Rates of Reactions -

Rate of chemical reactions

How fast the reactants are changed into products.

Factors that affect the rate of reaction

• Temperature

• Concentration (pressure for gases)

• Surface Area

• Catalysts

Collision Theory

• The collision frequency of reacting particles

• More there is in a certain period of time the faster the reaction is

• The energy transferred during a collision.

• Particles have to collide with enough energy to be successful

• To increase the probability of a successful collision you can increase the frequency of energy of the collisions

Temperature - Frequency

• When temp is increased the particles mover quicker as they have more kinetic energy

• This means there will collide more often and more collision means faster rate

Conc (Pressure) - Frequency

• If a solution is concentrated there are more particles of reactant knocking about between water molecules

• Which makes collisions between the important particles more likely

• In gas increasing pressure means the particles are squashed together so there is more frequent collisions

Surface Area - Frequence

• If one of the reactants is solid then breaking it into smaller pieces will increase its SA:V ration

• This mean that for the same vol of solid the particles around it will have more area to work on

• so there is more frequent collisions and the rate of reaction will be faster

Increasing the energy of collisons

• Higher Temp also increases the energy of collisions as well as frequency

• Reactions only happen if the particles collide with enough energy

• At higher temp there is more particles colliding with enough energy to make the reaction happen

Rate and Proportionality

• Directly proportional to frequency of successful collisions

• If frequency doubles rate will also double

Catalysts

A catalyst is a substance which can speed up a reaction without being changed or used up in the reaction. They do this by lowering the activation energy need for the reaction to occur. By providing an alternative pathway.

Mean rate of reaction formula

Mean rate of reaction = Quantity of reactant used or product formed/TIme

Measuring Formation of Product

• Precipitation

• Change in Mass (can plot a graph)

• Volume of Gas Given off (can plot a graph)

Precipitation - Measuring

• Record the visual change in the reaction if the intial solution is transparent and the product is a percipitate that clouds the solution.

• Observe a mark through solution and measure how long it takes for it to disappear

• Easy to do however results are subjective

Change in Mass - Measuring

• Measure speed of reaction that produces a gas using a mass balance

• Reactants into a conical flask and cotton wool in the neck

• As gas is released the mass disappearing is measured

• Gas is released straight into the room so dangerous if poisonous

Volume of Gas given off - Measuring

• Use a gas syringe to measure the volume of gas released during a reaction.

• The change in gas volume indicates the reaction rate,

Graphs Description

• Reactions start quickly because at the beginning there a lot of reactant particles around so collisions between them are frequent

• As it progresses the reactants get used up so number of particles decrease

• Meaning collisions between particles get less frequent and the reaction slows down

• Reaction stops when all of the particles from at least one of the reactants are used up

Comparing Rate of Reaction

• Fastest reaction will be the lines with the steepest slope at the beginning.

• Reactions that start off with the same amount of reactants will give lines that finish at the same level on the graph

Mean Rates from Graphs

• Change in Y/ Change in X

• Divide by Time in correct units

Rates and Tangents

• To find out rate at a particular point you need to know how steep the curve is

• You do this by drawing a tangent

Calculating Rates from Tangents

• find two points on the tangent

• gradient = change in y/change in x

• divide by time in correct units

RP5 - Magnesium Metal and Hydrochloric Acid

• Used to investigate the effect of increased reactant concentration of rate of reaction

• Reaction gives off hydrogen gas so you can measure rate by measuring volume of gas produced using a gas syringe (or mass scale)

  1. Meaure 50cm³ of dilute HCL using measuring cylinder and add to conical flask

  2. Add magnesium ribbon to acid and quickly attach and empty gas syringe

  3. Start stopwatch,take reading of volume at regular intervals until volume hasnt change for three reading in a row

  4. Repeat with more concetrated acid solutions. Variables such as mass and SA of magnesium ribbon the temp of reaction vol of acid (CV) only change acid concentration (IV)

RP5 - Sodium Thiosulfate and Hydrochloric Acid

• Both clear solutions that mix together to form a yellow percipitate of sulfur

• Add 50cm³ of dilute sodium thiosulfate solution to a flask

• Place the flask on a piece of paper with a black cross Add 10cm³ of dilute HCL to the flask give the flask a swirl and mix the reactant and start the stopwatch

• Black cross through cloudly sulfur and time how long it takes to disappear

• Reaction can be repeated with solutions of either reactant at different conc. The depth of liquid must be the same each time,use the same reaction flash each time, same person should observer cross each time. (CV)

Reversible Reaction

Reactions in which the products from the reaction can react together to form the original reactants. The direction of reversible reactions can be changed by changing the conditions.

Equilibrium

When a reversible reaction occurs in a closed system, equilibrium is reached when the forward and reverse reactions occur at exactly the same rate.

Equilibrium explanation

• As the reactants react in a reversible reaction their concentrations fall.So the forward reaction slows down

• As more products are made their concentrations rise the backward reaction speeds up

• After a while the forward and backward reaction wiull be going as the same rate meaning the system has reach equilibrium

• Reactionas are still happening however the overall effect is nil because the forward and reverse reactions cancel each other out

What side does the equilibrium lie on?

• If the equilibrium lies to the right the concentration of products is greater than that of reactants

• If the equilibrium lies to the left the concentration of reactants is greater than that of products

Energy transfer in reversible reactions

If the reaction is endothermic in one direction it will be exothermic in the other. The energy absorbed by the endothermic reaction is equal to the energy released by the exothermic reaction.

Le Chatelier’s Principle:

If a reaction at equilibrium is subjected to a change in concentration, temperature or pressure, the position of equilibrium will move to counteract the change.

Le Chatelier’s Principle - Pressure

• If you raise the temperature the yield of an endothermic reaction will increase and the yield of an exothermic reaction will decrease

• If you reduce the temperature the yield of the exothermic reaction will increase and the yield of the endothermic reaction will decrease

Le Chatelier’s Principle - Temeperature

• Raising the pressure favours the reaction which produces less volume(the fewest number of gas molecules)

• Lowering the pressure favours the reaction which produces more volume(the greatest number of gas molecules)

Le Chatelier’s Principle - Concentration

• If you increase the concentration of a reactant the system tries to decrease it by making more products

• If you decrease the concentration of a product the system tries to increase it again by reducing the amount of reactants