Edexcel GCSE Chemistry: Acids - Notes | Save My Exams PDF

Edexcel GCSE Combined Your notes Science: Chemistry Acids Contents Acids & Bases Hydrogen Ions & pH Core Practical: Investigating pH Acid Strength & Concentration Bases Reactions of Acids Test for Hydrogen & Carbon Dioxide C...

Edexcel GCSE Combined Your notes Science: Chemistry Acids Contents Acids & Bases Hydrogen Ions & pH Core Practical: Investigating pH Acid Strength & Concentration Bases Reactions of Acids Test for Hydrogen & Carbon Dioxide Core Practical: Preparing Copper Sulfate Prepare a Salt by Titration Solubility Rules Preparing an Insoluble Salt © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 1  Acids & Bases Your notes Defining Acids & Bases When acids are added to water, they form positively charged hydrogen ions (H+) The presence of H+ ions is what makes a solution acidic When alkalis are added to water, they form negative hydroxide ions (OH–) The presence of the OH– ions is what makes the aqueous solution an alkali The pH scale is a numerical scale which is used to show how acidic or alkaline a solution is, in other words it is a measure of the amount of the ions present in solution The pH Scale The pH scale goes from 0 – 14 (extremely acidic substances can have values of below 0) All acids have pH values of below 7, all alkalis have pH values of above 7 The lower the pH then the more acidic the solution is The higher the pH then the more alkaline the solution is A solution of pH 7 is described as being neutral The pH scale showing acidity, neutrality and alkalinity Indicators Two colour indicators are used to distinguish between acids and alkalis Many plants contain substances that can act as indicators and the most common one is litmus which is extracted from lichens Synthetic indicators are organic compounds that are sensitive to changes in acidity and appear different colours in acids and alkalis Phenolphthalein and methyl orange are synthetic indicators frequently used in acid- alkali titrations © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 2  Two Colour Indicators Table Your notes Synthetic indicators are used to show the endpoint in titrations as they have a very sharp change of colour when an acid has been neutralised by an alkali and vice-versa Litmus is not suitable for titrations as the colour change is not sharp and it goes through a purple transition colour in neutral solutions making it difficult to determine an endpoint Litmus is very useful as an an indicator paper and comes in red and blue versions, for dipping into solutions or testing gases © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 3  Hydrogen Ions & pH Your notes Hydrogen Ions & pH We have already seen that acids are substances that contain hydrogen ions in solution The more hydrogen ions the stronger the acid, but the lower the pH The higher the concentration of hydroxide ions in a solution the higher the pH So pH is a measure of the concentration of H+ ions in solution, but they have an inverse relationship The pH scale is logarithmic so that each change in pH is a tenfold change in hydrogen ion concentration The pH scale is logarithmic, meaning that each change of 1 on the scale represents a change in concentration by a factor of 10 © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 4 Therefore an acid with a pH of 3 has ten times the concentration of H+ ions than an acid of pH 4 Your notes An acid with a pH of 2 has 10 x 10 = 100 times the concentration of H+ ions than an acid with a pH of 4 From this we can summarize that for two acids of equal concentration, where one is strong and the other is weak, then the strong acid will have a lower pH due to its capacity to dissociate more and hence put more H+ ions into solution than the weak acid Examiner Tips and Tricks Acid strength is reflected in how many hydrogen ions are in solution. The more hydrogen ions the lower the pH and vice-versa. © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 5  Core Practical: Investigating pH Your notes Core Practical: Investigating pH Aim: To investigate the changes in pH of a fixed volume of dilute HCl on addition of varying amounts of a solid base Materials: Dilute HCl (0.5M or 1M), solid base such as CaO or Ca(OH)2 Conical flask, 25 cm3 or 50 cm3 volumetric pipette, glass rod Spatula and weighing boat pH probe or Universal Indicator paper Method: Use a pipette to measure a fixed volume of dilute HCl into a conical flask Add one spatula of calcium oxide or calcium hydroxide to the flask and swirl When all the base has reacted record the pH of the solution If using U.I. paper use the glass rod to extract a sample from the flask Repeat for different numbers of spatula (1-10) of solid but the same volume of HCl Record your results neatly in table format Analysis of results: Plot a graph of the amount of the base on the X-axis against the pH recorded on the Y- axis The resulting graph should look something like the one below © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 6  Your notes Investigating the change in pH during neutralisation of an acid Conclusion: The graph indicates a sudden change in pH which corresponds to the vertical section of the graph This indicates that the more solid base is added the higher the pH, therefore the base is neutralising the acid From the sample graph it can be seen that 6 spatulas of the base are required to completely neutralise the acid Hazards, risks and precautions © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 7 Hazard symbols to show substances that are corrosive, harmful to health and hazardous to the environment Your notes Copper(II) oxide can cause serious eye irritation and is a skin irritant. It is harmful if swallowed or inhaled and is toxic to aquatic life Dilute hydrochloric acid is not classified as hazardous at the concentrations typically used in this practical, however it may still cause harm to the eyes or the skin For both substances, avoid contact with the skin and use safety goggles For copper(II) oxide, care should be taken not to inhale the powder © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 8  Acid Strength & Concentration Your notes Acid Strength & Concentration Strong & Weak Acids can be either strong or weak, depending on how many ions they produce when they dissolve in water When added to water, acids ionise or dissociate to produce H+ ions: Hydrochloric acid: HCl ⟶ H+ + Cl– Nitric acid: HNO3 ⟶ H+ + NO3– Strong acids such as HCl and H2SO4 dissociate completely in water, producing solutions with a high concentration of H+ ions and thus a very low pH Weak acids such as ethanoic acid, CH3COOH and hydrofluoric acid, HF only partially ionize in water, producing solutions of pH values between 4 – 6 This data is summarized in the table below: Strong & Weak Acids Table For weak acids there is an equilibrium set-up between the molecules and their ions once they have been added to water Propanoic acid for example dissociates as follows: CH3CH2COOH ⇌ H+ + CH3CH2COO– The ⇌ symbol indicates that the process is reversible, as the products can react together forming the original reactants The equilibrium lies to the left, meaning there is a high concentration of intact acid molecules and therefore a low concentration of ions in solution, hence the pH is that of a weak acid and closer to 7 than a strong acid Concentrated & Dilute A solution is formed when a solute is dissolved in a solvent A dilute solution contains a small amount of solute in a given volume of solution A concentrated solution contains a large amount of solute in a given volume of solution © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 9  A concentrated solution of either an acid or a base is one that contains a high number of acid or base molecules per dm3 of solution Your notes A dilute acid or base solution is therefore one that has much fewer acid or base molecules per dm3 of solution Diagram illustrating how the concentration of a solution increases as more solute is added Examiner Tips and Tricks The terms strong and weak refer to the ability to dissociate and not concentration. A dilute solution of a strong acid can have a lower pH than a concentrated solution of a weak acid, due to the stronger acid undergoing complete dissociation. © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 10  Bases Your notes Bases What makes a base act like a base? Bases are substances which can neutralise an acid, forming a salt and water The term base and alkali are not the same A base which is water-soluble is referred to as an alkali So, all alkalis are bases, but not all bases are alkalis Alkalis have pH values of above 7 In basic (alkaline) conditions red litmus paper turns blue Bases are usually oxides, hydroxides or carbonates of metals The presence of the OH- ions is what makes the aqueous solution an alkali One unusual base is ammonia solution When ammonia reacts with water it produces hydroxide ions Some Common Alkalis and the Ions they Contain Examiner Tips and Tricks Aqueous ammonia and ammonium hydroxide are the same thing. When ammonia gas dissolves in water it forms ammonium hydroxide. Be careful to use the correct terminology: ammonia is the gas, NH3, ammonium is the ion present in ammonium compounds, NH4+ © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 11  Reactions of Acids Your notes Reactions of Acids Reactions of acids with metals Only metals above hydrogen in the reactivity series will react with dilute acids The more reactive the metal then the more vigorous the reaction will be Metals that are placed high on the reactivity series such as potassium and sodium are very dangerous and react explosively with acids When acids react with metals they form a salt and hydrogen gas: The general equation is: metal + acid ⟶ salt + hydrogen Some examples of metal-acid reactions and their equations are given below: Acid-Metals Reactions Table In general, we can summarise the reaction of a metal that forms a +2 ion as follows: Acids-Metals Summary Table Reaction of acids with oxides & hydroxides When an acid reacts with an oxide or hydroxide, a neutralisation reaction occurs © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 12  Metal oxides and metal hydroxides act as bases In all acid-base neutralisation reactions, a salt and water are produced: Your notes acid + base ⟶ salt + water The identity of the salt produced depends on the acid used and the positive ions in the base Hydrochloric acid produces chlorides, sulfuric acid produces sulfate salts and nitric acid produces nitrates The following are some specific examples of reactions between acids and metal oxides / hydroxides: 2HCl + CuO ⟶ CuCl2 + H2O H2SO4 + 2NaOH ⟶ Na2SO4 + 2H2O HNO3 + KOH ⟶ KNO3 + H2O In general, we can summarise the reaction of metals and bases as follows: Acids and Metals Oxides or Hydroxides Summary Table Reactions of Acids with Metal Carbonates Acids will react with metal carbonates to form the corresponding metal salt, carbon dioxide and water These reactions are easily distinguishable from acid – metal oxide/hydroxide reactions due to the presence of effervescence caused by the carbon dioxide gas Acids & Metal Carbonates Reactions Table © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 13  Your notes The following are some specific examples of reactions between acids and metal carbonates: 2HCl + Na2CO3 ⟶ 2NaCl + H2O + CO2 H2SO4 + CaCO3⟶ CaSO4 + H2O + CO2 Examiner Tips and Tricks If in an acid-base reaction there is effervescence produced then the base must be a metal carbonate which produces carbon dioxide gas. Neutralisation The chemistry of neutralisation reactions can be explained using ionic equations Ionic equations are used to show only the particles that chemically participate in a reaction The other ions present are not involved and are called spectator ions For example the neutralisation reaction between hydrochloric acid and sodium hydroxide: HCl + NaOH ⟶ NaCl + H2O If we write out all of the ions present in the equation we get: H+ + Cl- + Na+ + OH- ⟶ Na+ + Cl- + H2O The spectator ions are thus Na+ and Cl–. Removing these from the previous equation leaves the overall net ionic equation: H+ + OH- ⟶ H2O © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 14  The H+ ions come from the acid and the OH– ions come from the base, both combine to form the product water molecules Your notes This ionic equation is the same for all acid-base neutralisation Examiner Tips and Tricks Remember that although acids react with metals to form salts, that reaction is not neutralisation, but it counts as a redox reaction. Hazards Hazard symbols you may see in relation to acids The hazards associated with acids depend on the type and concentration of the acid Most dilute acids either require no hazard symbol or they are an irritant, so require the symbol to show they are harmful to health Eye protection should be worn when handling Moderately concentrated acids are often corrosive In addition to eye protection, gloves should also be worn Some concentrated acids, e.g. nitric acid, are oxidising which can cause or intensify a fire in contact with combustible materials Eye protection and gloves are necessary when handling concentrated acids and the use of a fume cupboard is often required © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 15  Test for Hydrogen & Carbon Dioxide Your notes Test for Hydrogen & Carbon Dioxide Testing Hydrogen The test for hydrogen consists of holding a burning splint held at the open end of a test tube of gas If the gas is hydrogen it burns with a loud “squeaky pop” which is the result of the rapid combustion of hydrogen with oxygen to produce water Be sure not to insert the splint right into the tube, just at the mouth, as the gas needs air to burn Diagram showing the test for hydrogen gas Examiner Tips and Tricks It is easy to confuse the tests for hydrogen and oxygen. Try to remember that a ligHted splint has a H for Hydrogen, while a glOwing splint has an O for Oxygen. Testing Carbon Dioxide The test for carbon dioxide involves bubbling the gas through an aqueous solution of calcium hydroxide (limewater) If the gas is carbon dioxide, the limewater turns milky or cloudy © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 16  Your notes Diagram showing the test for carbon dioxide gas Examiner Tips and Tricks Sometimes students think that extinguishing a burning splint indicates carbon dioxide gas. However, while it is a property of carbon dioxide, other gases, such as nitrogen, will also do this, so the test is not definitive and should not be quoted in an exam answer. © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 17  Core Practical: Preparing Copper Sulfate Your notes Core Practical: Preparing Copper Sulfate Aim: To prepare a pure, dry sample of hydrated copper(II) sulfate crystals Materials: 1.0 mol / dm3 dilute sulfuric acid Copper(II) oxide Spatula & glass rod Measuring cylinder & 100 cm3 beaker Bunsen burner Tripod, gauze & heatproof mat Filter funnel & paper, conical flask Evaporating basin and dish. © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 18  Your notes © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 19  Your notes The preparation of copper(II) sulfate by the insoluble base method Practical Tip: The base is added in excess to use up all of the acid, which would become dangerously concentrated during the evaporation and crystallisation stages Method: 1. Add 50 cm3 dilute acid into a beaker and warm gently using a Bunsen burner 2. Add the copper(II) oxide slowly to the hot dilute acid and stir until the base is in excess (i.e. until the base stops dissolving and a suspension of the base forms in the acid) 3. Filter the mixture into an evaporating basin to remove the excess base 4. Gently heat the solution in a water bath or with an electric heater to evaporate the water and to make the solution saturated 5. Check the solution is saturated by dipping a cold glass rod into the solution and seeing if crystals form on the end 6. Leave the filtrate in a warm place to dry and crystallise 7. Decant excess solution and allow the crystals to dry Results: Hydrated copper(II) sulfate crystals should be bright blue and regularly shaped Examiner Tips and Tricks Make sure you learn the names of all the laboratory apparatus used in the preparation of salts. © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 20 Hazards, risks and precautions Your notes Hazard symbols to show substances that are corrosive, harmful to health and hazardous to the environment Copper(II) oxide and solid copper(II) sulfate can cause serious eye irritation and is a skin irritant. It is harmful if swallowed or inhaled and is toxic to aquatic life For all substances, avoid contact with the skin and use safety goggles For copper(II) oxide, care should be taken not to breathe in the powder © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 21  Prepare a Salt by Titration Your notes Prepare a Salt by Titration If salts are prepared from an acid and a soluble reactant then a titration technique must be used In a titration, the exact volume of acid and soluble reactant are mixed in the correct proportions so that all that remains is the salt and water Preparing a Salt by Titration Aim: To prepare a sample of a dry salt starting from an acid and an alkali Diagram: © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 22  Your notes © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 23  Your notes Diagram showing the apparatus needed to prepare a salt by titration Method: Use a pipette to measure the alkali into a conical flask and add a few drops of indicator (phenolphthalein or methyl orange) Add the acid into the burette and note the starting volume Add the acid very slowly from the burette to the conical flask until the indicator changes to appropriate colour Note and record the final volume of acid in burette and calculate the volume of acid added (starting volume of acid - final volume of acid) Add this same volume of acid into the same volume of alkali without the indicator Heat to partially evaporate, leaving a saturated solution Leave to crystallise decant excess solution and allow crystals to dry Results: A dry sample of a salt is obtained Examiner Tips and Tricks When evaporating the solution some water is left behind to allow for water of crystallisation in some salts and also to prevent the salt from overheating and decomposing. © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 24  Solubility Rules Your notes Solubility Rules Ionic compounds are generally soluble in water compared to covalent substances, but there are exceptions A knowledge of the solubility of ionic compounds helps us to determine the most appropriate method for the preparation of salts The solubility of common ionic compounds is shown below: Solubility of the common salts Salts Soluble Insoluble Sodium, potassium All None and ammonium Nitrates All None Chlorides Most are soluble Silver and lead(II) Sulfates Most are soluble Barium, calcium and lead(II) Carbonates Carbonates of sodium, potassium and Most are ammonium insoluble Hydroxides Hydroxides of sodium potassium and Most are ammonium (calcium hydroxide is sparingly insoluble soluble) Note that calcium hydroxide is slightly soluble in water Examiner Tips and Tricks Calcium hydroxide solution is more commonly know as limewater and is used to test for carbon dioxide. Predicting Precipitates Some salts can be extracted by mining but others need to be prepared in the laboratory How the salt is made in the laboratory depends on whether the salt being formed is soluble or insoluble in water © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 25 To do this the balanced equation is written down to determine the identify of the salt product Your notes Then check the solubility of the salt using the solubility table If it is soluble in water, then it can be prepared by titration If it is insoluble then it can be prepared by precipitation For example a silver nitrate solution is mixed with a sodium chloride solution: AgNO3 (aq) + NaCl (aq) ⟶ AgCl (s) + NaNO3 (aq) From the table both AgNO3 and NaCl are water soluble but AgCl, silver chloride, is not and hence forms a precipitate Examiner Tips and Tricks The precipitation reaction by combining two soluble salts is also known as a double decomposition or double displacement reaction. © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 26  Preparing an Insoluble Salt Your notes Preparing an Insoluble Salt Aim: To prepare a dry sample of an insoluble salt, lead(II) sulfate Diagram: © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 27  Your notes The preparation of lead(II)sulfate by precipitation from two soluble salts Method: Measure out 25 cm3 of 0.5 mol dm3 lead(II) nitrate solution and add it to a small beaker Measure out 25 cm3 of 0.5 mol dm3 of potassium sulfate add it to the beaker and mix together using a stirring rod Filter to remove the precipitate from mixture © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 28  Wash the filtrate with distilled water to remove traces of other solutions Leave in an oven to dry Your notes Soluble salt 1 = lead(II) nitrate Soluble salt 2 = potassium sulfate Equation for the reaction: Pb(NO3)2 (aq) + K2SO4 (aq) → PbSO4 (s) + 2KNO3 (aq) lead(II) nitrate + potassium sulfate → lead(II) sulfate + potassium nitrate Examiner Tips and Tricks Care should be taken with handling lead salts as they are toxic. © 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 29