chemisty: salt & rate

  • Note:
    Salt is a compound made from acid where the hydrogen of the acid is replaced by metal or ammonium.

    Examples

    Salt

    Acid used to make the salt

    Sodium chloride (NaCl)

    Hydrochloric acid (HCl)

    Zinc sulphate (ZnSO₄)

    Sulphuric acid (H₂SO₄)

    Ammonium nitrate (NH₄NO₃)

    Nitric acid (HNO₃)

    Acid Reaction

    • Acid + (reactive) metal → salt + hydrogen

    • Acid + (metal) carbonate → salt + carbon dioxide + water

    • Acid + (metal) hydroxide / oxide → salt + water (neutralisation reaction between acid and alkali)

    • Examples of alkali are:

      • Ammonia

      • Group 1 compounds

      • Carbonate / hydroxide in oxide

      • Ammonium compounds of carbonate / hydroxide in oxide

      Solubility Rules

      Some salts are soluble and others are insoluble.
      The rules below will help you determine whether a salt is soluble or not:

      • All nitrate salts are soluble

      • All chloride salts are soluble except silver chloride and lead chloride

      • All sulfate salts are soluble except calcium sulfate, barium sulfate, silver sulfate, and lead sulfate

    • Note: Salt Preparation Method

                       Soluble salts
             /                      \
   Excess solid method         Titration method
 (acid + metal/oxide/base)   (acid + alkali)

      • Insoluble salts: by precipitation method

    • Steps in preparing soluble salt using excess solid:

      1. Add insoluble solid of metal or metal oxide/hydroxide/carbonate into acid (if needed, warm the acid to speed up the reaction).

      2. Add the solid until excess (no more bubbles produced or the solid no longer dissolves).

      3. Filter the mixture (to remove the excess solid).

      4. Heat the filtrate of salt solution until saturated (crystal form on edge of solution).

      5. Leave the saturated solution to cool to form crystals of salt.

      Steps in preparing soluble salt using titration:

      • Add alkali (from the burette) to a certain volume of acid in a conical flask that has been added acid-alkali indicator.

      • Use volumetric pipette to take certain volume accurately.

      • The alkali is added until the acid turns neutral, which is shown by the color change of the alkali indicator (e.g. green for universal indicator).

      • Add charcoal to the mixture in the flask to absorb the indicator.

      • Then filter the mixture to remove charcoal.

      • Heat the filtrate of salt solution until saturated.

      • Then leave it to form crystals of salt.

      There is also a small diagram showing: burette → volumetric pipette → conical flask

    • Note:

      The method to prepare insoluble salt is called precipitation method because it involves mixing two solutions (that are soluble) to produce insoluble solid.

      The reaction involved is not acid reaction, but double displacement reaction. However, acid might be used as one of the solutions.

      The two solutions involved in the method are:

      1. One solution that contains metal part of insoluble salt (usually metal nitrate).

      2. The other solution contains the last part of the insoluble salt (soluble compound usually group I compound or acid).

      Examples

      Insoluble

      Solution

      Silver chloride

      Silver nitrate + Sodium chloride

      Barium sulfate

      Barium nitrate + Sodium sulfate

      Lead iodide

      Lead nitrate + Sodium iodide

      Chemical equation for making silver chloride:
      Silver nitrate + sodium chloride → silver chloride + sodium nitrate

      AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)

      Steps in precipitation method:

      • Mix two solutions, stir, and give some time to make sure the reaction is complete.

      • Filter the mixture to obtain insoluble solid as residue on filter paper.

      • Rinse the residue with distilled water to make sure the insoluble salt is not contaminated with the solution.

    • Note: Some experiments that seem not following law of conservation of mass

      1. Reaction that produces gas conducted in not closed system

        • The mass after reaction decreases because the gas produced escapes into the air.

      2. Reaction involving gas in the air conducted in open air e.g. burning of magnesium

        • The mass after reaction increases because the oxygen from the air combines with magnesium to form magnesium oxide. This oxygen is not weighed at the beginning.

    • For reaction to happen, particles (of reactants) must collide with certain energy (called activation energy) to break bond of reactants. So the atoms can rearrange to become product.

    Reaction will be faster if the collision between particles happen more often and harder.

    Here are detail for the explanation of the effect of some factors to rate of reaction.

    Concentration

    Increasing concentration of reactants means more particles of reactants in the same volume (more crowded particles). Thus collision happen more often and speed up the reaction.

    Size of solid

    Smaller solid has bigger surface area (TBG p.117) for the collision happen more often and speed up the reaction.

    Temperature

    At higher temperature, particles move faster thus collide more often and harder and so the reaction happen faster.

    Catalyst

    Catalyst speed up the reaction by lowering the activation energy so more effective collision happen that will speed up the reaction.
    Catalyst is not used up during reaction thus can be used again.

  • Rates of reaction = how fast or slow a reaction is (e.g., burning, rusting, neutralization).

It is important to alter/change the rate of reaction especially in industry.

Some factors that affect rate of reaction

  • Concentration of reactant: Increase in concentration of reactant will speed up reaction.

  • Size of (solid) particle: the smaller the solid, the faster the reaction.

  • Temperature: the hotter, the faster reaction is.

  • Catalyst: is a substance that speeds up the reaction but does not change chemically after the reaction. The presence of catalyst speeds up the reaction.