C6 - Rate & Extent of Chemical Changes Flashcards
What does the rate of reaction mean?
How fast reactants are changed into products
On a graph, how can you tell how quick the reaction is by the line’s gradient?
The quickest reactions have the steepest gradients and become flat in the least time
What do flat lines show on a graph?
The reaction has finished
What is collision theory?
The more collisions there are, the faster the reaction is
What is the definition for activation energy?
The minimum amount of energy that particles need to react
What 4 things affect the rate of reaction?
- Temperature
- Concentration of solution / gas pressure
- Surface area
- Catalysts
What is a catalyst?
A substance that speeds up a reaction, without being used up in the reaction itself
How do catalysts work?
They work by decreasing the activation energy needed for the reaction to occur. They do this by providing an alternative reaction pathway with a lower activation energy
How can the rate of a reaction be observed?
- By how quickly the reactants are used up
- How quickly the products are formed
What is the equation for rate of reaction?
rate of reaction = amount of reactant used or amount of product formed ÷ time
What are the 3 different ways you can measure the rate of a reaction?
1) Precipitate and colour change - you can observe a mark through the solution and measure how long it takes to disappear. The faster the mark disappears, the quicker the reaction
If the reactants are coloured and the products are colourless (or vice versa), you can time how long it takes for the solution to lose (or gain its colour
2) Change in mass (usually gas given off) - as the gas is released, the mass disappearing is measured on the balance
The quicker the reading on the balance drops, the faster the reaction is
3) Volume of gas given off - the more gas given off during a given time interval, the faster the reaction
PRACTICAL: Describe how to investigate how magnesium and hydrochloric acid react to form hydrogen gas
1) Start by adding a set volume of dilute HCl to a conical flask
2) Add some magnesium ribbon to the acid and quickly attach an empty gas syringe to the flask
3) START THE STOPWATCH. Take recordings of the volume of gas in the gas syringe at regular intervals
4) Plot the results in a table
5) Plot this on a graph, with time on the x-axis and volume of gas produced on the y-axis
PRACTICAL: Describe how to investigate how sodium thiosulphate and hydrochloric acid produce a cloudy precipitate
1) Add a set volume of dilute sodium thiosulphate to a conical flask
2) Place the flask on a piece of paper with a black cross drawn on it
3) Add some dilute HCl to the flask and START THE STOPWATCH
4) Watch the black cross disappear through the cloudy sulphur and time how long it takes to go
PRACTICAL: Describe how you can investigate how the concentration of acid affects rate
1) The reaction can be repeated with solutions of either reactants at different concentrations (only change the concentration of one reactant at a time). The depth of the liquid must be kept the same each time
2) These results show the effect of increasing the concentration of HCl on the rate of reaction, when added to an excess of sodium thiosulphate
3) The higher the concentration, the quicker the reaction and therefore the less time it takes for the mark to disappear
How do you find the mean rate of reaction from a graph?
1) To find the mean rate for the whole reaction, you just work out the overall change in the y-value and then divide this by the whole time taken for the reaction
Eg: time taken = 20s, change in y-value = 25cm3
25cm3 ÷ 20s = 1.25cm3/s
2) You can also use the graph to find the mean rate of reaction between any two points in time
Eg: Find the mean rate of reaction between 20s and 40s
(19cm3 - 15cm3) ÷ (40s - 20s) = 0.2cm3/s
