AS 90944: Acids and Bases

Properties of acids

  • Weak acids have a sour taste
  • Acids change the colour of indicators such as phenolphthalein, litmus and universal indicator.
  • Acids react with certain metals to form hydrogen gas (zinc, magnesium, aluminium ,iron and other reactive metals but not copper) and a salt
  • Acids react with carbonates and hydogen carbonates (old fashioned name bicarbonates)
  • Acids react with bases or alkalis to form a salt and water.
  • Acids release hydrogen ions when they are dissolved in water.

Properties of  Bases and Alkalis

All metal oxides and hydroxides are bases. Metal oxides don’t dissolve in water. Sodium hydroxide, potassium hydroxide, calcium hydroxide and ammonium hydroxide dissolve in water. Bases like these that dissolve in water are called alkalis.

Most household cleaning products contain contain bases. Oven cleaners contain sodium hydroxide (sometimes called caustic soda). Sodium hydroxide reacts with fat and grease to break them down. Soap is made when sodium hydroxide reacts with fats or oils.

Sodlum hydroxide feels soapy to touch because it reacts with fat and oil in your skin

Toothpaste contains a weak base to neutralise the acid produced by mouth bacteria.

Antacid tablets contain carbonates to react with and neutralise excess acid in the stomach.

  • Weak bases have a bitter taste and feel slippery to touch.
  • Bases change the colour of indicators (litmus phenolphthalein and universal indicator)
  • Bases react with acids to form a salt and water
  • Soluble bases (Alkalis) release hydroxide ions in solution) 

Homework

Watch the video about using universal indicator to test the pH of different solutions. Watch also how the pH changes during a neutralisation reaction.

Answer the questions after it. To be handed in on Tuesday…..have a good weekend.

  1. why should you wear gloves when using Mr Muscle?
  2. What pH would you predict for toothpaste? What colour would universal indicator turn when it is used to test the pH of toothpaste?
  3. What colour is phenolphtalein in acid solution?
  4. What colour is phenolphthalein in basic solution
  5. What colour does universal indicator turn when it is added to vinegar? What is the pH of vinegar?
  6. What colour does universal indicator turn when it is added to Mr Muscle. What is the pH of Mr Muscle?
  7. What colour does universal indicator turn when it is added to baking soda (sodium hydrogen carbonate) in solution. What is the pH of baking soda solution?
  8. What colour does universal indicator turn when it is added to lemon juice. What is the pH of lemon juice?
  9. What colour does universal indicator turn when it is added to a solution of table salt. What is the pH of salt solution? Why does salt solution have this pH?
  10. What is the PH of sodium hydroxide solution. Is it a strong or weak base?
  11. What is the pH of hydrochloric acid solution? Is it a strong or weak acid?
  12. When vinegar is neutralised with sodium hydroxide the hydrogen ions are removed and the pH of the solution changes till it reaches a pH of 7 (completely neutral) Describe the colour changes you would see when vinegar is neutralised with sodium hydroxide solution. (Use the colours from the colour chart.
  13. Writing fully balanced chemically equations.

………but first a reminder about salts and word equations

When hydrochloric acid reacts with a base and is neutralised salts known as chlorides are produced.

When sulphuric acid reacts with a base and is neutralised salys known as sulphates are produced.

When nitric acid reacts with a base and is neutralised salts known as nitrates are produced.

Here are the general word equations for reactions you need to know about.

acid + base —-> salt + water (bases are metal oxides and hydroxides)

acid + carbonate —–> salt + carbon dioxide + water

acid + hydrogen carbonate —–> salt + carbon dioxide + water

acid + metal —-> salt + hydrogen

Now complete each word equation, write down the chemical formulae and finish with a fully balanced chemical equation. 

copper oxide + nitric acid —->

sodium hydroxide + sulphuric acid —->

calcium carbonate + hydrochloric acid —->

magnesium + hydrochloric acid

hydrochloric + sodium hydrogen carbonate —->

Use the table of ions from your text book to help write chemical formulae

 

 

AS 90944: Rate of reaction homework,effect of heat on the thiosulphate reaction.

Thiosulphate (doesn’t matter what the formula is but thio as a prefix indicates a sulphur compound) ions react with the hydrogen ions in acids to produce a thick precipitate of sulphur. The reaction happens faster at higher temperatures.

We can compare speeds of sprinters by measuring how long it takes for them to run a fixed distance, 100m. The world record held by Usain Bolt is quoted as a time, 9.58 seconds. The smaller the time taken to cover 100 meters the faster the speed.

We can time how quickly enough sulphur is produced by the thiosulphate ions to completely block a black band on the back of a test tube.The equation for this reaction can be written as:

Thiosulphate ions     +     hydrogen ions     —–>     sulphur     +     other products

Homework:

Watch the video. Decide how long it takes for each reaction in the video to produce enough sulphur to hide the black mark.Draw up a table of results. Plot a graph of temperature against time taken for the reaction( temperature goes on the horizontal axis, this is the variable you have control over.

Do an experimental write under the headings (on loose paper)

Aim

equipment 

Diagram

Method

table of results 

graph 

Conclusion. What happens to the rate of reaction as the temperature increases?

Discussion: Use particle theory to explain the effect of heat on the rate of reaction with thiosulphate and hydrogen ions. Words to use in the discussion include activation energy, kinetic energy, effective collisions, frequency of collisions.

 

AS 90944 Rate of Reaction homework. The effect of a catalyst on the rate of a reaction

Homework Part 1:The Effect of a catalyst.

Reactive metals will dissolve in mineral acids (sulphuric, nitric, hydrochloric) to produce a salt and hydrogen gas.

Watch the video of the experiment you carried out last lesson and do an experimental write up under these headings.

Aim

Equipment

Diagrams (neat and labelled)

Method : bullet points giving clear instructions so that someone else could come along and repeat your experiment

Results

Discussion : What is the role of a catalyst and why were copper shavings placed in a test tube with acid before adding the zinc?

 

Homework Part 2. Acid and metal word equations.

In the video we saw that reactive metals dissolved in mineral acids to form a salt and release hydrogen gas.

Copy down these word equations after your experimental write up and complete them The first one has been done for you.

Zinc      +      sulphuric acid     —–>     zinc sulphate      +     Hydrogen

Zinc     +     nitric acid           —–>     

Magnesium     +     hydrochloric acid     —–>

Iron      +      sulphuric acid     —–>

Aluminium     +     nitric acid     —–>

Copper     +     hydrochloric acid     —–>

 

Homework part 3: describe carefully how you would test for hydrogen gas

AS 91161 Carry out a quantitative analysis. titrations

Play  the presentation Chemical equations and calculations. Pay particular attention to the worked example before doing the example at the end of this post

 

Example to be handed in tomorrow

Hydrochloric acid solution was titrated with standard sodium hydroxide solution.

Before the titration the acid was diluted by making 1ml of solution up to 20ml(1:20) in a volumetric flask

20 ml samples of the acid in conical flasks were  titrated by running standard sodium hydroxide solution ( 0.15 molL-1 concentration) in from a burette using phenolphthalein indicator

Titration Results


Volume of hydrochloric acid=20ml

Titration

rough

titration 1

titration 2

titration 4

titration 5

Final volume (ml)

26.35

49.60

24.50

50.00

47.80

Initial volume (ml)

00.00

25.20

00.20

27.00

23.30

Titre (ml)

26.35

24.40

24.30

23.00

23.50


On your own paper:

  1. write the heading Titration Calculation
  2. Copy the table of results beneath
  3. select three concordant results and calculate the average titre
  4. Write down the balanced chemical equation for the reaction
  5. Use the method you have been shown to calculate the concentration of the hydrochloric acid
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