topic 3: physical chemistry

Question 1

exothermic reaction

Answer 1

gives out energy to its surroundings in the form of heat

shown as a rise of temperature

Question 2

endothermic reaction

Answer 2

takes in the energy from the surroundings in the form of heat

shown as a fall in temperature

Question 3

what is enthalpy change

Answer 3

the overall change in energy!!!

Question 4

exothermic enthalpy change

Answer 4

value will be negative (giving out energy = losing energy)

Question 5

endothermic enthalpy change

Answer 5

value will be positive (taking in energy = gaining energy)

Question 6

initial rise in energy (reaction profiles)

Answer 6

shows the minimum amount of energy the reactants need to collide with each other and react

represented as the initial rise in energy

Question 7

components in a reaction profile

Answer 7

reactants
products
activation energy
energy released/absorbed (negative or a positive enthalpy??)

Question 8

calorimetry

Answer 8

the process of measuring the amount of heat released or absorbed during a chemical reaction

Question 9

calorimetry practical (dissolving, displacement and neutralisation)

Answer 9

put 25cm^3 of hydrochloric acid and sodium hydroxide in separate beakers

place the beakers in a water bath and set the heat to 25°C until they are both at the same temperature

add the HCl followed by the NaOH to a polystyrene cup with a lid

take the temperature of the mixture every 30 seconds, and record the highest temperature

Question 10

calorimetry practical (combustion)

Answer 10

put 50g of water in a copper can and record its temperature

weigh the spirit burner and the lid

put the spirit burner underneath the can, and light the wick. heat the water, stirring constantly until the temperature reaches about 50°C

put out the flame using the burner lid, and measure the final temperature of the water

weigh the spirit burner and the lid again

use the measurements youve taken to calculate the enthalpy change

Question 11

ways to prevent energy loss (calorimetry practical - displacement, dissolving + neutralisation)

Answer 11

cotton wool in the beaker gives more insulation

lid prevents loss through evaporation

Question 12

why is a copper container used (calorimetry practical - combustion)

Answer 12

copper conducts heat effecticely

Question 13

ways to ensure heat goes fully into the water (calorimetry practical - combustion)

Answer 13

reduce draughts by using a screen to act as a draught excluder

Question 14

heat energy change

Answer 14

heat energy transferred (J) = mass of liquid being heated (g) x specific heat capacity (J/g/°C) x change in temperature of the liquid (°C)

Question 15

specific heat capacity

Answer 15

the amount of energy needed to raise the temperature of 1 gram of water by 1°C.

e.g the specific heat capacity of water is 4.2J/g/°C so it takes 4.2 joules of energy to raise the temperature of 1 gram of water by 1°C

Question 16

how to calculate the molar enthalpy change

Answer 16

first calculate the heat energy change

find the Mr

using mass = mr x mol calculate the number of mols to find out the heat transferred by 1 mol of fuel

Question 17

molar enthalpy change

Answer 17

the enthalpy change given out by one mole of the reactant

Question 18

collision theory

Answer 18

the more collisions there are the faster the reaction is

particles have to collide with enough energy for the collision to be successful

Question 19

factors that determine the rate of reactions

Answer 19

temperature
the concentration of a solution
surface area
pressure
presence of a catalyst

Question 20

temperature (reaction rate)

Answer 20

particles move faster when temperature is increase, which means there are more collisions

higher temperatures also increase the energy of the collisions. collisions require a certain energy to be successful

temperature increase means that there are more successful collisions

Question 21

concentration (reaction rate)

Answer 21

when a solution is more concentrated it means there are more particles of reactant in the same volume

collisions are more likely so the reaction rate increases

Question 22

pressure (reaction rate)

Answer 22

in a gas, increasing the pressure means that the particles are more crowded

the frequency of collisions between particles will increase, so the rate of reaction also increases

Question 23

smaller solid particles/more surface area (reaction rate)

Answer 23

if one reactant is a solid, breaking it into smaller pieces will increase its surface area to volume ratio

the particles around it will have more area to work on so the frequency of collisions will increase

this means that the rate of reaction will be faster

Question 24

catalyst (reaction rate)

Answer 24

a catalyst is a substance which increases the rate of a reaction

this is done through providing an alternative reaction pathway that has a lower activation energy, so more of the particles have the minimum amount of energy for a reaction to occur

Question 25

rate of reaction formula

Answer 25

amount of reactant used or amount of product formed / time

Question 26

precipitation to measure rate of reaction

Answer 26

mix the two reactant solutions and put the flask on the piece of paper that has a mark on it mix the two reactant solutions and put the flask on a piece of paper that has a mark on it observe the mark through the mixture and measure how long it takes for the mark to be obscured. the faster it disappears, the faster the reaction

Question 27

change in mass to measure rate of reaction

Answer 27

as the gas is released, the lost mass is easily measured on the balance. the quicker the balance drops, the faster the reaction when the reading stops, the balance stops changing

Question 28

the volume of gas to measure rate of reaction

Answer 28

using a gas syringe, you measure the volume of gas given off the more gas given off during a set time interval, the faster the reaction you can tell the reaction has finished when no more gas is produced

Question 29

measure how surface area affects rate

Answer 29

set up the apparatus (conical flask, bung, gas syringe) add marble chips and hydrochloric acid into the flask take readings at regular time intervals by reading the gas syringe (the volume of gas) repeat the experiment with the same volume + acid concentration and exactly the same mass of marble chips, but the marble more crunched up then repeat with powdered chalk (same compound)

Question 30

reaction rate is affected by temperature

Answer 30

sodium thiosulfate and hydrochloric acid react together to form a yellow precipitate of sulfur you can measure the rate by watching a black mark disappear through the cloudy sulfur and measuring how long it takes to go this is repeated at different temperatures

Question 31

measure how using catalysts affect reaction rate (practical)

Answer 31

hydrogen peroxide naturally decomposes, giving off oxygen gas which provides an ideal way to measure the rate of reaction using a gas syringe magnesium (IV) oxide catalyses this reaction set up the apparatus (conical flask filled with hydrogen peroxide, rubber bung connected to a gas syringe) measure the volume of gas at regular time intervals and record the results in the table repeat the experiment with exactly the same volume and concentration, but using a different catalyst e.g copper oxide plot the different points on a graph.

Question 32

reversible reactions

Answer 32

products of the reaction can react with each other and convert back to the original reactant

Question 33

thermal decomposition of ammonium chloride

Answer 33

ammonium chloride is a white solid. when its heated it breaks down into ammonia gas and hydrogen chloride (forward) if you let it cool the ammonia and hydrogen chloride react to reform the solid - this is the backward reaction