GCSE Chemistry

Acids and Energetics

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  • Created by: Farrah
  • Created on: 21-03-11 10:08

Indicators

Hydrochloric acid (Acid):
- Litmus - RED
- Methyl Orange - RED
- Phenolphthalein - CLEAR

- Universal Indicator - RED

Water (Neutral):

- Litmus - N/A
- Methyl Orange - ORANGE
- Phenolphthalein - CLEAR

- Universal Indictaor - GREEN

Sodium Hydroxide (Alkali):

- Litmus - BLUE
- Methyl Orange - YELLOW
- Phenolphthalein - PINK
- Universal Indicator - BLUE 

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Reactions of Acids

Metal + Acid -> Salt + Hydrogen
Magnesium is the most reactive.

e.g. zinc + sulphuric acid -> zinc sulpahte + hydrogen 

Carobnate + Acid -> Salt + Water + Carbon dioxide
copper(II) carbonate + sulphuric acid -> copper sulphate + water + carbon dioxide 

Alkali + Acid -> Salt + Water
Alkalis (hydroxide) are soluble bases
e.g. sodium hydroxide + hydrochoric acid -> sodium chloride + water

Copyright S-cool (http://www.s-cool.co.uk/gcse/assets/learn_its/gcse/chemistry/acids-and-alkalis/neutralisation/image004.gif)

Base + Acid -> Salt + Water
A base is a metal oxide
copper(II) oxide + sulphuric acid -> copper sulphate + water 

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Properties of Acids

 

1. They are liquids.

2. They are solutions of compounds in water.

3. If concentrated they can be corrosive.

4. Acids taste sour (for example, vinegar).

5. Turn blue litmus paper red - this is an easy test for an acid!

6. Usually react with metals to form salts.

7. Acids contain hydrogen ions.

8. Turn Universal Indicator from green to red, and have a pH less than 7.

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Common Acids used in Laboratories

1. Hydrochloric acid, HCl(aq)

2. Nitric acid, HNO3(aq)


3. Sulphuric acid, H2SO4(aq)

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Properties of Alkalis

1. They feel soapy to touch.

2. They are soluble bases.

3. Like acids, they can burn the skin.

4. They turn red litmus blue - this is how you test for an alkali!

5. Alkalis contain hydroxide ions (OH-).

6. They taste bitter.

7. Turns Universal Indicator from green to blue or purple.

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Common Alkalis used in Laboratories

1. Sodium hydroxide, NaOH(aq)


2. Ammonia, NH3NH4OH(aq)


3. Calcium hydroxide, Ca(OH)2(aq)

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Common Examples of Neutral substances

1. Water

2. Sodium chloride solution, NaCl(aq)(common salt)


3. Sugar solution C6H12O6(aq)

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Acid and Alkali Molecules

In a strong acid, nearly all the acid molecules form ions.


In a weak acid, only some of the acid molecules form ions.


The more OHions (hydroxide ions), the more alkaline an alkali will be.


In other words, the more OH- ions there are the higher the pH number.

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Neutralisation

Bases and alkalis both neutralise acids - to form salt and water ONLY.

The alkali has neutralised the acid by removing its H+ ions, and turning them into water.

Neutralisation reactions involve a temperature rise as heat energy is released. These types of reactions are examples of exothermic reactions.

Acidic industrial waste is often neutralised using calcium oxide. This is sometimes called lime.

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Salt Preparation

Soluble salts can be produced by adding an insoluble metal, metal oxide or metal carbonate to an acid


1. Measure out 25cm³ of acid using a measuring cylinder

2. Add excess solid to the acid and stir

3. Filter off the excess solid

4. Pour the filtrate into an evaporating basin

5. Heat to partially evaporate the solution

6. Leave the solution to cool and crystallise

7. Decant the excess solution and blot the crystals dry on a paper towel

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Choosing a Method for Soluble Salt Preparation

METAL + ACID
The metal must be of medium reactivity for this method to work

BASE + ACID
The base must be INSOLUBLE for this method to work

CARBONATE + ACID
The carbonate must be INSOLUBLE for this method to work

ALKALI + ACID
An alkali (SOLUBLE base) must be available

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Titration

Are used to:
1) to find the concentration of an unknown substance - in analysis
2) to find the exact reacting volumes of an acid and an alkali when used to make salt crystals
(http://t1.gstatic.com/images?q=tbn:ANd9GcRuouHVrc2aeStdBENErp0Z9XaIapLb6aWNIYE8V9gvXcTA3CvMTw) 

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Titration Method

1) Fill the burette with dilute hydrochloric acid (using a funnel). Remember to fill the dead space below the tap and make sure there are no air bubbles.

2) Use a pipette and safety filler to measure 10cm³ of sodium hydroxide solution into at least 3 conical flasks.

3) Add 3 drops of a suitable indicator (e.g. phenolphthalein) to each (NOT U.I.)

4) Record the initial burette reading and add the acid into the sodium hydroxide solution until the phenolphthalein turns clear

5) Record the final burette reading and calculate the volume used

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Precipitation Reacton

When two soluble ionic salt solutions are mixed, the ions involved may re-combine to produce an insoluble salt

Sodium, Potassium & Ammonium salts: ALL soluble

Nitrates: ALL soluble

Chlorides: soluble except for silver, lead and chloride

Sulphates: soluble except for barium and calcium sulphate

Carbonates: insoluble except for sodium, potassium & ammonium carbonates

e.g. lead nitrate + sodium chloride -> lead chloride + sodium nitrate

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Calculating Energy Changes

Temperature change in a reaction can be converted into an enthalpy change in Joules using the following equation:

Energy change = mass of solution being heated x specific heat capacity of solution x temp. change

Q = m c ΔT

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ΔH for Exothermic and Endothermic Reactions

Exothermic reaction:

  • More energy is released when bonds are made
  • Less energy is taken in when bonds break
  • ΔH = negative

(http://t0.gstatic.com/images?q=tbn:ANd9GcS05aPIqTgoyQCGQ2y4vPki1KBuuu-VmKBesNsbe5YFbPUkt69B)

ΔH = bonds broken - bonds formed 

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Enthalpy Changes ΔH

Enthalpy (H) is the internal chemical energy stored within a compound.

Exothermic reactions: ΔH = negative because make>break

  • Products are more stable than the reactants - they have stronger bonds
  • Energy is released as heat when bonds are made

Endothermic reactions: ΔH = positive because break>make

  • Products are less stable than the reactants - weaker bonds
  • Energy is taken in from the surroundings when bonds are made
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