RATES OF REACTION

Chemistry

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  • Created by: katie
  • Created on: 17-05-12 15:35

rates of reaction

Reactions can go at all sorts of different rates - 1) one of the slowest if the rusting of iron, 2) a moderate speed reaction is a metal reacting with a acid to produce a gentle stream of bubbles, 3) a really fast reaction is an explosion.

The rate of a reaction depends of FOUR things:

1) temperature

2)concentration (or pressure for gases)

3) catalyst

4) size of particles (or surface area)

Typical graphs for rate of reactions: The increases rate on a graph could be due to: increase in temperature, increase i concentration (or pressure), a catalyst was added or a solid reactant crushed up into smaller bits.

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measuring rates of reaction

Three ways to measure the speed of a reaction - can be observed either by how quickly the reactants are used up or how quickly the products are formed. Usually alot easier to measure products forming.

Rate of reaction = amount of reactant used or amount of product found/ time

1) PRECIPITATION -> when the product of reaction is a precipitate which clouds the situation, observe a marker through a solution and measure how long it takes to dissapear, the quicker it goes the quicker the reaction, only works for reactions where the intial solution is rather see-through.

2)CHANGE I MASS (usually a gas given off) -> measuring the speed of a reaction that produces a gas can be carried out of a mass balance, as the gas is releases, the mass dissapearing is easily measured on the balance, the quicker the reading on the balance drops, the faster the reaction.

3)THE VOLUME OF GAS GIVEN OFF -> involves the use of a gas syringe to measure volume of gas given off, the more gass iven off during a given time interval the faster the reaction. They are very accurate to the nearest millilire, i the reaction is too vigorous you can easily blow the plunger out the end of the syringe.

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rates of reaction experiments

1) reaction of hydrochloric acid and marble chips - measure the volume of gas evolved with a gas syringe and take readings at regular times, make a table of readings and plot them as a graph, choose regular time intervals so time is the indepentant variable and volume is the dependant varibale, repeat experiment with same volume of acid and mass of marble chips but marbles more crunches up, then repeat with mass mass of powdered chalk instead of marble chips.

2) reaction of magnesium metal with dilute HCl - reaction is good for measuring the effects of increases concentration, reaction gives off hydrogen gas which can measure with mass balancem time is independant and mass loss is the dependant variable.

3) sodium thiosulfate and HCl produce a cloudy precipitate - two chemica;s are both clear solutions, react together to form yellow precipitate of sulfur, involved watching black mark dissapear through cloudy sulfur and timing how lon it takes to go, can be repeated at different temperatures (best way to use a waterbath so its correct temperature), depth of liquid must be kept same each time.

4) the decomposition of hydrogen peroxide - good reaction for showing the effect of catalysts, normally quite slow but sprinkle of manganese(IV) oxide catalyst speeds it up no end and other catalysts which work are potato pee and blood, oxygen gas is given off which provides ideal way to measure rate of reaction using gas syringe.

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collision theory

- solid saMore collisions increases the rate of reaction:

1) HIGHER TEMPERATURE increases collisions - when temperature is increases the particles move quicker, if they move quicker they have more collisions

2) HIGHER CONCENTRATION (or PRESSURE) increases collisions - solution made more concentrated means more particles reactant knoking about between the water molecules which makes collisions between the important particles most likely

3) LARGER SURFACE AREA increases collisions - one of the reactants is a solid then breaking it up into smaller pieces will increase it surface aream means particles has more area to work on, more useful collisions

4) CATALYSTS increase the number of SUCCESSFUL collisions - solid catalyst works by giving the reacting particles a surface to stick to, they increase the number of SUCCESSFUL collisions by lowering the activiation energy.

Faster collisions increase the rate of reaction -> FASTER COLLISIONS ARE ONLY CAUSES BY INCREASING TEMPERATURE

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catalysts

A catalyst is a substance which changes the speed of a reaction without being changed or used up in the reaction.

1) catalysts lower the activation energy - activation energy is the minimum amount of energy needed for a reaction to happen, bit like having to climb up one side of a hill before you ski down the other, catalysts lower the activation energy of reactions making it easier for them to happen, means a lower temp can be used.

2) solid catalysts work best when they have a big surface area - usually used as a powder or pellets or a fine gauze, gives them a very large surface area to enable the reacting particles to meet up and do the business, transition metals are common catalysts in many industrial reactions.

3) catalysts help reduce costs in industrial reactions - very important for commerical reasons, catalysts increase rate of reaction which saves money, allows them to work a lower temp, which reduces energy used up in reaction (sustainable) BUT they are expensive to buy but you can use them again and again. Different reactions use different catalysts and they can be poisoned by impurities.

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energy transfer in reactions

In an Exothermic reaction, heat is given out:

1) burning fuels - best example of exothermic reaction is burning fuels also called COMBUSTION because it gives out a lot of heat

2) neutralisating reactions - an acid and a alkali are also exothermic

3) oxidation reaction - addition of water to anhydrous copper (II) sulfate to turn it blue hydrated copper sulfate cyrstals produce hear so it exothermic

In an Endothermic reaction, heat is taken out: They are less common reactions, thermal decompositions are a good example:

Thermal decomposition of calcium carbonate - heat must be supplied to make the compound decompose to make quickline, a lot of energy is needed to make this happen, calcium carbonate needs to be heating in a kiln and kept at 800 degrees.

Thermal decomposition of hydrated copper sulfate - heat blue hydrated copper (II) sulfate cyrstals it drives the water off and leaves white anhydrous copper (II) sulfate powder = ENDO ,add couple drops of water to white powder and you get blue cystals back = EXO (reversibile reaction)

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reversible reactions

A reversible reaction is one where the profucts of the reaction can themselves react to the original reactants.

They will reach a DYNAMIC EQUILIBRIUM = the reactions are still taking place in both directions

Chaning temperature and pressure to get more product - in a reversible reaction the 'position of equilibrium' depends strongly on the temperature and pressure surrounding the rection, if you alter them you can move the 'position of the equilibrium' to give more product and less reactants.

Temperature = All reactions are exothermic in one direction and endothermic in the other. If you raise the temp, the endo reaction with increase to use up the extra heat. If you reduce the temp, the exo reaction will increase to give out more heat

Pressure = Many reactions have a greater volume on one side either of products or reactants. If you raise the pressure, it will encourage the reaction which produces less volume. If you lower the pressure it will encourage the reaction which produces more volume.

Adding a CATALYST doesnt change the equilibrium position: catalysts speed up both the forward and backward reactions by the same amount so adding a catalyst means the reaction reaches equilibrium qucker but you end up with the same emount of product as you would without a catalyst.

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the haber process

Nitrogen and hydrogen are needed to make ammonia

1) the nitrogen is obtained easily from the air which is 78% nitrogen (and 21& oxygen)

2) the hydrogen comes from natural gas or other sources like crude oil

3) because the reaction is reversible - it occurs in both directions - not all of the nitrogen and hydrogen will convert to ammonia. The reaction reaches a dynamic equilibrium.

Higher pressures favour the forward reaction, pressure is set as high as possible to give best % yield, forward reaction is exo, which means that increasing temp will move the eqilibrium the wrong way. Lower temps mean a slower rate of reaction though. The 450 degrees is a compromise between maxium yeild and speed of reation. Ammonia ia formed as a gas but as it cools in the condenser it liquefies and is removed. The unused hydrogen and nitrogen are recycled so nothing is wasted.

 

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