Physical Chemistry Flashcards
What pH is the strongest acid?
pH 0
What pH is the strongest alkali?
pH 14
What pH is a neutral substance?
ph 7
What are two examples of substances at pH 1?
car battery acid
stomach acid
What are two examples of substances at pH 3?
vinegar
lemon juice
What is an example of a substance at pH 4?
acid rain
What is an example of a substance at pH 5?
normal rain
What is an example of a substancce at pH 7?
pure water
What is an example of a substancce at pH 8.5?
washing-up liquid
What is an example of a substancce at pH 10?
pancreatic juice
What is an example of a substancce at pH 11?
soap powder
What is an example of a substancce at pH 12?
bleach
What is an example of a substancce at pH 13.5?
caustic soda (drain cleaner)
What is an indicator?
a dye that changes colour depending on the pH of a solution
Is universal indicator a useful indicator? Why?
it is a very useful combination of dyes which gives a wide range of colours
What does litmus paper test? What is its colour change?
litmus paper tests whether a solution is acidic or alkaline because it changes colour at about pH 7
acidic: red
neutral: purple
alkaline: blue
What is the colour change of phenolphthalien?
acidic: colourless
alkaline: bright pink
What is the colour chang of methyl orange?
acidic: red
alkaline: yellow
An acid is source of … ions
Acids have a pH of … than 7
An acid is source of hydrogen ions (H+)
Acids have a pH of less than 7
A base is a substance that can … an acid
Alkalis are … bases
An alkali is a source of … ions and has a pH of … than 7
A base is a substance that can neutralise an acid
Alkalis are soluble bases
An alkali is a source of hydroxide ions (OH-) and has a pH of greater than 7
What is the reaction between an acid and a base? What is this called?
acid + base → salt + water
this is called neutralisation
Neutralisation can also be see in terms of ions. What is this equation?
H+ (aq) + OH-(aq) → H2O (l)
What does it mean if we have indigestion? What do indigestion tablets contain?
indigestion means you have too much hydrochloric acid in your stomach
indigestion tablets contain bases that neutralise the acid
Metal oxides are generally …
This means they will react with acids to form … and …
Metal oxides are generally bases
This means they will react with acids to form a salt and water
If the acid is hydrochloric acid, the salt will be a metal chloride
If the acid is sulphuric acid, the salt will be a metal sulphate
If the acid is nitric acid, the salt will be a metal nitrate
What does the reaction of acid and metal carbonates produce?
acid + metal carbonate → salt + water + carbon dioxide
If the acid is hydrochloric acid, the salt will be a metal chloride
If the acid is sulphuric acid, the salt will be a metal sulphate
If the acid is nitric acid, the salt will be a metal nitrate
Salts can be … or …
Salts can be soluble or insoluble
Sodium, potassium and ammonium salts are soluble/insoluble
Sodium, potassium and ammonium salts are soluble
Nitrates are soluble/insoluble
Nitrates are soluble
Most carbonates are soluble/insoluble - except for …
Most carbonates are insoluble - except sodium, potassium and ammonium carbonates
Most chlorides are soluble/insoluble - except for …
Most chlorides are soluble - except for silver and lead chloride
Most sulphates are soluble/insoluble - except for …
Most sulphates are soluble - except for lead (II), barium and calcium sulphate
What two ways can you make soluble salts?
using acids and insoluble bases
using an alkali
How do you make soluble salts using acids and insoluble salts?
- You need to choose the right acid, and an insoluble base (most metal oxides, metal carbonates and metal hydroxides are insoluble)
- Add the metal oxide, carbonate or hydroxide to the acid - the solid will dissolve in the acid as it reacts. You’ll know when all the acid has been neutralised because the excess solid will sink to the bottom of the flask and remain there
- You can filter out the excess base to get the salt solution
- To get pure, solid crystals of the salt, evaporate off the water
Which acid and which insoluble base would you choose to make copper nitrate? What is this equation?
react nitric acid and copper carbonate
copper carbonate + nitric acid → copper nitrate + carbon dioxide + oxygen
CuCO3 (s) + 2HNO3 (aq) → Cu(NO3)2 (aq) + CO2 (g) + H2O (l)
Why can’t you use the reacting acids with insoluble bases method if you want to neutralise an acid with an alkali?
alkalis are soluble so you can’t filter them out if you add too much - in fact you can’t tell when you’ve added too much
How do you make soluble salts using an alkali?
- You have to add exactly the right amount of alkali to just neutralise the acid - you need to use an indicator to show when the reaction is finished
- The best way of doing this is to do a titration
- Then, repeat using exactly the same volumes of alkali and acid but without any indicator so the salt isn’t contaminated
How do you make insoluble salts? What is this called?
precipitation reactions
- Choose two solutions that contain the ions you need
- Mix these two solutions together
- Filter out the salt
When making a soluble salt you filter the … at the end but if you’re making an insoluble salt you filter the … at the end
When making a soluble salt you filter the base at the end but if you’re making an insoluble salt you filter the salt at the end
What are titrations used for?
titrations allow you to find exaclty how much acid is needed to neutralise a quantity of alkali (or vice versa)
In a titration, do you add an alkali to an acid or an acid to an alkali?
you can do titratiosn either way round
How do you carry out a titration?
- Using a pipette add some alkali (usually about 25 cm3) to a conical flask, along with two or three drops of indicator
- Fill a burette with the acid. Make sure to do this below eye level - you don’t want to be looking up if some acid spills over
- Using the buretter, add the acid to the alkali a bit at a time - giving the conical flask a regular swirl. Go especially slowly when you think the end-point (colour change) is about to be reached
- The indicator changes colour when all the alkali has been neutralised
- Record the volume of acid used to neutralise the alkali. It is best t orepeat this process a few times, making sure you get concordant results - this makes for more reliable results
Would you use universal indicator when doing titrations? Why?
no
it’s too hard to tell accurately when the reaction is over - you want a sudden colour change
You start with 25 cm3 of sodium hydroxide solution in your flask, and you know that its concentration is 0.1 moles per dm3. During your titration, you find it takes 30 cm3 of sulphuric acid (of unknown concentration) to neutralise the sodium hydroxide.
Find the concentration of the acid
1. Number of moles = concentration x volume
= 0.1 x (25/1000)
=0.0025
2. The equation for the reation is:
2NaOH + H2SO4 → Na2SO4 + 2H2O
sodium hydroxide : sulphuric acid
2 : 1
0.0025 : (0.0025 ÷ 2)
moles of sulphuric acid = 0.00125 moles
3. concentration = moles ÷ volume
= 0.00125 ÷ (30/1000)
= 0.0417 moles per dm3
What four things does the rate of reaction depend on?
temperature
concentration (or pressure for gases)
catalyst
size of particles (or surface area)
How can you tell the fastest reaction on a graph?
whichever line flattens out first must have the steepest slope compared to the others and is therefore the fastest
What three ways can you measure the speed of a reaction?
precipitation
change in mass (usually gas given off)
the volume of gas given off
How do you calculate the rate of reaction?
rate of reaction = amount of reactant used or amound of product formed ÷ time
How do you measure the speed of a reaction by precipitation?
- This is when the product of the reaction is a precipitate which clouds the solution
- OBserve a marker through the solution and measure how long it takes for it to disappear
- The quicker the marker the disappears, the quicker the reaction
When does a precipitation work to measure the speed of a reaction? Is it a good test?
this only works for reactions where the initial solution is rather see-through
the result is very subjective - different people might not agree over the exact point when the mark ‘disappears’
How you measure the speed of a reaction by change in mass?
- Measuring the speed of a reaction that prodcues a gas can be carried out on a mass balance
- As the gas is released the mass disappearing is easily measured on the balance
- The quicker the reading on the balance drops, the faster the reaction
- When the mass stops changing, the reaction has finished
How accurate is measuring the speed of a reaction by change in mass? Why? Are there any disadvantages?
very accurate because the balance is very accurate
the disadvantages are that you release the gas straight into the room and that if the flask is hot then you lose mass by evaporation
How do you measure the speed of a reaction by the volume of gas given off?
- This involves the use of a syringe to measure the volume of gas given off
- The more gas given off during a given time, the faster the reaction
- When the gas stops being produced, the reaction has finished
How accurate is measuring the speed of a reaction by the volume of gas given off? Why? What do you have to be careful of?
quite accurate because gas syringes usually give volumes accurate to the nearest millimetre
you have to be careful because if the reaction is too vigorous, you can easily blow the plunge out of the end of the syringe
What is the collision theory?
in order for their to be a reaction, particles have to collide and have to collide hard enough (with enough energy)
What two things does the rate of reaction depend on?
how often and how hard the reacting particles collide with each other
What does a larger in surface do to the rate of reaction? Why?
an increase in surface area causes more frequent collisions because there is more are to work on, so the rate of reaction is faster
What does using a higher concentration or pressure do to the rate of reaction? Why?
if a solution is made concentrated it means there are more particles to react knowkcing about between the water molecules which make collisions between the important particles more likely
in a gas, increasing the pressure means the particles are more squashed up together so they collide more frequently
as the reaction preogresses there are fewer and fewer reactant particles so they collide less frequently and the reaction rate slows down
What does a higher temperature do to the rate or reaction? Why?
when the temperature is increased the particles have more kinetic energy and move quicker
if they’re moving quicker, they’re going to collide more frequently
What does using a catalyst do t the rate of reaction? Why?
a solid catalyst works by giving the reacting particles a surface to stick to
they increase the successful collisions by lowering the activation energy (which is needed to break the initial bonds)
How do you make the collisions of particles faster?
faster collisions are only caused by increasing the temperature
Bond breaking is …. What is required to do this?
Bond breaking is endothermic
energy must be supplied to do this
Bond making is …. What is required to do this?
Bond making is exothermic
energy is released to do this
An exothermic reaction is one which … to the surroundings, usually in the form of … and usually shown by a …
An exothermic reaction is one which gives energy out to the surroundings, usually in the form of heat and usually shown by a rise in temperature
An endothermic reaction is one which … to the surroundings, usually in the form of … and usually shown by a …
An endothermic reaction is one which takes energy to the surroundings, usually in the form of heat and usually shown by a fall in temperature
The overall change in energy in a reaction is called the … change. It has the symbol …. The units of this are …
The overall change in energy in a reaction is called the enthalpy change. It has the symbol ΔH. The units of this are kJ/mol - the amount of energy in kilojoules per mole of reactant
In exothermic reactions the products are at a higher/lower energy than the reactants
They have a positive/negative energy change
In exothermic reactions the products are at a lower energy than the reactants
They have a negative energy change
In endothermic reactions the products are at a higher/lower energy than the reactants
They have a positive/negative energy change
In endothermic reactions the products are at a higher energy than the reactants
They have a positive energy change
Do catalysts affect the overall energy change of a reaction?
No, they only low the activation energy
How do you calculate the enthalpy change ( ΔH) of a reaction?
enthalphy change (ΔH) = total energy absorbed to break bonds - total energy released in making bonds
How can you find out the enthalpy changes in the lab? What two ways is this done?
calorimetry:
dissolving, displacement and neutralisation reactions
combustion
WHat is the biggest problem in calorimetry experiments? How is this reduced?
energy is lost to the surroundings
this is reduced by putting the polystyrene cu into a beaker of cotton wool to give more insulation, and putting a lid on the cup to reduce energy lost by evaporation
How do you carry out calorimetry by combustion?
To measure the amount of energy produced when a fuel is burnt, you can simply burn the fuel and use the flame to heat up some water. This method uses a metal container, usually made of copper because copper conducts heat so well
- Reduce druaghts by using a screen t oact as a draught excluder
- Put 50g of water in the copper can and record its temperature
- Weigh the spirit burner
- Put the spirit burner underneath the can, and light the wick. Heat the water, stirring constantly, until the temperature reaches about 50oC
- Put out the flame using the burner lid, and measure the final temperature of the water
- Weigh the spirit burner and lid again
- Calculate the enthalpy change
How do you calculate the molar enthalpy change?
- Calculate the amount of energy transferred
- Find out how many moles if fuel produced this heat
- heat produced by 1 mole of fuel = amount of energy transferred (remember to make it negative becaue combustion is an exothermic reaction) ÷ moles of the fuel
