Y8: Knowledge organisers These slides contain the facts you need to learn for each topic. Of course in your assessments the questions don’t just ask you recall based questions. Most of your questions will ask you to apply knowledge or evaluate procedures however you cannot do these kinds of questions without first knowing the facts.

Chemical and physical changes: In a physical change no new substances are made. In a chemical change the atoms in the reacting substances are split up and rearranged into the products. New substances are made. Physical change Chemical change State changes eg, melting Combustion Dissolving Neutralisation

Acids and alkalis: definitions A substance that produces H+ ions when dissolved in water. Base A substance that is able to neutralise an acid Alkali A soluble base that forms OH- ions in water pH A measure of how strong/weak and acidic or alkaline substance is Examples of acids HCl = hydrochloric acid HNO3 = nitric acid H2SO4 = sulfuric acid Examples of alkalis NaOH = sodium hydroxide Examples of bases Metal oxides Metal carbonates CuO = copper oxide CaCO3 = Calcium carbonate NH3 = ammonia

Acids and alkalis: The pH scale The pH scale is a numerical scale which shows how strong or weak and acid or alkali is. 0- <4 4- <7 7 >7-10 >10-14 Strong acid Weak acid Neutral Weak alkali Strong alkali There are 2 ways to determine the pH of a substance. Both are used with substances dissolved in water. pH meter Indicator Gives a numerical result on a digital readout. Changes colour at different pHs. The colour must be compared to a comparison chart to determine the pH.

Acids and alkalis: household substances Common household substances and their pHs Acids Vinegar 2-4 Lemon juice 3-5 Neutral substances Pure water 7 Salt water Alkalis Toothpaste 8 Oven cleaner 10

Acids and alkalis: using indicators Indicators are substances that have different colours at different pHs. They are available in solution and paper form. Some indicators have general uses whereas others are used for very specific purposes. Universal indicator An indicator made from a mixture of different indicators which gives a rainbow of colours at different pH values. Litmus An indicator that is blue in alkali and red in acid. Most often found in paper form (red litmus paper or blue litmus paper). Phenolphthalein A solution indicator which is colourless in acid and pink in alkali.

Acids and alkalis: making a natural indicator Lots of plants are able to act as indicators as they contain pH sensitive coloured pigments. Examples: Red cabbage and poinsettia 1. Cut up the plant into small pieces. Put it in a beaker. 2. Pour water over the pieces. 3. Boil the mixture for several minutes until a deep colour is obtained. 4. Filter off the pieces of plant to leave a solution of indicator. Remember that natural indicators will have unique colours at different pHs, they won’t give the same colours as universal indicator.

Acids and alkalis: neutralisation Bases and alkalis can neutralise acids. The H+ ions that cause the acidity react, and the H atoms become part of a new water molecule. The H+ ions are used up and the acidity ‘cancelled out’ or neutralised. Ionic equation for neutralisation H+ + OH-  H2O This just shows what happens to the ions. When we look at all the substances involved we can write word equations for the different neutralisations. These reactions form soluble salts. Acid + alkali Salt + water Acid + base Specific examples can be found in the salt formation section.

Acids and alkalis: monitoring neutralisation If we are doing a neutralisation reaction we have to have some way of knowing when the neutralisation process is finished. Otherwise we might add too much alkali. A pH curve is a graph which shows what happens to pH when acids and alkalis react. The long straight, vertical portion of the graph is the neutralisation point. Most commonly we use indicators to monitor a neutralisation reaction and stop adding when a particular colour change occurs. Not all indicators are suitable for this purpose Good Phenolphthalein Sharp colour change (colourless to pink or pink to colourless) at neutralisation. Bad Universal indicator Has a range of colours and neutral colour would be green. Difficult to judge if it is a yellow-green (so slightly acidic) or green-blue (slightly alkaline) shade.

Reactions of acids: salt formation General word equations for acid reactions Reactants Products Acid + Alkali Salt Water Base Metal carbonate Carbon dioxide Reactive metal Hydrogen How to work out the name of the salt formed A salt has 2 parts to its name A metal part (1st word) A non metal part (2nd word) The metal part comes from the base/alkali/carbonate/metal. Eg, sodium hydroxide gives salts where the 1st word is sodium. The non metal part comes from the acid. Hydrochloric acid ~chloride Sulfuric acid ~sulfate Nitric acid ~nitrate

Experiment: Salt formation from acid and alkali Measure a volume of acid with a syringe and put in a small beaker Add 2 drops of phenolphthalein Using a syringe add small portions of alkali until the solution turns pink Note the volume of alkali used Repeat the experiment but leave out the indicator Evaporate the resulting solution to get crystals of salt

Experiment: Salt formation from acid and metal oxide Measure a volume of acid. Add the metal oxide to excess (so solid can be seen) Heat the reaction mixture. If all the solid dissolves add more to ensure an excess is used. Filter off the excess solid Evaporate the solution to obtain the salt crystals Precaution Reason Using excess metal oxide To make sure all the acid is neutralised (so it doesn’t get heated up in the next step) so a high yield of crystals will be obtained. Heating up To make sure the reaction is complete (lots of experiments are very slow at room temperature)

Chemical formulae Most elements in the Periodic Table are atomic Fe, Cu, Na, Ne, Pd, Si Some elements are diatomic Elements in group 7 F2, Cl2, Br2, I2 Hydrogen, oxygen and nitrogen H2, O2, N2 Common acids Hydrochloric acid HCl, sulfuric acid H2SO4, nitric acid HNO3 Bases and alkalis Ammonia NH3, Sodium hydroxide NaOH Other substances Carbon dioxide CO2, Water H2O

Chemical equations: what they show A convenient way of describing a chemical reaction is to write an equation. There are 2 types – Word equations Balanced symbol equations When chemical reactions happen the atoms in the reactants (shown on the left of the equation) are broken up and rearranged to make the products (shown on the right of the equation).

Word equations: Word equations are useful for remembering the general patterns of reactions Eg, you remember that acid + alkali gives salt + water You can then apply this knowledge to a specific example given in a question instead of learning lots of individual pairs of reactants and their products. Apart from this they are not very useful because chemists acros the world have different names for substances.

Law of conservation of mass “The law of conservation of mass states that mass is neither created nor destroyed. In a closed system, the mass of the reactants is equal to the mass of the products.” In simple terms: 1. During any chemical reaction no particles are created or destroyed: the atoms are simply rearranged from the reactants to the products. 2. Mass is never lost or gained in chemical or physical changes (although it may seem that it is from measurements taken during these processes).

Conservation of mass in chemical change Mass is conserved in chemical change Some reactions seem like they don’t conserve mass, these tend to be those that produce gases (so the gases float off and aren’t measured by the balance)

Conservation of mass in physical change Melting A 2g ice cube weighs the same when it is melted 2g water Dissolving 1g sugar cubes 50g water Sugar solution – 51g

Chemical formulae A chemical formula shows the number of atoms of each elements in a substance. Each element is represented using its symbol (found in the Periodic Table). Remember that these atom symbols also start with a capital letter. Some may be just a capital letter e.g. S = sulfur others may be a capital letter and a lowercase letter e.g. Si = silicon If one atom of an element is present then there is no number in the formula. E.g. NaOCl In this formula there is one atom of each Na (sodium), O (oxygen) and Cl (chlorine) When there is more than one atom of an element, a subscript (smaller and lower down) number is put to the right of the symbol. This multiplies the atom to the left of it. E.g. H2O In this formula there are 2 atoms of hydrogen and one of oxygen. There may also be brackets in a formula. In this case the subscript number multiplies everything in the brackets to the left. E.g. Mg(OH)2 E.g. Cu(NO3)2 In this formula there is one atom of magnesium and 2 each of oxygen and hydrogen In this formula there is one atom of copper, 2 atoms of nitrogen and (2 x 3 = 6) atoms of oxygen

Conservation of mass: balanced chemical equations A symbol equation shows how reactants turn into products. To obey the law of conservation of mass they must have the same number of atoms of each element on both the left and right hand side of the equation. Na + Cl2 NaCl Count the atoms on the left and right hand sides of the equation given above. This equation is unbalanced. There are 2 atoms of chlorine on the left and one on the right. We must balance the equation. You might think the easiest thing to do is to make the chlorine into just Cl rather than Cl2 but this isn’t allowed, we can’t just change the formula of a substance to make our balancing easier! RULES: To balance an equation we put big numbers in front of a formula. These multiply the whole formula to the right. Going back to our equation – to make 2 Cl on the right hand side we put a 2 in front of the NaCl Na + Cl2 2 NaCl But this also changes the number of Na atoms so this isn’t yet finished. So we need a 2 in front of the Na as well. The balanced equation is : 2Na + Cl2 2NaCl

Some equations to practice with…

Combustion – reactions with oxygen When a substance is burned it combines with oxygen in the air. The new compound is called an oxide. Air is a mixture of gases, only around 18% is oxygen, the majority of air is the unreactive gas nitrogen. Combustion reactions can take place in room air or in pure oxygen. The observations may be different depending on this.

Metal and non metals: where we find them on the Periodic Table We draw a stepped line under boron. All the metals are to the left and all the non-metals to the right of this line.

Metals and non metals: properties Typical metal properties Typical non-metal properties Shiny Dull High melting point (tend to be solid at room temperature) Lower melting points Conduct electricity Not usually conductors, often insulators Conduct heat Tend not to conduct heat Sonorous

Metalloids and elements with inbetween properties Some elements have atypical (not typical) properties for their classification. E.g. Mercury is classed as a metal but it is liquid at room temperature, it has a low melting point E.g. carbon graphite is classed as a non metal but it is shiny and can conduct electricity The elements either side of the stepped line we draw on the Periodic Table are sometimes called metalloids or semi-metals. They tend to have a mix of properties, some that would be typical for metals and some that would be typical for non-metals.

Metals and non metals: Burning metals in air

Metals and non metals: burning metals in excess oxygen Observation Magnesium Burns with a bright white flame Sodium Burns with a bright yellow/orange flame Potassium Burns with a lilac flame

Metals and non metals: burning non metals in oxygen Observation Carbon Bright yellow flash Sulfur Burns with a blue flame

Metals and non metals: acidic and basic properties of the oxides Metal oxides are alkaline Non metal oxides are acidic (Remember that we need to dissolve the oxide to test its pH, either with a pH meter or with a suitable indicator.)

Metals and non metals: Reactive metals and water