Topic 3 - Chemical Changes Flashcards
What do acids and alkalis combine to form ?
They combine to form water, which is neutral.
What does the pH scale measure ?
The pH scale is used to measure how acidic or alkaline a substance is
What is formed when alkalis dissolve in water (aqueous solutions) ?
Alkalis are substances that form hydroxide (OH-) ions
What do acids form when they dissolve in water (aqueous solutions) ?
Acids are substances that form hydrogen (H+) ions
Thinking about what ions form when they dissolve in water, why does mixing an acid and an alkali create a neutral substance?
Acids form hydrogen (H+) ions when they dissolve in water.
Alkalis form hydroxide (OH-) ions when they dissolve in water.
H+ and OH- ions combine to form H2O, otherwise known as water!
Water is a neutral substance so an acid and an alkali neutralise each other
How is the strength of an acid determined ?
by how much it ionises in water.
What happens to all acids in aqueous solutions ?
All acids ionise (become ions)
What happens to strong acids in aqueous solutions ?
They Fully ionise in water, meaning that all of the acid molecules break up to form H+ ions.
E.g. hydrochloric acid:
HCl → H+ + Cl-
Sulfuric and nitric acid are also strong acids.
What happens to weak acids in aqueous solutions?
Only partially ionise in water, meaning that not all of the acid molecules break up to form H+ ions.
E.g. ethanoic acid (the main ingredient in vinegar):
CH3COOH ⇌ CH3COO- + H+
Most acids are weak acids.
How is the acidity or alkalinity of a solution is measured using what ?
the pH scale
What is a neutral pH ?
pH of exactly 7
What is the pH of an acid ?
pH less than 7 = acid.
What is the pH of strong acids ?
Strong acids have a pH close to 0.
What is the pH of alkalis ?
pH more then 7
What is the pH of a strong alkali ?
A pH close to 14
What do acids form when dissolved in water ?
Acids form hydrogen (H+) ions when they dissolve in water.
What do alkalis form when dissolved in water ?
Alkalis form hydroxide (OH-) ions when they dissolve in water
For every pH increase of 1 what happens to the concentration of H+ ions ?
For every increase of 1 in pH, the concentration of H+ ions gets divided by 10:
At pH 0, the concentration of H+ ions is 1 mol/dm3.
At pH 1, the concentration of H+ ions is 0.1 mol/dm3.
At pH 2, the concentration of H+ ions is 0.01 mol/dm3.
Etc.
The pH of lemon juice is about 2, whereas tomato juice’s pH is about 4. Approximately how much of an increase in hydrogen ion concentration is there between tomato juice and lemon juice?
100x
What does concentrated acid mean ?
the more H+ ions are dissolved per unit of volume. The more substance dissolved in a solvent the more concentrated it is.
What happens to the concentration of H+ ions every time the pH increases by 1?
Divides by 10
What are the two main ways to measure pH of a substance ?
Universal indicator
pH probe
How does the universal indicator measure pH ?
The indicator changes to a particular colour depending on the pH
How does a pH probe measure the pH of a solution ?
The probe returns a number that represents the pH of a solution.
What type of indicator is used in measuring pH?
Universal
What are examples of indicators ?
Methyl orange
Phenolphthalein
Litmus paper
What colour is methyl orange in acid ?
Red in acid
What is methyl orange in alkali ?
Yellow in alkali
What colour is Phenolphthalein in acid ?
Colourless in acid.
What colour is Phenolphthalein in alkali ?
Pink in alkali.
What colour is litmus paper in acid ?
Red in acid.
What colour is litmus paper in alkali ?
Blue in alkali.
Fact ?
Red cabbage contains a chemical called anthocyanin. They act as natural litmus paper. Their molecules change shape in acidic or alkaline conditions and this changes the molecules’ colour.
Do indicators show pH ?
No they only show if a substance is acid or alkaline
How can we measure the change in pH ?
by adding a solid base to a fixed volume of acid and measuring the pH
What is method 1 for measuring change in pH ?
1) Use a measuring cylinder to add a set volume of acid.
For example 100cm3 of hydrochloric acid.
2) Estimate and record the pH of the contents of the beaker.
3) Put a piece of universal indicator paper onto a white tile.
4) Dip the end of the glass rod into the liquid, then tap it onto some universal indicator paper.
5) Wait 30 seconds, then match the colour to the appropriate pH on a pH colour chart.
6) Rinse the glass rod with water.
What is method 2 for measuring change in pH ?
1) Measure out a mass of a base onto a piece of paper or a ‘weighing boat’.
2) Add the powdered base to the beaker.
3) Stir, and then estimate and record the pH of the mixture.
4) Repeat until a maximum pH of 14.
What are ways that you can improve the measuring of change in pH ?
You can improve this method by using a pH probe data logger.
This will mean you don’t have to judge the colour of each value of pH. It gives a digital value to 2 decimal places.
What are the dependent and independent variables in the experiment measuring a change in pH?
Independent: amount of base added
Dependent: pH of liquid
What is a ‘concentrated’ solution ?
A solution that contains a relatively large amount of the solute in the same volume of solvent
What is a ‘dilute’ solution ?
A solution that contains a relatively small amount of the solute in a given volume of solvent.
Why is tap water an example of a dilute solution ?
because it contains very small quantities of dissolved minerals.
When acids react with metals, what are always formed ?
a hydrogen and a salt are always formed
What are examples of metals reacting with acids ?
magnesium + hydrochloric acid → magnesium chloride + hydrogen
iron + sulfuric acid → iron sulfate + hydrogen
zinc + sulfuric acid → zinc sulfate + hydrogen
How can acids be neutralised ?
by reacting with bases (an alkali is a base that dissolves in water).
Metal carbonates, metal oxides and metal hydroxides are all examples of bases
What’s an alkali ?
a base that dissolves in water
Acid + oxide → ? + ?
Acid + oxide → salt + water.
E.g. hydrochloric acid + magnesium oxide → magnesium chloride + water.
Acid + hydroxide → ? + ?
Acid + hydroxide → salt + water.
E.g. hydrochloric acid + sodium hydroxide → sodium chloride + water.
Acid + carbonate → ? + ? + ?
Acid + carbonate → salt + water + carbon dioxide.
E.g. hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide
Balance this equation for the reaction between zinc oxide and nitric acid:
ZnO + 2HNO3 → Zn(NO3)2 + H2O
What is the test for hydrogen ?
Place a lit splint close to the mouth of a test tube that contains a gas.
If the gas is hydrogen, a “squeaky pop” sound will be produced.
This noise is generated because hydrogen burns rapidly in the presence of oxygen to give H2O.
What is the test for carbon dioxide ?
Take an aqueous solution of limewater (calcium hydroxide) and bubble through the gas.
If the gas is carbon dioxide, the limewater solution will turn from clear to cloudy.
What is the apparatus needed to test for carbon dioxide ?
Limewater
Test tubes
What are soluble salts ?
salts which dissolve in water
How do you produce soluble salts ?
through a reaction between acids and solid, insoluble substances such as pure metals, metal oxides, metal hydroxides, or metal carbonates.
How are blue copper sulfate crystals produced ?
adding black copper oxide to sulfuric acid:
CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)
copper oxide+sulfuric acid→ ? + ?
Copper sulfate + water
What is the aim of the Required Practical - Separating Mixtures ?
to produce a pure, dry sample of a soluble salt from an insoluble oxide or carbonate.
What are the steps for the required practical - separating mixtures ?
Preparation:
Use a Bunsen burner to gently warm the acid
Add the insoluble solid (with stirring) until the reaction no longer happens
Filtration:
Filter the solution to remove the excess insoluble solid
This will leave a solution of the salt dissolved in water
Crystallisation:
Heat the solution in an evaporating basin above a beaker of water.
The ‘water bath’ ensures gentle heating.
Let the solution cool and allow more water to evaporate.
As water evaporates, the solution will become more concentrated and the salt will begin to crystallise.
What does crystallisation do ?
separates a dissolved solid (the solute) from a liquid (the solvent).
What’s a base ?
base is any substance that reacts with an acid to form a salt and water only
What happens to soluble salts in water ?
Soluble salts dissociate (break up) into their anion and cation as the giant ionic lattice breaks.
What happens to insoluble slats in water ?
Insoluble salts do not break up, so their giant ionic lattice stays intact (together).
What salts are soluble in water ?
All common sodium, potassium and ammonium salts
All nitrates
Most chlorides
Most sulfates
Sodium, potassium and ammonium carbonates
Sodium, potassium and ammonium hydroxides
What salts are insoluble in water ?
Silver, lead chlorides
Lead, barium, calcium sulfates
Most carbonates
Most hydroxides
If a reaction produces an insoluble solid, what is this called ?
It is called a precipitate
E.g. NaCl(aq) + AgNO3(aq) → NaNO3(aq) + AgCl(s).
Silver chloride is a precipitate.
E.g. 2KI(aq) + Pb(NO3)2(aq) → PbI2(s) + 2KNO3(aq).
Lead iodide is a precipitate.
How can acids be neutralised ?
by reacting with bases
What are 3 example of bases ?
Metal carbonates, metal oxides and metal hydroxides
When acids are neutralised by metal carbonates, what is formed ?
Salt + water + carbon dioxide
When acids are neutralised by oxides what is produced ?
Salt + water
When acids are neutralised by hydroxides what is produced ?
Salt + water
What’s a neutralisation reaction ?
A neutralisation reaction happens when an alkali is reacted with an acid. The hydrogen ions from the acid combine with hydroxide ions from the alkali to give water
What is titration ?
a technique that measures the volumes of acid and alkali solutions that are needed in a neutralisation reaction
What are the key variables in a titration ?
If we were neutralising an alkali with an acid, titration could be used to calculate the concentration of the alkali if we knew:
The concentration of the acid to be added.
The volume of the alkali.
What do indicators show in a titration ?
Indicators show the end point of titrations by suddenly changing colour when the solution turns from acidic to alkaline, or alkaline to acidic.
When choosing an indicator, the strength of the acid and alkali must be considered.
If both the acid and alkali are strong, what would the suitable indicators be ?
Methyl orange (yellow in alkali → red in acid). Phenolphthalein (pink in alkali → colourless in acid).
Which point of neutralisation reactions do titration indicators help us see?
The end point
What are the steps of a titration ?
1) Using a pipette, add a set volume of a strong alkali to a conical flask.
2) Add a few drops of a suitable indicator.
3) Place a strong acid (sulfuric, hydrochloric or nitric acid) of known concentration in a burette.
4) Take an initial reading of the volume of acid in the burette.
5) Gradually add the acid to the alkali. Make sure that you are regularly swirling the conical flask as you add the acid. As you get closer to the end point, start to add the acid at a slower rate.
6) As soon as the indicator changes colour, stop adding acid.
7) Take a final reading of the volume of acid in the burette.
8) Subtract the final reading from the initial reading to calculate the volume of acid needed to neutralise the alkali.
In a titration experiment, how much indicator is needed?
A few drops
If we were neutralising an alkali with an acid, titration could be used to calculate the concentration of the alkali if we knew what ?
The concentration of the acid to be added
The volume of the alkali
In a titration experiment, at what point should you stop adding acid?
As soon as the indicator changes colour
Hydrochloric acid can be described as a strong acid. What does this mean?
Hydrochloric acid fully dissociates in water
What is electrolysis ?
A process in which electrical energy, from a direct current supply, decomposes electrolytes
What are electrolytes?
electrolytes are ionic compounds in the molten state or dissolved in water
When a voltage (direct current) is applied across an electrolyte, how are the charged ions attracted to the electrodes ?
Charged ions are attracted with the opposite charge to the ion
What electrodes are positively charged ions (cations) attracted to ?
the negative electrode (cathode).
What electrodes are negatively charged ions (anions) attracted to ?
the positive electrode (anode).
What’s are the electrolysis apparatus ?
2 electrodes are submerged in an electrolyte, and a voltage is applied across the electrodes using a power supply.
What happens when we electrolyse molten copper oxide (CuO)?
Positively charged copper ions (Cu2+) are attracted to the cathode.
A pinkish coat of copper will appear on the cathode.
Negatively charged oxygen ions (O2-) are attracted to the anode.
Oxygen atoms will form at the anode and combine to give O2 gas bubbles.
If copper ions (Cu2+) are in a solution and a voltage is applied, which electrode will they move towards and what will happen when they touch that electrode?
When a voltage is applied across an electrolyte, the charged ions are attracted to the oppositely charged electrodes. Copper ions (Cu2+) are positively charged, so will move towards the cathode (negative electrode).
When the copper ions (Cu2+) touch the cathode, each ion will gain 2 electrons, forming copper metal:
Cu2+(aq) + 2e- → Cu(s)
