how is speed of reaction indicated
rate at which bubbles of hydrogen are given off
more reactive the metak, the faster the reaction will go
how could you investigate the reactivity of metals
measure temperature change of reaction with acid/ water over set time period
use same mass and surface area of metal each time
more reactive metal gives greater temperature change
metal + water ?
what metals react with water
metal hydroxide + hydrogen
e.g. potassium, sodium, lithium, calcium
explain oxidation
commmon metals react with oxygen to form oxides , they are often the ores that metals need to be extracted from
= gain of oxygen
explain reduction
a reaction that separates a metal from its oxide is called a reduction reaction
loss of oxygen
how can metals be extracted from ores
chemically by reduction using carbon
the ore is reduced as oxygen is removed from it
carbon gains oxygen so its oxidised
position of metal in reactivity series determines whether it can be extracted by reduction with carbon
metals higher than carbon is extracted with electrolysis(expensive)
-below carbon, extracted with carbon as carbon can only take oxygen away from metals less reactive than carbon itself
redox reactions
oxidation
reduction
oxidation is loss of electrons
gain of electrons is reduction
OILRIG
Give an example of a redox reaction
reduction and oxidation can occur at the same time
iron atoms are oxidised to give fe 2+ ions when reacting with dilute acid
Fe+2H+->Fe2+ +H2
iron atoms lose electrons, oxidised by hydrogen ions
Fe-2e- ->Fe2+
Hydrogen ions gain electrons, reduced by iron atoms 2H+ + 2e- -> H2
explain displacement reactions
a more reactive metal will displace a less reactive metal from its compound
e.g iron in copper sulfate solution, iron displaces copper= iron sulfate solution + copper
iron loses 2 electrons (2+ion)-oxidised
copper gains 2 electrons = reduced
metal ion always gains electrons and is reduced, metal atom loses electrons and is oxidised
what happens during electrolysis
elecric current passsed through electrolyte (molten or dissolved ionic compound) ion moves towards electrodes , where they react and compound decomposes
what happens in ions in electrolysis
positive ions in electrolyte move near cathode (-ve ) and gain electrons where reduced
negtive ions move to anode (+ve) and loses electrons =oxidised
creates flow of charge through electrolyte as ions travel to electrodes
= uncharged elements formed and discharged from electrolyte
why should electrodes be inert
why do ionic compounds have to be molten
electrodes should be inert so they dont react with electrolyte
molten ionic compounds can be electrolysed as ions can move freely to conduct electricity
molten ionic liquids are always broken up into elements
positive metal ions are reduced to element at cathode
negative non metal ions are oxidised at anode
