C2.4 Rates and Reactions

Measuring the amount of REACTANTS whcih are used up over time or the amount of PRODUCT made over time are two ways of finding out the RATE of a reaction. The SLOPE of the lines on the graph drawn from these experiments tells us about the rate at any given TIME.

WAYS OF DETECTING CHANGES IN REACTANTS OR PRODUCTS:

• measure the DECREASING mass of a reaction mixture
• measure the INCREASING VOLUME of gas given off
• measure the DECREASING LIGHT through a mixture.

GRADIENT:

• STEEPER slope- FASTER reaction
• LOWER slope- SLOWER reaction

RATE OF REACTION: amount of reactant used or amount of product formed

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                                                                             time

Particles can react with each other only when they COLLIDE with sufficient ENERGY. Reaction rates increase when collisions are more energetic, and happen more FREQUENTLY. The MINIMUM amount of energy needed fror particles is known as ACTIVATION ENERGY.

REACTIONS MORE LIKELY TO HAPPEN:

• if you INCREASE the CHANCE of reacting particles colliding with each other
• if you INCREASE the ENERGY that they have when they collide

• Factor for INCREASING the rate of reaction
• POWDERED surfaces INCREASE surface area, so there is more access for the reactant to react
• Also larger surface area INCREASES the frequency of Collisions NOT the energy that they collide with.
• HOWEVER: for a lump of solid- LESS surface area- so less surface for reactant to reach to.

INCREASING TEMP....

• INCREASES frequency of the collisions- more TEMP, means that particles have MORE energy, so they move QUICKER and COLLIDE more.
• INCREASES the energy that the particles collide with-so COLLIDE harder so more energy GIVEN OFF.

When we increase the temperature of a reacting mixture, we increase its RATE of reaction. The higher temperature makes the particles move more QUICKLY so they COLLIDE more often and the collisions have more ENERGY. At room temperature, a temperature RISE of about 10oC roughly DOUBLES  the reaction rate. This explains why we use fridges and freezers. REDUCING the temperature DECREASES the rate of reactions which make food go OFF.

The RATE of a reaction is affected by the CONCENTRATION of reactants in solutions and by the PRESSURE if the reactants are gases. Both of these tell us that the NUMBER of particles that there are in a certain VOLUME of the reaction mixture. Increasing these will increase the FREQUENCY of COLLISIONS between reacting particles, making FASTER reactions.

INCREASING CONCENTRATION:

• more PARTICLES are moving about in the same volume of solution. so tehy are more CROWDED- more likely to COLLIDE. Frequent collision - FASTER reaction.
• the partciles DO NOT GAIN more ENERGY.

• They are remain UNCHANGED CHEMICALLY at the end of the reaction
• lasts a LONG time as used OVER and OVER again
• e.g. IRON Catalyst used to make Ammonia. PLATINUM catalys used to make nitic acid.
• Usually used as POWDERS- thsi gives them a LARGE SA, so further increases the Rate of reaction
• only SMALL amount of catalyst needed to speed up the reaction.
• ESSENTIALLY: they only LOWER the ACTIVATION ENERGY- so less energy needed by the particles to collide.

A catalyst INCREASES the rate of a chemical reaction. However it is not USED up and REMAINS the same chemically after the REACTION.

• Often EXPENSIVE and PRECIOUS metals
• ususally cheaper to use expensive catalyst rather than using EXTRA ENERGY. To get the same rate of reaction with out a catalyst- High temperatures and Pressures needed.
• save MONEY and ENVIRONMENT- as high temperatures other wise burn a lot of FOSSIL fuel
• Industry catalysts- often TRANSITION METALS, they are TOXIC- so if they slowly escape into the atmosphere then BUILD up in organisms- poisons them slowly.
• To reduce some of the nasty effects of Catalysts, OTHER chemicals added- so to improve it.

FUTURE:

• Nanoparticles also used in Catalysts- SMALL mass but LARGE surface Area.

MEDICINE:

• Previously TRANSITION METALS were used to make catalysts, but now NO metals- safer

ENZYMES:

• Biological Catalysts- can be used in soldids so Enzymes are not wasted, easier separate

Chemical reactions inolve energy CHANGES. When a reaction releases energy we say the reaction is EXOTHERMIC. Two important examples of thsi type of reaction are NEUTRALISATION and OXIDATION. When a reaction takes in energy we say the reaction is ENDOTHERMIC. An important example of tshi type of reaction is THERMAL DECOMPOSITION.

EXOTHERMIC REACTIONS:

• GIVES out heat
• measure a RISE in temperature
• Unusual example: RESPIRATION. Glusose iS OXIDISED.
• Warm blooded animals rely on EXOTHERMIC reactions to MAINTAIN body temperature.

ENDOTHERMIC REACTION:

• TAKES IN heat
• measure a DECREASE in temperature
• LESS common
• OFTEN need energy to start off the reaction

REVERSIBLE REACTION:

• One way EXOTHERMIC
• Other way ENDOTHERMIC- absorbs the SAME amount of energy that is RELEASED
• Energy CANNOT be CREATED or DESTROYED

COPPER SULPHATE:

• When HEAT copper sulphate - FORMS crystals, drives off water off the crystals,
• PRODUCES: ANHYDROUS( without water) crystals- white crystals
• Copper Sulphate solution was initially HYDRATED- had water.
• Changes COLOUR- useful test for water

WATER PAPER:

• Filter paper with COBALT CHLORIDE- leave to dry.
• When water added to the COBALT CHLORIDE PAPER- turns pink.

HAND WARMERS:

• Exothermic reaction
• Some are ONCE use only or MULTIPLE uses
• Reusuable- involves crystallisation of SUPERSATURATED SOLUTION
• Once heaters- give off more energy

SELF HEATING CANS:

• Calcium oxide + water = Calcium Hydroxide
• Break seal to allow the reactants to MIX- Exothermic reaction

COLD PACKS:

• Ammonia Nitrate + water = ENDOTHERMIC REACTION
• Used in emergency situations- as use ONCE
• When reactants mixed temperature drops- taking energy from the person's skin- REDUCES swelling and pain.
• Same principel for keeping drinks COLD.