chemistry - rate of reactions Flashcards
What does rate of chemical reaction mean
How fast the reactants change into products
How could you find the speed of a reaction
By recording the amount if product for or the amount of reactant used up over time
What does the shape of a graph tell you about the rate of reaction
The steeper the line on the graph, the faster the rate of reaction. Overtime the line becomes less steep as the reactants are used up. The quickest reactions have the steepest lines and become flat in the shortest time.
Colision theory
The rate of reaction depends on
- the collision frequency of reacting particles. The more collisions there are the faster the reaction is.
- The energy transferred during a collision - particles have to collide with enough energy for the collision to be successful.
Activation energy
The amount of energy particles need to break the bonds in the reactants and start the reaction
What are the four things that the rate of a reaction depends on
- temperature
- the concentration of a solution or the pressure of gas
- surface area
- the presence of a catalyst
how does an increase in temperature effect the RoR
- when the temp is increased the particles all move faster
- If they move faster, they’re going to collide more frequently
- Also the faster they move the more energy they have, so more of the collisions will have enough energy to make the reaction happen
how does increasing the concentration or pressure effect the RoR
- If a solution is made more concentrated, it means there are more particles moving about in the same volume of water
- similarly when the pressure of gas is increased, it means there are the same number of particles in a smaller space
- this makes the collisions between the reactant particles more frequent
how does increasing the surface area effect the RoR
- if one of the reactants is solid, then breaking it up into smaller pieces will increase it surface area to volume ratio
- this means that for the same volume of the solid, the particles around the it will have more area to work on - so there will be collisions more frequently
ways to measure the RoR
- precipitation and colour change
- change in mass
- the volume of gas given off
equation for rate of reaction
amount of reactant used or amount of product formed divided by time
How to measure the rate of reactions using precipitation and colour change
Add a set volume of dilute sodium thiosulphate to a conical flask
Place the flask ion a piece of paper with a black cross drawn on it
Add some dilute HCl to a flask and start the stopwatch
Watch the black cross disappear through the cloudy sulphur and time how ling it takes to go
The reaction can be repeated with solutions of either reactants at different concentrations.
limitations to the precipitate and colour change practical
- doesn’t give a set of graphs
- results are very subjective
How does using a catalyst effect the RoR
A catalyst is a substance that speeds up a reaction without being used up in the reaction itself. They decrease the activation energy needed for the reaction to occur. They do this by providing an alternative reaction pathway.
how to measure the rat of reactions using magnesium and HCl
Start by adding a set volume of dilute hydrochloric acid to a conical flask and carefully place on a mass balance
Add some magnesium ribbon to the acid and quickly plug the flask with cotton wool
Start the stopwatch and record the mass on the balance. Take readings of the mass at regular intervals
Plot the results in a table and work out the mass lost for each reading.
Plot a graph with time on the x axis and loss of mass on the y axis
Repeat with more concentrated acid solution
