topic 7 - rates of reaction and energy changes

Question 1

precipitation reactions for determining rate of reaction

Answer 1

• mix 2 transparent solutions on a piece of paper with a mark on it
• observe the mark through the mixture and time how long it takes for the mark to be obscured
• the result is subjective - people may disagree on when the mark disappears

Question 2

rate of reaction equation

Answer 2

rate of reaction = amount of reactant used or product formed ÷ time

Question 3

reactants for rate experiments involving gases

Answer 3

marble chips and hydrochloric acid

volume of gas produced against time

Question 4

reagents for rate experiment involving precipitation

Answer 4

sodium thiosulphate and hydrochloric acid (how it changes with temp)

time for X to be obscured against temp

Question 5

activation energy

Answer 5

the minimum energy needed for bonds to break and a reaction to start

on a reaction profile - the energy difference between the reactant and the highest point on the curve

Question 6

exothermic reactions

Answer 6

energy is transferred to the surroundings - temp of surrounding increases

energy is released when bonds in products are formed - exothermic

energy take in to break < energy released when formed

Question 7

endothermic reactions

Answer 7

energy is taken from surroundings, so there is a fall in the temperature of surroundings

energy has to be supplied to break bonds - endothermic process

energy taken in to break > energy released when formed

Question 8

exothermic reaction profile

Answer 8

products are at a lower energy than reactants - energy is lost to surroundings

Question 9

endothermic reaction profile

Answer 9

products are at a higher energy than reactants - energy is taken in

Question 10

measuring temperature changes

Answer 10

• put a polystyrene cup into a large beaker of cotton wool
• add a known volume of the first reagent
• measure starting temp
• add measured volume/mass of second reagent and stir
• add a lid to reduce energy lost by evaporation
• record the max/min temp and calculate the change

Question 11

temp change when dissolving salts in water

Answer 11

measure temp of water then after the salt dissolves

endothermic - ammonium chloride
exothermic - calcium chloride

Question 12

temp change with neutralisation reactions

Answer 12

most are exothermic

but the one between ethanoic acid and sodium carbonate is endothermic

Question 13

are precipitation reactions endothermic or exothermic

Answer 13

exothermic, increase in temperature of surroundings