Chemistry - Rate of chemical change Flashcards
How can the rate of reaction be found?
The rate of reaction can be found by measuring the amount of product formed in a certain period of time
What are the three main methods of measuring rate?
-measuring the volume of gas given off by a reaction over time
-measuring the loss of mass of a reaction over time when a gas is produced
-measuring the amount of light that passes through a reaction mixture (as a precipitate forms) over time – this can be done by inspection or using a light sensor and data logger
How is the mass of a substance measured?
The mass of a substance – solid, liquid or gas – is measured with a balance. This mass balance could be attached to a data logger to measure the decrease in mass against time continuously and produce a graph instantly. The data logger does not make the mass balance any more accurate. Cotton wool is used in the neck of the flask as it allows the gas to escape.
How is the volume of a gas measured?
The volume of a gas is usually measured with a gas syringe (or sometimes with an upside-down measuring cylinder) when the chemical reaction produces a gas, eg metal reacting with acid to produce hydrogen.
How can you calculate rate were a precipitate forms using inspection?
The rate can be measured by placing the reaction container on top of a black ‘X’ before adding the chemicals together (example shown in diagram below). If you time how long it takes until you can no longer see the X due to the formation of the precipitate, you can calculate the rate using the equation.
rate = 1 ÷ (time taken for X to be obscured)
How do you measure the rate where a precipitate forms using a light sensor?
The amount of light passing through the reaction can be measured directly using a light sensor and a data logger. The rate can then be calculated by using the light sensor to work out how long it takes for the light level to stop decreasing.
What are the advantages/disadvantages for light sensor?
Disadvantages
-Light sensors are more expensive and so may not be readily available
-Light from the surrounding area could possibly interfere with results from light sensor
Advantages
-light sensors can provide data for the entire reaction, allowing the rate of reaction to be calculated for any point in the reaction
-the rate of reaction can be determined more accurately
What is collision theory?
For a chemical reaction to occur, the reactant particles must collide with each other.
- Two pairs of particles move towards each other
- The pairs collide and reform so that each member of the original pair joins with a member of the other pair, forming two new pairs
- The new pairs are now moving away from each other
What happens when there is too little energy in a collision (collision theory)?
A collision with too little energy will not produce a reaction.
- Two pairs of particles move towards each other
- The pairs are now touching
- The pairs are now moving apart from each other
What is activation energy?
The minimum amount of energy for a collision to be successful is called the activation energy.
What does the rate of reaction depend on?
The rate of a reaction depends on the rate of successful collisions between reactant particles. The more successful collisions there are, the faster the rate of reaction.
How does temperature increase effect the rate of reaction?
-The reactant particles move more quickly
-They have more energy
-The particles collide successfully more often
-The rate of reaction increases this is due to more particles having energy greater than or equal to the activation energy
How does the increase in concentration or pressure effect the rate of reaction?
-There are more reactant particles per unit volume (ie the reactant particles become more crowded)
-There is a greater chance of the particles colliding, which leads to more successful collisions per second
-The rate of reaction increases
How does surface area effect the rate of reaction?
The rate of a chemical reaction can be raised by increasing the surface area of a solid reactant. This is done by cutting the substance into small pieces, or by grinding it into a powder.
To summarise:
large pieces = small surface area
small pieces (powder) = large surface area
How does the increase in surface area effect the rate of reaction?
-More particles are exposed to the other reactant
-There is a greater chance of particles colliding, which leads to more successful collisions per second
-The rate of reaction increases
