Rates of Reaction and Energy Changes Flashcards
How do measure rate of reaction?
amount of reactant used/product formed
time
What are three examples of how rate of reaction can be measured?
- precipitation
- change in mass
- volume of gas given off
How can the rate of a reaction be measured using precipitation?
- mix two transparent solutions in a flask over a piece of paper with a mark on it
- observe how long it takes for the mark to be obscured (by the precipitate) - the faster it takes the faster the reaction
Why is using precipitation a bad way of measuring rate of reaction?
it is subjective - people might not agree on when the mark can no longer be seen
How can the rate of a reaction be measured using change in mass?
- put solution on a mass balance - as it reacts gas is released meaning that mass will be lost - the quicker it is lost, the faster the reaction - the gas released may be harmful so use a fume cupboard
How can the rate of a reaction be measured using the volume of gas given off?
- use gas syringe to measure volume of gas given off
- the more gas collected during a set time interval - the faster the reaction - be careful the right size gas syringe is used for the experiment
How can you measure how surface area affects rate of reaction?
- set up a conical flask with marble chips in dilute HCl attached to a gas syringe which will measure how much CO2 is produced
- take readings of how much CO2 is uptaken at regular time intervals
- repeat experiment with same volume and concentration of acid but with smaller marble chips and then with marble powder to see how surface area affects rate of reaction
What affects the rate of reaction? (3)
- using finer particles gives marble larger surface area
- more concentrated solutions have higher rates of reaction (or higher pressure for gases)
- higher temperature means a higher rate of reaction
How can you measure how rate of reaction changes with temperature?
(using a precipitation reaction)
- measure out fixed volumes of Sodium Thiosulfate and Hydrochloric Acid (transparent solutions) in a measuring cylinder
- use water bath to gently heat the solutions to desired temperature (before you mix them)
- mix solutions in a conical flask and place over a piece of paper with a mark and watch it disappear through the cloudy, yellow sulfur produced and time this
- repeat at different temps to see how rate of reaction varies
Can you use graphs to calculate rates of reaction?
(pg 131 - incl. tangents to find gradient of a curve)
What is the collision theory?
the rate of a reaction depends on the collision frequency of reacting particles (obvious)
What is the activation energy?
the minimum energy that particles need for a successful collision
Why does increasing the temperature of particles increase rate of reaction? (2 ways)
- particles move faster at a higher temp so they’re more likely to collide
- increases energy of collisions - more successful collisions as they’re more likely to reach the activation energy
Why does increasing concentration (or pressure in a gas) increase rate of reaction?
more concentrated solution means more particles in the same volume so more collisions will occur
Why does using smaller particles in a solid increase rate of reaction?
breaking a large solid into smaller pieces increases its surface area to volume ratio and so frequency of collisions will increase as there is more area to collide with
What is a catalyst?
a substance which increases rate of reaction without being chemically changed or used up in the reaction
How do catalysts work?
catalysts provide an alternative reaction pathway which has a lower activation energy - so more particles will have at least the minimum amount of energy needed for a collision
Do you know what reaction profiles look like?
pg 133
What do enzymes do?
biological catalysts - catalyse chemical reactions in living cells (e.g. respiration/photosynthesis/protein synthesis catalysed)
What do enzymes in yeast cells do?
used in the fermentation process in alcohol - catalyse the reaction that converts sugars into ethanol and CO2
What is an exothermic reaction?
a reaction that gives out energy to the surroundings usually by heat
What is an endothermic reaction?
a reaction that takes in energy from the surroundings e.g. by heat
What do reaction profiles show?
the amount of energy levels of the reactants and products in a reaction - e.g. exothermic moves down as reactants have more energy than the products (endothermic vice versa)
What is activation energy?
the minimum amount of energy needed for bonds to break and a reaction to start
How is activation energy shown on reaction profiles?
activation energy is the difference between the reactants and the highest point of energy on the graph
