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Topic 7a - Rates of reaction - Coggle Diagram
Topic 7a - Rates of reaction
collision theory
Increasing collisions
Temperature
Increasing the temperature means the particles have more kinetic energy, so move faster
Frequency of collisions increases and the energy of collisions increases, so a greater proportion of collisions are successful
Concentration
More concentrated solutions mean more particles in a given volume so collisions are more likely
Frequency of successful collisions increases as pressure increases
Surface area
Increasing the surface area to volume ratio means that for the same volume of the solid, the particles in the solution have more particles to collide with
This will lead to more frequent and successful collisions and the rate will increase
Chemical reaction sonly occur when the reacting particles collide with enough energy, the minimum amount of energy is the activation energy
To increase the rate of reaction, you need to increase the frequency of successful collision so more reach the activation energy
Rate of reaction graphs
When comparing graphs you can see which is the faster one because it plateus earlier
The fastest reaction has the steepest slope at the beginning
Reactions that start at the same amount of reactants, will finish at the same level
To find the rate of reaction at a give point, draw a tangent to the graph, and the gradient is the rate of reaction
Catalysts
Enzymes
Generally only catalyse a specific molecule
Catalyse chemical reactions inside living organisms
Biological catalyst
A substance that speeds up a reaction without being chemically changed or used up, and without changing the reaction products
Don't appear in chemical equations, instead they are written above the arrow
Decrease the activation energy by providing an alternative reaction pathway
Calculating rates of reaction
Measuring the rate of reaction = Quantity of reactant or product made/ Time
Measured in units like: cm3 / seconds
Measures: product produced in a given time
Methods to determine rate of reaction
Precipitate
Quicker the cross disappears, quicker the reaction
Time taken for cross underneath to become obscure is measure
Initial solution is transparent , product is a precipitate which clouds the solution, making it obscure
Recording a visual change
Change in mass
Reaction in finished when the mass is constant
Reaction is finished when the mass is constant
Measuring the speed of a reaction that produces a gas
Volume of gas given off
The more gas given off, the faster the reaction
Reaction is finished when no more gas is produced
Using a gas syringe to measure the volume of gas given off
Core Pactical - Effect of surface area on rate
Use the reaction of marble chips with dilute HCl to investigate the effect of surface area of the rate of reaction
2HCl + CaCO3 --> CaCl2 + H2O + CO2
The reaction produces CO2, so you can follow the rate by measuring the volume of gas given off
Method
1) Pour some HCl into a connical flask
2) Weigh 2g of marble chips and add it to the acid
3) Start the Stop watch
4) Take the readings of the gas volume at regular intervals until the volume remains constant
5) Use a pestle and mortar to lightly crush 2g of marble chips and repeat using the same volume and concentration of acid
Reord your results in a table, time on the x-axis and volume of gas produced on the y-axis
Results
Smaller particles have a larger surface area to volume ratio
As you go from larger chhips to small ones, the surface area to volume ratio increases
The powdered chalk would give the fastest rate
Core Practical - Investigating the effect of temperature
Use the reaction between sodium thiosulfate and Hcl
They are both colourless solutions, which form a yellow precipitate
2HCl + Na2S2O3 --> 2NaCl + S + H2O
Method
1) Start by measuring 50cm3 of sodium thiosulfate and 10cm3 of HCl
2) Gently heat both solutions in a water bath
3) Mix solutions in a conical flask, place it on a piece of paper with a black cross on it
4) Start the stopwatch and measure the time for the cross to disappear
5) Repeat the experiment at different temperatures and carry out each expriment multiple times to make the results more accurate
6) The depth of the solutions must be constant, and the same person must observe the cross as it is subjective when it disappeares
Results
Reactions that happen a lower temperatures take longer
Reactions at higher temperatures finished sooner
Increasing the temperature increases the rate of reaction