Chemistry

Rate of Reaction

Reactions can go at all different sorts of rates!

Slowest - Rusting Iron

Moderate - Magnesuim reacting with an Acid

Fast - Explosion

The Rate of Reaction depends on 4 Factors;

• Temperature
• Concerntration
• Catalyst
• Surface Area of Solid

Rate of Reaction = Amount of Reactant used of Amount of Product Formed / Time

Measuring Rate of Reaction

There are different ways the Rate of Reaction can be measured;

• Precipitation - This is when the product of the reaction is a precipitate which clouds the solution. Observe a mark through the solution and measure how long it takes to dissapear. The quicker the mark dissapears, the quicker the reaction.
• Change in Mass - Measuring the speed of a reaction that produces a gas can be carried out on a mass balance. The quicker the reading on the mass balance drops, the faster the reaction.
• The Volume of Gas Given Off - This involves the use of a Gas Syringe to measure the volume of Gas given off. The more Gas given off during the time interval, the faster the reaction.

Rate of Reaction Experiments

• Reaction of Hydrochloric Acid and Marble Chips - This experiment is often used to demonstrate the effect of breaking up the solid into small bits. Measure the volume of gas evolved with the Gas Syringe and take readings at regular intervals. Repeat the experiment with the same volume of acid and same mass of marble chips but with the marble more crunched up. This experiement measures the effect of Larger Surface Area.

• Reaction of Magnesuim Metal with Dilute Hydrochloric Acid - This recation is good for measuring the effect of Increased Concerntration. This reaction gives of Hydrogen Gas, which we can measure with a mass balance. Take readings of mass at regular time intervals. Repeat with more concertrated acid sloutions but always with the same amount of magnesium.

More Rate of Reaction Experiments

• Soduim Thiosulfate and Hydrochloric Acid Produce a Cloudy Precipitate - These two chemicals are both clear solutions. They react together to form a Yellow Precipitate of Sulfur. The experiment involves watching a black mark dissapear and time how long to takes to go. The higher the temperature, the quicker the reaction therefore it takes less time for the mark to dissapear.
• Decomposition of Hydrogen Peroxide - This is a good reaction for showing the effect of different Catalysts. This reaction is normally quite slow but a sprinkle of Maganese Oxide Catalyst which can be found in Potato Peel and Blood. Oxygen is then given off, which provides an ideal way to measure the reaction. Better Catalyst give quicker reactions. 

Collision Theory

• More collisions increase the Rate of Reaction - The effects of Temperature, Surface Area and Concerntration.
• Higher Temperature - Particles move quicker therefore collide more often.
• Higher Concerntration - When a solution is made more particles of Reactant about between the Water Molecules therefore collisions between important molecules are more likely.
• Larger Surface Area - If one of the reactants is Solid then breaking it up into smaller pieces will Increase Total Surface Area meaning there willl be more Frequent Collisions.

Collision Theory and Catalysts

• Increasing the temperature causes Faster Collisions.
• Reactions only happen if particles collide with enough energy and the minimum amount of energy needed is known as Activation Energy.
• A Catalyst is a substance which speeds up a reaction, without being changed or used up.
• How does a Solid Catalyst Work? - By giving the Reacting Particles a surface to stick to. This increases the number of Successfull Collisions.
• How do Catalysts reduce cost in Industry? - Catalysts increase the Rate of Reaction, which saves alot of money. They also allow the reaction to work at a much lower temperature which reduces the energy cost.
• Disadvantages of Catalysts? Expensive, Different Reactions need Different Catalysts and Catalysts can be posioned by Impurities.

Energy Transfers In Reactions

An Exothermic Reaction is one which transfers energy into its surroudings,usually in the form of heat and is usually shown by a rise in temperature.

• Burning of Fuels.
• Neutralisation Reactions.

An Endothermic Reaction is one that takes in energy from its surroudings,usually in the form of heat and is usually shown by a fall in temperature.

• Thermal Decompostion - CaCo3 - CaO + Co2
• Sports Injury Pack

Reversible Reactions can be Endothermic and Exothermic

• If the reaction is Endothermic in one different and Exothermic in the other
• Hydrated Copper Sulfate - Anhydrous Copper Sulfate + Water

Acids and Alkalis

• The strongest Acid has a pH 0.
• The strongest Alkali has a pH 14.
• An Acid is a substance with a pH less than 7. Acids form H+ ions in water.
• A Base is a substance of a pH greater than 7.
• An Alkali is a base that dissolves. Alkalis form OH- ions in water.
• So, H+ ions make solutions Acidic and OH- ions make it an Alkaline.

The Reaction between Acids and Bases is called Neutralisation;

• Acid + Base - Salt + Water

In terms of H+ and OH-;

• H+ (Aq) + OH- (Aq) - H2O (L)

State Symbols

• (S) - Solid
• (L) - Liquid
• (G) - Gas
• (Aq) - Dissolved in Water

Acids Reacting with Metals

Acid + Metal - Salt + Hydrogen

• The more Reactive the metal, the Faster the Reaction will go.
• Copper does not react at all because Copper is less reactive than Hydrogen.

The Speed of the Reaction is indicated by the rate at which the Hydrogen Bubbles are given off . The Hydrogen is confirmed by the Burning Splint Test.

• The name of the Salt depends on which Metal and which Acid is used.

For Example;

• Hydrochloric Acid always produces Chloride Salts.

Oxides, Hydroxides and Ammonia

Metal Oxides and Metal Hydroxides dissolve in water. These soluble compounds are Alkalis otherwise known as bases.

• Acid + Metal Oxide - Salt + Water
• Acid + Metal Hydroxide - Salt + Water

The combination of Metal and Acid decides the Salt;

• Hydrochloric Acid + Copper Oxide - Copper Chloride + Water
• Sulfuric Acid + Zinc Oxide - Zinc Sulfate + Water
• Nitric Acid + Magnesuim Oxide - Magnesuim Nitrate + Water

Ammonia dissolves in water to make an Alkaline Solution. When it reacts with Nitric Acid, you produce a neutral salt. Ammonium Nitrate and this is good for many reasons including;

• Good fertiliser because it has Nitrogen from two different sources and plants need Nitrogen to make proteins.

Making Salts

Metal + Acid - Salt +Hydrogen

Soluble salts can be made by reacting Acids with either Soluble or Insoluble Bases.

If you are using an Alkali ,which is a soluble base then you need to add just enough acid to make a neutral solution (check a small sample with Universal Indicator). Then warm the Salt Solution to evaporate the water.

If you sre using an Acid add the Metal until no more bubles can be seen. Then filter out the excess Metal and leave to Crytalise.

Making Salts

Insoluble Salt = Metal Oxide + Acid - Salt + Water

This work s for all Metal Oxides and is useful for making Salts out of Unreactive Metals.

• Add the Metal Oxide to the Acid until no more will dissolve.
• Filter to remlve access Metal to be left with the Salt.
• Leave to Crystalise.

Electrolysis

This is when you pass an Electric Current through an Ionic Substance thats Molten or an Solution breaking down the Elements. This requires a liquid to conduct the electricity, this is called an Electrolyte these contain free ions.

• Reduction - Gain of Electrons.
• Oxidation - Loss of Electrons.

1. Positively charged ions move to the Negetive Electrode during Electrolysis. They receive electrons and are Reduced.
2. Negatively charged ions move to the Positive Electrode during Electrolysis. They lose electrons and are Oxidised. The substance that is broken down is called the electrolyte.

• Metal is formed at Negetive Electrode.
• Non-Metal at the Positive Electrode.