Answer question #3. Make sure your reactions are balanced. Write them out below

Question

Answer question #3. Make sure your reactions are balanced. Write them out below and classify the redox reactions as combination,decomposition, disproportional, metal displacement, halogen displacement or hydrogen displacement. 14) What is the standard solution in a titration used for and how can you know your titration is complete? 3. The Marsh test for arsenic involves the following steps: (a) The generation of hydrogen gas when sulfuric acid is added to zinc. (b) The reaction of hydrogen with As(III) oxide to produce arsine. (c) Conversion of arsine to arsenic by heating. Write equations repre- ting these steps and identify the type of the reaction in each step. sen

Explanation / Answer

a) The generation hydrogen gas when sulphuric acid added to zinc

Zn + H2SO4 à ZnSO4 + H2

This reaction is oxidation reaction because Zn lost its electrons.

Net reaction    Zn Zn2+ + 2 e

b) The reaction of hydrogen with As(III)oxide to produce arsine

         As2O3 + 6 H2 2 AsH3 + 3 H2O

This reaction is reduction reaction

Net reaction As2O3 + 12 e + 6 H+ 2 As3 + 3 H2O

Overall, we can write like this

As2O3 + 6 Zn + 6 H+ 2 As3 + 6 Zn2+ + 3 H2O

As2O3 + 6 Zn + 6 H2SO4 2 AsH3 + 6 ZnSO4 + 3 H2O

c) Conversion of arsine to arsenic by heating.

2AsH3 2As + 3H2

This reaction is elimination reaction

AsH3 is unstable compound, it decomposes to As by heating.

14) Standard solution is a solution, it has a precisely known concentration of a substance. This solution we are in titration to determine the unknown solution concentrations.

If you use any indicators, at end point colour will change means titration is completed.

We can use pH meter also to determine the end point. At end point values will change drastically.

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